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The transient storage zone processes are investigated in a small second order stream with a 2.2 square kilometer watershed. The presence of transient storage zones in small streams impacts the available flow paths for water and results in a wider range of residence times for water and dissolved chemicals than would be predicted by considering only the main channel flow path. Residence times can be used to quantify the health of a stream as several biogeochemical and ecological processes occur in water slowed by transient storage. Sections of the studied stream are impacted by varying types of stream restoration practices and watershed management practices. These practices lead to the presence of a wide range of transient storage zone types. A combination of data collection, field experimentation, and data analysis is used to characterize the variability in transient storage zone processes and residence times in the small stream and to relate the identified storage zone processes to physical characteristics of the stream. Field experimentation took place along measured sections and included geophysical investigations of the subsurface and continuous-injection tracer studies using a conservative tracer with sampling from the main channel flow and from identified transient storage zones following the continuous injection of tracer. Data analysis leads to a preliminary understanding of how various stream and watershed management practices can impact the presence of transient storage zones and the range of residence times and flow paths in small streams.

Hyporheic exchange flow (HEF) at the streambed–water interface (SWI) has been shown to impact the pattern and rate of discharging groundwater flow (GWF) and the consequential transport of heat, solutes and contaminants from the subsurface into streams. However, the control of geographic and hydromorphological catchment characteristics on GWF–HEF interactions is still not fully understood. Here, the spatial variability in flow characteristics in discharge zones was investigated and averaged over three spatial scales in five geographically different catchments in Sweden. Specifically, the deep GWF discharge velocity at the SWI was estimated using steady-state numerical models, accounting for the real multiscale topography and heterogeneous geology, while an analytical model, based on power spectral analysis of the streambed topography and statistical assessments of the stream hydraulics, was used to estimate the HEF. The modeling resulted in large variability in deep GWF and HEF velocities, both within and between catchments, and a regression analysis was performed to explain this observed variability by using a set of independent variables representing catchment topography and geology as well as local stream hydromorphology. Moreover, the HEF velocity was approximately two orders of magnitude larger than the deep GWF velocity in most of the investigated stream reaches, indicating significant potential to accelerate the deep GWF velocity and reduce the discharge areas. The greatest impact occurred in catchments with low average slope and in reaches close to the catchment outlet, where the deep GWF discharge velocity was generally low., QC 20230629

Rivers have an important role in the global carbon cycle, emitting large quantities of greenhouse gases, including methane (CH4). In rivers, most CH4 is produced in the hyporheic zone and emitted to the atmosphere, likely mostly via diffusion. Because of the several times higher warming potential of CH4 compared to CO2, it is essential to understand how, where and when carbon is transformed to CH4 and emitted into the atmosphere. I investigated spatio-temporal variations of CH4 concentration and fluxes in two free-flowing braided river sections of the Vjosa River and their relation to seasonal shifts of discharge, which drive sediment deposition and surface drying in interaction with geomorphology. Localized sampling in the hyporheic zone allowed to investigate the potential dependence of CH4 concentration on oxygen and abundance of alternative electron acceptors as well as organic matter. Compared to other river systems, I found very low CH4 concentrations and fluxes. Measurements along a hyporheic depth gradient revealed seasonal differences: I found differences of concentration along sediment depth in June and between habitats differing in surface water availability (dry vs. wet) in September. Surface drying also led to increased or even negative fluxes. I was not able to define an environmental variable able to universally explain the occurrence of CH4. My results indicate a major impact of small-scale habitat heterogeneity driven by sediment composition and fine sediment deposition on the sediment surface and in the hyporheic zone. This affects oxygen availability, electron acceptors and organic matter making the habitat structure suitable for CH4 production. My work underscores ample knowledge gaps in the understanding of CH4 dynamics and their drivers for rivers in general and especially in braided river systems., Barbara Jechsmayr, BSc, Zusammenfassung in deutscher Sprache, Masterarbeit Universität Innsbruck 2023

Understanding and predicting the geographic distribution of taxa in hierarchical stream landscapes is a cornerstone of river ecology. A central issue is to tease apart the unique and shared effects of local and catchment predictors over species distributions. Here, we tested Hynes's influential hypothesis (1975, Baldi Memorial Lecture) that ‘In every respect, the valley rules the stream'. We predicted that if catchment features exert a major control on in-stream local conditions, the shared effect of local and catchment predictors should largely surpass their unique effects. To test this prediction, we used logistic regression models and variation partitioning to quantify the unique and shared effects of local and catchment predictors on the distribution of two hyporheic crustacean taxa (Bogidiellidae, Amphipoda and Anthuridae, Isopoda) in streams of New Caledonia. We sampled the two taxa at 228 sites. At each site, we quantified nine local predictors related to habitat area and stability, sediment metabolism and water origin, and eight catchment predictors related to geology, area, primary productivity, land use and specific discharge. When analyzed separately, the two predictor types explained the same amount of model variation in occurrence in both taxa. When analyzed jointly, the shared effects of the two predictor types explained twice as much model variation as the unique effect of each. The overriding contribution of shared effects was notably due to controls exerted by catchment area and geology on local habitat size and sediment metabolism, respectively. For both taxa, a model with only these two catchment predictors provided occurrence distribution as reliable as models containing only local predictors or both predictor types. Our findings pave the way for predicting reliably from catchment predictors alone the geographic distribution in local occurrence of taxa in difficult-to-access habitats and landscapes, such as here the hyporheic zone of tropical str

Understanding and predicting the geographic distribution of taxa in hierarchical stream landscapes is a cornerstone of river ecology. A central issue is to tease apart the unique and shared effects of local and catchment predictors over species distributions. Here, we tested Hynes's influential hypothesis (1975, Baldi Memorial Lecture) that ‘In every respect, the valley rules the stream'. We predicted that if catchment features exert a major control on in-stream local conditions, the shared effect of local and catchment predictors should largely surpass their unique effects. To test this prediction, we used logistic regression models and variation partitioning to quantify the unique and shared effects of local and catchment predictors on the distribution of two hyporheic crustacean taxa (Bogidiellidae, Amphipoda and Anthuridae, Isopoda) in streams of New Caledonia. We sampled the two taxa at 228 sites. At each site, we quantified nine local predictors related to habitat area and stability, sediment metabolism and water origin, and eight catchment predictors related to geology, area, primary productivity, land use and specific discharge. When analyzed separately, the two predictor types explained the same amount of model variation in occurrence in both taxa. When analyzed jointly, the shared effects of the two predictor types explained twice as much model variation as the unique effect of each. The overriding contribution of shared effects was notably due to controls exerted by catchment area and geology on local habitat size and sediment metabolism, respectively. For both taxa, a model with only these two catchment predictors provided occurrence distribution as reliable as models containing only local predictors or both predictor types. Our findings pave the way for predicting reliably from catchment predictors alone the geographic distribution in local occurrence of taxa in difficult-to-access habitats and landscapes, such as here the hyporheic zone of tropical str

Hyporheic zones increase freshwater ecosystem resilience to hydrological extremes and global environmental change. However, current conceptualizations of hyporheic exchange, residence time distributions, and the associated biogeochemical cycling in streambed sediments do not always accurately explain the hydrological and biogeochemical complexity observed in streams and rivers. Specifically, existing conceptual models insufficiently represent the coupled transport and reactivity along groundwater and surface water flow paths, the role of autochthonous organic matter in streambed biogeochemical functioning, and the feedbacks between surface-subsurface ecological processes, both within and across spatial and temporal scales. While simplified approaches to these issues are justifiable and necessary for transferability, the exclusion of important hyporheic processes from our conceptualizations can lead to erroneous conclusions and inadequate understanding and management of interconnected surface water and groundwater environments. This is particularly true at the landscape scale, where the organizational principles of spatio-temporal dynamics of hyporheic exchange flow (HEF) and biogeochemical processes remain largely uncharacterized. This article seeks to identify the most important drivers and controls of HEF and biogeochemical cycling based on a comprehensive synthesis of findings from a wide range of river systems. We use these observations to test current paradigms and conceptual models, discussing the interactions of local-to-regional hydrological, geomorphological, and ecological controls of hyporheic zone functioning. This improved conceptualization of the landscape organizational principles of drivers of HEF and biogeochemical processes from reach to catchment scales will inform future river research directions and watershed management strategies.

Hyporheic zone reaction rates are highest just below the sediment-water interface, in a shallow region called the benthic biolayer. Vertical variability of hyporheic reaction rates leads to unexpected reaction kinetics for stream-borne solutes, compared to classical model predictions. We show that deeper, low-reactivity locations within the hyporheic zone retain solutes for extended periods, which delays reactions and causes solutes to persist at higher concentrations in the stream reach than would be predicted by classical approaches. These behaviors are captured by an upscaled model that reveals the fundamental physical and chemical processes in the hyporheic zone. We show how time scales of transport and reaction within the biolayer control solute retention and transformation at the stream scale, and we demonstrate that accurate assessment of stream-scale reactivity requires methods that integrate over all travel times.

The environmental damage suffered by urban water bodies and the need for public water supply result in a greater interest in techniques that enable water treatment in an efficient and ecological way, such as River Bank Filtration (RBF). This technique uses the soil as a filtering medium, as well as the biological activities of organisms that dwell in the Hyporheic Zone (HZ), the zone of interaction between the surface water body and its underlying aquifer. Knowledge of sediments and hyporheic organisms is indispensable to study RBF. The present paper aimed to characterize the HZ of the middle section of Beberibe river (Pernambuco State, Brazil) in its sedimentological and biological aspects, with sampling druing the rainy and dry seasons, in two distinct sampling sites, one in a conserved area and the other in a highly urbanized area. Biological characterization was performed at the level of large taxonomic groups of meiofauna, accounting for 982 individuals, with the three most abundant taxa being Nematoda, Annelida, and Rotifera. Permutational Analysis of Variance (PERMANOVA) statistical tests were performed, showing significant differences for the season and point factors (p < 0.05) in relation to abundance. The highest concentration of individuals and total organic matter were seen in the rainy season, especially at the point located in the urbanized area. With sedimentological characterization by grain size tests of the hyporheic sediments, the predominance of silt was observed during the rainy season, and sandy during the dry season. It was concluded that the main factor that influenced the structure of the meiofauna community was the release of domestic effluents. The information obtained by the present work helps to understand some features of the HZ, which is essential for RBF or other techniques that use the interstitial matrix., Os danos ambientais sofridos pelos corpos hídricos urbanos e a necessidade de suprir o abastecimento público das cidades resultam em um maior interesse por técnicas que possibilitem o tratamento de água de maneira eficiente e ecológica, como a Filtração em Margem (FM). Essa técnica utiliza o próprio solo como meio filtrante, além das atividades biológicas dos organismos que ocupam a Zona Hiporreica (ZH), zona de interação entre o manancial superficial e o aquífero subjacente. É indispensável o conhecimento acerca dos sedimentos e dos organismos hiporreicos para o estudo da FM. Este artigo objetivou caracterizar a ZH do trecho médio do rio Beberibe (Pernambuco, Brasil) em seus aspectos sedimentológicos e biológicos, com coletas nos períodos chuvoso e seco, em dois pontos distintos, um em uma área conservada, e o outro em área altamente urbanizada. A caracterização biológica foi realizada em nível de grandes grupos taxonômicos da meiofauna, contabilizando-se 982 indivíduos, sendo os três táxons mais abundantes o Nematoda, a Annelida e a Rotifera. Realizaram-se testes estatísticos de Análise Permutacional de Variância (PERMANOVA), constatando-se diferenças significativas para os fatores período e ponto (p < 0,05) em relação à abundância. A maior concentração de indivíduos e de matéria orgânica total ocorreu no período chuvoso, sobretudo no ponto localizado na área urbanizada. Por meio da caracterização por ensaios granulométricos dos sedimentos hiporreicos, observou-se a predominância arenosa no período seco, e de silte durante o período chuvoso. Concluiu-se que o principal fator que influenciou na estrutura da comunidade da meiofauna foi o lançamento de efluentes domésticos. As informações obtidas com o presente trabalho auxiliam a compreensão de algumas características da ZH, a qual é essencial para a FM ou outras técnicas ambientais que utilizem a matriz intersticial.

The environmental damage suffered by urban water bodies and the need for public water supply result in a greater interest in techniques that enable water treatment in an efficient and ecological way, such as River Bank Filtration (RBF). This technique uses the soil as a filtering medium, as well as the biological activities of organisms that dwell in the Hyporheic Zone (HZ), the zone of interaction between the surface water body and its underlying aquifer. Knowledge of sediments and hyporheic organisms is indispensable to study RBF. The present paper aimed to characterize the HZ of the middle section of Beberibe river (Pernambuco State, Brazil) in its sedimentological and biological aspects, with sampling druing the rainy and dry seasons, in two distinct sampling sites, one in a conserved area and the other in a highly urbanized area. Biological characterization was performed at the level of large taxonomic groups of meiofauna, accounting for 982 individuals, with the three most abundant taxa being Nematoda, Annelida, and Rotifera. Permutational Analysis of Variance (PERMANOVA) statistical tests were performed, showing significant differences for the season and point factors (p < 0.05) in relation to abundance. The highest concentration of individuals and total organic matter were seen in the rainy season, especially at the point located in the urbanized area. With sedimentological characterization by grain size tests of the hyporheic sediments, the predominance of silt was observed during the rainy season, and sandy during the dry season. It was concluded that the main factor that influenced the structure of the meiofauna community was the release of domestic effluents. The information obtained by the present work helps to understand some features of the HZ, which is essential for RBF or other techniques that use the interstitial matrix., Os danos ambientais sofridos pelos corpos hídricos urbanos e a necessidade de suprir o abastecimento público das cidades resultam em um maior interesse por técnicas que possibilitem o tratamento de água de maneira eficiente e ecológica, como a Filtração em Margem (FM). Essa técnica utiliza o próprio solo como meio filtrante, além das atividades biológicas dos organismos que ocupam a Zona Hiporreica (ZH), zona de interação entre o manancial superficial e o aquífero subjacente. É indispensável o conhecimento acerca dos sedimentos e dos organismos hiporreicos para o estudo da FM. Este artigo objetivou caracterizar a ZH do trecho médio do rio Beberibe (Pernambuco, Brasil) em seus aspectos sedimentológicos e biológicos, com coletas nos períodos chuvoso e seco, em dois pontos distintos, um em uma área conservada, e o outro em área altamente urbanizada. A caracterização biológica foi realizada em nível de grandes grupos taxonômicos da meiofauna, contabilizando-se 982 indivíduos, sendo os três táxons mais abundantes o Nematoda, a Annelida e a Rotifera. Realizaram-se testes estatísticos de Análise Permutacional de Variância (PERMANOVA), constatando-se diferenças significativas para os fatores período e ponto (p < 0,05) em relação à abundância. A maior concentração de indivíduos e de matéria orgânica total ocorreu no período chuvoso, sobretudo no ponto localizado na área urbanizada. Por meio da caracterização por ensaios granulométricos dos sedimentos hiporreicos, observou-se a predominância arenosa no período seco, e de silte durante o período chuvoso. Concluiu-se que o principal fator que influenciou na estrutura da comunidade da meiofauna foi o lançamento de efluentes domésticos. As informações obtidas com o presente trabalho auxiliam a compreensão de algumas características da ZH, a qual é essencial para a FM ou outras técnicas ambientais que utilizem a matriz intersticial.

Rivers and aquifers are continuously exchanging water, driven by processes that occur on various temporal and spatial scales, ranging from small streambed features to large geological structures. The interaction between these two components occurs in permeable sediments below the stream channel, called the hyporheic zone. This zone is an important ecotone in which water, energy, and solutes originating from groundwater and stream water mix. The exchange fluxes through the hyporheic zone are controlled by a distribution of hierarchically nested flow cells of different sizes that are generated by a spectrum of spatial scales of the hydraulic head condition. Thus, a multiscale mathematical approach is required to reach a comprehensive understanding of the hyporheic exchange processes. Therefore, this thesis investigates the roles of regional groundwater flow and hyporheic fluxes in a nested flow system within the streambed sediment. Next, the study assesses the importance of regional and local parameters in generalizing the surface water and groundwater interaction. This division of the top-boundary condition in two scale-intervals of the sub-surface flow is arbitrary but facilitates the analytical procedure. The regional groundwater flow field is evaluated using numerical modeling, accounting for the site-specific landscape morphology and geological heterogeneity of a Swedish boreal catchment. An exact spectral solution is applied to the hyporheic flow with account taken to local streambed topography fluctuation. Combinatorial sampling of the modeled flow data and a Monte Carlo simulation are used in a sensitivity analysis to address the uncertainty in hydrostatic and dynamic head contributions to the hyporheic flow field. Then, the impact of the regional groundwater and the hyporheic flows on the nested flow system in aquatic sediment are studied through superpositioning of the flow fields. This is an efficient approach to analyze the nested flow system because the i, Vattendrag och akviferer utbyter vatten på olika tids- och längdskalor på grund av bottenformationer och det omgivande landskapets topografi. Samspelet av flöden på olika skalor sker i de permeabla sedimenten under det strömmande ytvatten, i den så kallade hyporheiska zonen. Den hyporheiska zonen är en viktig ekoton där vatten, energi och lösta ämnen som kommer från grundvatten och strömmande ytvatten blandas. Flödet i den hyporheiska zonen är uppdelat på flödesceller i ett hierkiskt storlekssystem (spektrum) av olika storlekar som omfattar från flodbäddens formationer till det regionala landskapets topografi. Det innebär att vattendragets hyporeiska zon påverkar det storskaliga grundvattnets utströmning och tvärtom att grundvattnets utströmning påverkar den hyporeiska zonen. Således måste matematiska analyser av vattenströmningen täcka hela det breda intervallet av skalor för att uppnå en tillräcklig förståelse av det hyporheiska utbytesprocesser. I denna avhandling gjorde jag en uppdelning av det regionala grundvattenflödet och det hyporheiskt flödet och undersökte deras respektive roller för strömningsinteraktionen i vattendragens sediment, samt bedömde vikten av regionala och lokala parametrar för att generalisera ytvatten- och grundvatteninteraktionen. En särskilt viktig fråga var att dela upp randvillkoret för grundvattenytan på de två skalintervallen och samtidigt ta hänsyn till infiltrationens begränsning för grundvattenytan. Det regionala grundvattenflödet utvärderades med hjälp av numerisk modellering med hänsyn till platsspecifik landskapsmorfologi, geologisk heterogenitet i ett svenskt borealt avrinningsområde och hydrologisk information, som infiltration och vattendrasnätverket. En exakt spektral lösning applicerades på det hyporeiska flödet med hänsyn till vattendragets lokala topografi på skalor som inte förekom i den regionala grundvattenanalysen. Graden av osäkerhet i det hydrostatiska och dynamiska bidragen till det hyporheiska flödet undersöktes g, QC 20211104

Rivers and aquifers are continuously exchanging water, driven by processes that occur on various temporal and spatial scales, ranging from small streambed features to large geological structures. The interaction between these two components occurs in permeable sediments below the stream channel, called the hyporheic zone. This zone is an important ecotone in which water, energy, and solutes originating from groundwater and stream water mix. The exchange fluxes through the hyporheic zone are controlled by a distribution of hierarchically nested flow cells of different sizes that are generated by a spectrum of spatial scales of the hydraulic head condition. Thus, a multiscale mathematical approach is required to reach a comprehensive understanding of the hyporheic exchange processes. Therefore, this thesis investigates the roles of regional groundwater flow and hyporheic fluxes in a nested flow system within the streambed sediment. Next, the study assesses the importance of regional and local parameters in generalizing the surface water and groundwater interaction. This division of the top-boundary condition in two scale-intervals of the sub-surface flow is arbitrary but facilitates the analytical procedure. The regional groundwater flow field is evaluated using numerical modeling, accounting for the site-specific landscape morphology and geological heterogeneity of a Swedish boreal catchment. An exact spectral solution is applied to the hyporheic flow with account taken to local streambed topography fluctuation. Combinatorial sampling of the modeled flow data and a Monte Carlo simulation are used in a sensitivity analysis to address the uncertainty in hydrostatic and dynamic head contributions to the hyporheic flow field. Then, the impact of the regional groundwater and the hyporheic flows on the nested flow system in aquatic sediment are studied through superpositioning of the flow fields. This is an efficient approach to analyze the nested flow system because the i, Vattendrag och akviferer utbyter vatten på olika tids- och längdskalor på grund av bottenformationer och det omgivande landskapets topografi. Samspelet av flöden på olika skalor sker i de permeabla sedimenten under det strömmande ytvatten, i den så kallade hyporheiska zonen. Den hyporheiska zonen är en viktig ekoton där vatten, energi och lösta ämnen som kommer från grundvatten och strömmande ytvatten blandas. Flödet i den hyporheiska zonen är uppdelat på flödesceller i ett hierkiskt storlekssystem (spektrum) av olika storlekar som omfattar från flodbäddens formationer till det regionala landskapets topografi. Det innebär att vattendragets hyporeiska zon påverkar det storskaliga grundvattnets utströmning och tvärtom att grundvattnets utströmning påverkar den hyporeiska zonen. Således måste matematiska analyser av vattenströmningen täcka hela det breda intervallet av skalor för att uppnå en tillräcklig förståelse av det hyporheiska utbytesprocesser. I denna avhandling gjorde jag en uppdelning av det regionala grundvattenflödet och det hyporheiskt flödet och undersökte deras respektive roller för strömningsinteraktionen i vattendragens sediment, samt bedömde vikten av regionala och lokala parametrar för att generalisera ytvatten- och grundvatteninteraktionen. En särskilt viktig fråga var att dela upp randvillkoret för grundvattenytan på de två skalintervallen och samtidigt ta hänsyn till infiltrationens begränsning för grundvattenytan. Det regionala grundvattenflödet utvärderades med hjälp av numerisk modellering med hänsyn till platsspecifik landskapsmorfologi, geologisk heterogenitet i ett svenskt borealt avrinningsområde och hydrologisk information, som infiltration och vattendrasnätverket. En exakt spektral lösning applicerades på det hyporeiska flödet med hänsyn till vattendragets lokala topografi på skalor som inte förekom i den regionala grundvattenanalysen. Graden av osäkerhet i det hydrostatiska och dynamiska bidragen till det hyporheiska flödet undersöktes g, QC 20211104

Metoprolol is widely used as a beta-blocker and considered an emerging contaminant of environmental concern due to pseudo persistence in wastewater effluents that poses a potential ecotoxicological threat to aquatic ecosystems. Microbial removal of metoprolol in the redox-delineated hyporheic zone (HZ) was investigated using streambed sediments supplemented with 15 or 150 mu M metoprolol in a laboratory microcosm incubation under oxic and anoxic conditions. Metoprolol disappeared from the aqueous phase under oxic and anoxic conditions within 65 and 72 days, respectively. Metoprolol was refed twice after initial depletion resulting in accelerated disappearance under both conditions. Metoprolol disappearance was marginal in sterile control microcosms with autoclaved sediment. Metoprolol was transformed mainly to metoprolol acid in oxic microcosms, while metoprolol acid and alpha-hydroxymetoprolol were formed in anoxic microcosms. Transformation products were transient and disappeared within 30 days under both conditions. Effects of metoprolol on the HZ bacterial community were evaluated using DNA- and RNA-based time-resolved amplicon Illumina MiSeq sequencing targeting the 16S rRNA gene and 16S rRNA, respectively, and were prominent on 16S rRNA rather than 16S rRNA gene level suggesting moderate metoprolol-induced activity-level changes. A positive impact of metoprolol on Sphingomonadaceae and Enterobacteriaceae under oxic and anoxic conditions, respectively, was observed. Nitrifiers were impaired by metoprolol under oxic and anoxic conditions. Collectively, our findings revealed high metoprolol biodegradation potentials in the hyporheic zone under contrasting redox conditions associated with changes in the active microbial communities, thus contributing to the attenuation of micropollutants.

Hyporheic exchange (HE) contributes to the biogeochemical turnover of macro- and micro-pollutants in rivers. However, the spatiotemporal complexity and variability of HE hinder understanding of its role in the overall functioning of riverine ecosystems. The present study focuses on investigating the role of bacterial diversity and sediment morphology on HE using a multi-flume experiment. A fully coupled surface–subsurface numerical model was used to highlight complex exchange patterns between surface water and the underlying flow field in the sediments. Under the experimental conditions, the surface water flow induced by bedforms has a prominent effect on both local trajectories and residence time distributions of hyporheic flow paths, whereas mean hyporheic retention times are mainly modulated by average surface flowrates. In case of complex bedform morphologies, the numerical model successfully reproduces the HE estimated by means of salt dilution tests. However, the 2D numerical representation of the system falls short in predicting HE in absence of bedforms, highlighting the intrinsic complexity of water circulation patterns in real scenarios. Finally, results show that higher bacterial diversities in the stream sediments can significantly reduce hyporheic fluxes. This work provides a framework to interpret micropollutants turnover in light of the underlying physical transport processes in the hyporheic zone. The study emphasizes the importance of better understanding the tradeoff between physically driven transport processes and bacterial dynamics in the hyporheic zone to quantify the fate of pollutants in streams and rivers.

The hyporheic zone is the reactive interface between surface water and groundwater found beneath streams and rivers, where chemical gradients and an abundant biological presence allow beneficial attenuation of contaminants. Such attenuation often requires reactants from surface water and groundwater to mix, but few studies have explored the controls on mixing of upwelling groundwater water in the hyporheic zone and its potential to foster mixing-dependent reactions. The goals of this dissertation are therefore to evaluate the effects of (1) hydraulic controls and (2) reaction kinetic controls on hyporheic mixing and mixing-dependent reactions, and (3) use two-dimensional visualization techniques to quantify patterns of hyporheic mixing and mixing-dependent reactions. These objectives were addressed by hyporheic zone simulations using a laboratory sediment mesocosm and numerical models. In the laboratory, a hyporheic flow cell was created to observe both conservative dye mixing and abiotic mixing-dependent reaction. The numerical models MODFLOW and SEAM3D were then used to simulate the experimental data to better understand hydraulic and transport processes underlying laboratory observations and provide sensitivity analysis on hydraulic and reaction kinetic parameters. Visualization techniques showed a distinct mixing zone developing over time for both conservative and reactive conditions. Mixing zone thickness in both conditions depended on surface water head drop and the ratio of boundary inflows of surface water and groundwater (inflow ratios). The abiotic reaction caused the mixing zone to shift even under steady-state hydraulics indicating that hyporheic zone mixing-dependent reactions affect the location of mixing as chemical transformations take place. The numerical model further showed the production zone to be thicker than the mixing zone and located where reactants had already been depleted. Finally, mapping of two-dimensional microbial respiration (i.e., e

Os danos ambientais sofridos pelos corpos hídricos urbanos e a necessidade de suprir o abastecimento público das cidades resultam em um maior interesse por técnicas que possibilitem o tratamento de água de maneira eficiente e ecológica, como a Filtração em Margem (FM). Essa técnica utiliza o próprio solo como meio filtrante, além das atividades biológicas dos organismos que ocupam a Zona Hiporreica (ZH), zona de interação entre o manancial superficial e o aquífero subjacente. É indispensável o conhecimento acerca dos sedimentos e dos organismos hiporreicos para o estudo da FM. Este artigo objetivou caracterizar a ZH do trecho médio do rio Beberibe (Pernambuco, Brasil) em seus aspectos sedimentológicos e biológicos, com coletas nos períodos chuvoso e seco, em dois pontos distintos, um em uma área conservada, e o outro em área altamente urbanizada. A caracterização biológica foi realizada em nível de grandes grupos taxonômicos da meiofauna, contabilizando-se 982 indivíduos, sendo os três táxons mais abundantes o Nematoda, a Annelida e a Rotifera. Realizaram-se testes estatísticos de Análise Permutacional de Variância (PERMANOVA), constatando-se diferenças significativas para os fatores período e ponto (p < 0,05) em relação à abundância. A maior concentração de indivíduos e de matéria orgânica total ocorreu no período chuvoso, sobretudo no ponto localizado na área urbanizada. Por meio da caracterização por ensaios granulométricos dos sedimentos hiporreicos, observou-se a predominância arenosa no período seco, e de silte durante o período chuvoso. Concluiu-se que o principal fator que influenciou na estrutura da comunidade da meiofauna foi o lançamento de efluentes domésticos. As informações obtidas com o presente trabalho auxiliam a compreensão de algumas características da ZH, a qual é essencial para a FM ou outras técnicas ambientais que utilizem a matriz intersticial., The environmental damage suffered by urban water bodies and the need for public water supply result in a greater interest in techniques that enable water treatment in an efficient and ecological way, such as River Bank Filtration (RBF). This technique uses the soil as a filtering medium, as well as the biological activities of organisms that dwell in the Hyporheic Zone (HZ), the zone of interaction between the surface water body and its underlying aquifer. Knowledge of sediments and hyporheic organisms is indispensable to study RBF. The present paper aimed to characterize the HZ of the middle section of Beberibe river (Pernambuco State, Brazil) in its sedimentological and biological aspects, with sampling druing the rainy and dry seasons, in two distinct sampling sites, one in a conserved area and the other in a highly urbanized area. Biological characterization was performed at the level of large taxonomic groups of meiofauna, accounting for 982 individuals, with the three most abundant taxa being Nematoda, Annelida, and Rotifera. Permutational Analysis of Variance (PERMANOVA) statistical tests were performed, showing significant differences for the season and point factors (p < 0.05) in relation to abundance. The highest concentration of individuals and total organic matter were seen in the rainy season, especially at the point located in the urbanized area. With sedimentological characterization by grain size tests of the hyporheic sediments, the predominance of silt was observed during the rainy season, and sandy during the dry season. It was concluded that the main factor that influenced the structure of the meiofauna community was the release of domestic effluents. The information obtained by the present work helps to understand some features of the HZ, which is essential for RBF or other techniques that use the interstitial matrix.

Os danos ambientais sofridos pelos corpos hídricos urbanos e a necessidade de suprir o abastecimento público das cidades resultam em um maior interesse por técnicas que possibilitem o tratamento de água de maneira eficiente e ecológica, como a Filtração em Margem (FM). Essa técnica utiliza o próprio solo como meio filtrante, além das atividades biológicas dos organismos que ocupam a Zona Hiporreica (ZH), zona de interação entre o manancial superficial e o aquífero subjacente. É indispensável o conhecimento acerca dos sedimentos e dos organismos hiporreicos para o estudo da FM. Este artigo objetivou caracterizar a ZH do trecho médio do rio Beberibe (Pernambuco, Brasil) em seus aspectos sedimentológicos e biológicos, com coletas nos períodos chuvoso e seco, em dois pontos distintos, um em uma área conservada, e o outro em área altamente urbanizada. A caracterização biológica foi realizada em nível de grandes grupos taxonômicos da meiofauna, contabilizando-se 982 indivíduos, sendo os três táxons mais abundantes o Nematoda, a Annelida e a Rotifera. Realizaram-se testes estatísticos de Análise Permutacional de Variância (PERMANOVA), constatando-se diferenças significativas para os fatores período e ponto (p < 0,05) em relação à abundância. A maior concentração de indivíduos e de matéria orgânica total ocorreu no período chuvoso, sobretudo no ponto localizado na área urbanizada. Por meio da caracterização por ensaios granulométricos dos sedimentos hiporreicos, observou-se a predominância arenosa no período seco, e de silte durante o período chuvoso. Concluiu-se que o principal fator que influenciou na estrutura da comunidade da meiofauna foi o lançamento de efluentes domésticos. As informações obtidas com o presente trabalho auxiliam a compreensão de algumas características da ZH, a qual é essencial para a FM ou outras técnicas ambientais que utilizem a matriz intersticial., The environmental damage suffered by urban water bodies and the need for public water supply result in a greater interest in techniques that enable water treatment in an efficient and ecological way, such as River Bank Filtration (RBF). This technique uses the soil as a filtering medium, as well as the biological activities of organisms that dwell in the Hyporheic Zone (HZ), the zone of interaction between the surface water body and its underlying aquifer. Knowledge of sediments and hyporheic organisms is indispensable to study RBF. The present paper aimed to characterize the HZ of the middle section of Beberibe river (Pernambuco State, Brazil) in its sedimentological and biological aspects, with sampling druing the rainy and dry seasons, in two distinct sampling sites, one in a conserved area and the other in a highly urbanized area. Biological characterization was performed at the level of large taxonomic groups of meiofauna, accounting for 982 individuals, with the three most abundant taxa being Nematoda, Annelida, and Rotifera. Permutational Analysis of Variance (PERMANOVA) statistical tests were performed, showing significant differences for the season and point factors (p < 0.05) in relation to abundance. The highest concentration of individuals and total organic matter were seen in the rainy season, especially at the point located in the urbanized area. With sedimentological characterization by grain size tests of the hyporheic sediments, the predominance of silt was observed during the rainy season, and sandy during the dry season. It was concluded that the main factor that influenced the structure of the meiofauna community was the release of domestic effluents. The information obtained by the present work helps to understand some features of the HZ, which is essential for RBF or other techniques that use the interstitial matrix.

Rivers and aquifers are continuously exchanging water, driven by processes that occur on various temporal and spatial scales, ranging from small streambed features to large geological structures. The interaction between these two components occurs in permeable sediments below the stream channel, called the hyporheic zone. This zone is an important ecotone in which water, energy, and solutes originating from groundwater and stream water mix. The exchange fluxes through the hyporheic zone are controlled by a distribution of hierarchically nested flow cells of different sizes that are generated by a spectrum of spatial scales of the hydraulic head condition. Thus, a multiscale mathematical approach is required to reach a comprehensive understanding of the hyporheic exchange processes. Therefore, this thesis investigates the roles of regional groundwater flow and hyporheic fluxes in a nested flow system within the streambed sediment. Next, the study assesses the importance of regional and local parameters in generalizing the surface water and groundwater interaction. This division of the top-boundary condition in two scale-intervals of the sub-surface flow is arbitrary but facilitates the analytical procedure. The regional groundwater flow field is evaluated using numerical modeling, accounting for the site-specific landscape morphology and geological heterogeneity of a Swedish boreal catchment. An exact spectral solution is applied to the hyporheic flow with account taken to local streambed topography fluctuation. Combinatorial sampling of the modeled flow data and a Monte Carlo simulation are used in a sensitivity analysis to address the uncertainty in hydrostatic and dynamic head contributions to the hyporheic flow field. Then, the impact of the regional groundwater and the hyporheic flows on the nested flow system in aquatic sediment are studied through superpositioning of the flow fields. This is an efficient approach to analyze the nested flow system because the i, Vattendrag och akviferer utbyter vatten på olika tids- och längdskalor på grund av bottenformationer och det omgivande landskapets topografi. Samspelet av flöden på olika skalor sker i de permeabla sedimenten under det strömmande ytvatten, i den så kallade hyporheiska zonen. Den hyporheiska zonen är en viktig ekoton där vatten, energi och lösta ämnen som kommer från grundvatten och strömmande ytvatten blandas. Flödet i den hyporheiska zonen är uppdelat på flödesceller i ett hierkiskt storlekssystem (spektrum) av olika storlekar som omfattar från flodbäddens formationer till det regionala landskapets topografi. Det innebär att vattendragets hyporeiska zon påverkar det storskaliga grundvattnets utströmning och tvärtom att grundvattnets utströmning påverkar den hyporeiska zonen. Således måste matematiska analyser av vattenströmningen täcka hela det breda intervallet av skalor för att uppnå en tillräcklig förståelse av det hyporheiska utbytesprocesser. I denna avhandling gjorde jag en uppdelning av det regionala grundvattenflödet och det hyporheiskt flödet och undersökte deras respektive roller för strömningsinteraktionen i vattendragens sediment, samt bedömde vikten av regionala och lokala parametrar för att generalisera ytvatten- och grundvatteninteraktionen. En särskilt viktig fråga var att dela upp randvillkoret för grundvattenytan på de två skalintervallen och samtidigt ta hänsyn till infiltrationens begränsning för grundvattenytan. Det regionala grundvattenflödet utvärderades med hjälp av numerisk modellering med hänsyn till platsspecifik landskapsmorfologi, geologisk heterogenitet i ett svenskt borealt avrinningsområde och hydrologisk information, som infiltration och vattendrasnätverket. En exakt spektral lösning applicerades på det hyporeiska flödet med hänsyn till vattendragets lokala topografi på skalor som inte förekom i den regionala grundvattenanalysen. Graden av osäkerhet i det hydrostatiska och dynamiska bidragen till det hyporheiska flödet undersöktes g, QC 20211104

Rivers and aquifers are continuously exchanging water, driven by processes that occur on various temporal and spatial scales, ranging from small streambed features to large geological structures. The interaction between these two components occurs in permeable sediments below the stream channel, called the hyporheic zone. This zone is an important ecotone in which water, energy, and solutes originating from groundwater and stream water mix. The exchange fluxes through the hyporheic zone are controlled by a distribution of hierarchically nested flow cells of different sizes that are generated by a spectrum of spatial scales of the hydraulic head condition. Thus, a multiscale mathematical approach is required to reach a comprehensive understanding of the hyporheic exchange processes. Therefore, this thesis investigates the roles of regional groundwater flow and hyporheic fluxes in a nested flow system within the streambed sediment. Next, the study assesses the importance of regional and local parameters in generalizing the surface water and groundwater interaction. This division of the top-boundary condition in two scale-intervals of the sub-surface flow is arbitrary but facilitates the analytical procedure. The regional groundwater flow field is evaluated using numerical modeling, accounting for the site-specific landscape morphology and geological heterogeneity of a Swedish boreal catchment. An exact spectral solution is applied to the hyporheic flow with account taken to local streambed topography fluctuation. Combinatorial sampling of the modeled flow data and a Monte Carlo simulation are used in a sensitivity analysis to address the uncertainty in hydrostatic and dynamic head contributions to the hyporheic flow field. Then, the impact of the regional groundwater and the hyporheic flows on the nested flow system in aquatic sediment are studied through superpositioning of the flow fields. This is an efficient approach to analyze the nested flow system because the i, Vattendrag och akviferer utbyter vatten på olika tids- och längdskalor på grund av bottenformationer och det omgivande landskapets topografi. Samspelet av flöden på olika skalor sker i de permeabla sedimenten under det strömmande ytvatten, i den så kallade hyporheiska zonen. Den hyporheiska zonen är en viktig ekoton där vatten, energi och lösta ämnen som kommer från grundvatten och strömmande ytvatten blandas. Flödet i den hyporheiska zonen är uppdelat på flödesceller i ett hierkiskt storlekssystem (spektrum) av olika storlekar som omfattar från flodbäddens formationer till det regionala landskapets topografi. Det innebär att vattendragets hyporeiska zon påverkar det storskaliga grundvattnets utströmning och tvärtom att grundvattnets utströmning påverkar den hyporeiska zonen. Således måste matematiska analyser av vattenströmningen täcka hela det breda intervallet av skalor för att uppnå en tillräcklig förståelse av det hyporheiska utbytesprocesser. I denna avhandling gjorde jag en uppdelning av det regionala grundvattenflödet och det hyporheiskt flödet och undersökte deras respektive roller för strömningsinteraktionen i vattendragens sediment, samt bedömde vikten av regionala och lokala parametrar för att generalisera ytvatten- och grundvatteninteraktionen. En särskilt viktig fråga var att dela upp randvillkoret för grundvattenytan på de två skalintervallen och samtidigt ta hänsyn till infiltrationens begränsning för grundvattenytan. Det regionala grundvattenflödet utvärderades med hjälp av numerisk modellering med hänsyn till platsspecifik landskapsmorfologi, geologisk heterogenitet i ett svenskt borealt avrinningsområde och hydrologisk information, som infiltration och vattendrasnätverket. En exakt spektral lösning applicerades på det hyporeiska flödet med hänsyn till vattendragets lokala topografi på skalor som inte förekom i den regionala grundvattenanalysen. Graden av osäkerhet i det hydrostatiska och dynamiska bidragen till det hyporheiska flödet undersöktes g, QC 20211104

Rivers and aquifers are continuously exchanging water, driven by processes that occur on various temporal and spatial scales, ranging from small streambed features to large geological structures. The interaction between these two components occurs in permeable sediments below the stream channel, called the hyporheic zone. This zone is an important ecotone in which water, energy, and solutes originating from groundwater and stream water mix. The exchange fluxes through the hyporheic zone are controlled by a distribution of hierarchically nested flow cells of different sizes that are generated by a spectrum of spatial scales of the hydraulic head condition. Thus, a multiscale mathematical approach is required to reach a comprehensive understanding of the hyporheic exchange processes. Therefore, this thesis investigates the roles of regional groundwater flow and hyporheic fluxes in a nested flow system within the streambed sediment. Next, the study assesses the importance of regional and local parameters in generalizing the surface water and groundwater interaction. This division of the top-boundary condition in two scale-intervals of the sub-surface flow is arbitrary but facilitates the analytical procedure. The regional groundwater flow field is evaluated using numerical modeling, accounting for the site-specific landscape morphology and geological heterogeneity of a Swedish boreal catchment. An exact spectral solution is applied to the hyporheic flow with account taken to local streambed topography fluctuation. Combinatorial sampling of the modeled flow data and a Monte Carlo simulation are used in a sensitivity analysis to address the uncertainty in hydrostatic and dynamic head contributions to the hyporheic flow field. Then, the impact of the regional groundwater and the hyporheic flows on the nested flow system in aquatic sediment are studied through superpositioning of the flow fields. This is an efficient approach to analyze the nested flow system because the i, Vattendrag och akviferer utbyter vatten på olika tids- och längdskalor på grund av bottenformationer och det omgivande landskapets topografi. Samspelet av flöden på olika skalor sker i de permeabla sedimenten under det strömmande ytvatten, i den så kallade hyporheiska zonen. Den hyporheiska zonen är en viktig ekoton där vatten, energi och lösta ämnen som kommer från grundvatten och strömmande ytvatten blandas. Flödet i den hyporheiska zonen är uppdelat på flödesceller i ett hierkiskt storlekssystem (spektrum) av olika storlekar som omfattar från flodbäddens formationer till det regionala landskapets topografi. Det innebär att vattendragets hyporeiska zon påverkar det storskaliga grundvattnets utströmning och tvärtom att grundvattnets utströmning påverkar den hyporeiska zonen. Således måste matematiska analyser av vattenströmningen täcka hela det breda intervallet av skalor för att uppnå en tillräcklig förståelse av det hyporheiska utbytesprocesser. I denna avhandling gjorde jag en uppdelning av det regionala grundvattenflödet och det hyporheiskt flödet och undersökte deras respektive roller för strömningsinteraktionen i vattendragens sediment, samt bedömde vikten av regionala och lokala parametrar för att generalisera ytvatten- och grundvatteninteraktionen. En särskilt viktig fråga var att dela upp randvillkoret för grundvattenytan på de två skalintervallen och samtidigt ta hänsyn till infiltrationens begränsning för grundvattenytan. Det regionala grundvattenflödet utvärderades med hjälp av numerisk modellering med hänsyn till platsspecifik landskapsmorfologi, geologisk heterogenitet i ett svenskt borealt avrinningsområde och hydrologisk information, som infiltration och vattendrasnätverket. En exakt spektral lösning applicerades på det hyporeiska flödet med hänsyn till vattendragets lokala topografi på skalor som inte förekom i den regionala grundvattenanalysen. Graden av osäkerhet i det hydrostatiska och dynamiska bidragen till det hyporheiska flödet undersöktes g, QC 20211104

Interaction between surface water and groundwater plays a fundamental role in influencing aquatic chemistry, where hyporheic exchange processes, distribution of flow paths and residence times within the hyporheic zone will influence the transport of mass and energy in the surface-water/groundwater system. Geomorphological conditions greatly influence hyporheic exchange, and heterogeneities such as rocks and clay lenses will be a key factor for delineating the hyporheic zone. Electrical resistivity tomography (ERT) and ground-penetrating radar (GPR) were used to investigate the streambed along a 6.3-m-long reach in order to characterise geological layering and distinct features which may influence parameters such as hydraulic conductivity. Time-lapse ERT measurements taken during a tracer injection demonstrated that geological features at the meter-scale played a determining role for the hyporheic flow field. The penetration depth of the tracer into the streambed sediment displayed a variable spatial pattern in areas where the presence of highly resistive anomalies was detected. In areas with more homogeneous sediments, the penetration depth was much more uniformly distributed than observed in more heterogeneous sections, demonstrating that ERT can play a vital role in identifying critical hydraulic features that may influence hyporheic exchange processes. Reciprocal ERT measurements linked variability and thus uncertainty in the modelled resistivity to the spatial locations, which also demonstrated larger variability in the tracer penetration depth, likely due to local heterogeneity in the hydraulic conductivity field., QC 20240110

Hyporheic zones (HZs) are reactive transition regions between rivers and aquifers which are thought to play an important role in the attenuation of micropollutants. Micropollutants are chemical substances such as pharmaceuticals, industrial chemicals or personal care products that are found in trace concentrations in the environment and that can be harmful to organisms. This thesis aimed to narrow the knowledge gap on the environmental fate of wastewater-derived polar organic micropollutants in the aquatic environment, with a specific emphasis on the hyporheic zone. In Paper I an efficient workflow was developed for the in-situ characterization of polar organic micropollutants and their transformation products (TPs) in the hyporheic zone at high spatial and temporal resolution and with minimal disturbance of natural flow paths. A low volume sampling device was combined with a newly developed high throughput-direct injection-UHPLC-MS/MS method. Application in the field revealed significant differences in micropollutant concentrations that varied over small time- and spatial scales. In Paper II the results of a comprehensive field study performed in the urban lowland river Erpe in Berlin, Germany, are presented. The work provided data on in-situ attenuation behavior of 24 micropollutants and TPs, along with novel insights into the spatially- and temporally varying environmental factors which play a major role in controlling in-stream attenuation of micropollutants. Paper III describes a novel, multi-flume experiment designed to investigate the influence of hyporheic exchange flow and sediment bacterial diversity on dissipation half-lives of 31 micropollutants and associated TPs. Attenuation and transformation of most substances increased significantly with bacterial diversity; fewer compounds responded to both bacterial diversity and hyporheic exchange flow. In addition to the discovery of several novel TPs, a number of bacterial strains were identified that might be, At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.

Ibuprofen, a non-steroidal anti-inflammatory pain reliever, is among pharmaceutical residues of environmental concern ubiquitously detected in wastewater effluents and receiving rivers. Thus, ibuprofen removal potentials and associated bacteria in the hyporheic zone sediments of an impacted river were investigated. Microbially mediated ibuprofen degradation was determined in oxic sediment microcosms amended with ibuprofen (5, 40, 200, and 400 µM), or ibuprofen and acetate, relative to an un-amended control. Ibuprofen was removed by the original sediment microbial community as well as in ibuprofen-enrichments obtained by re-feeding of ibuprofen. Here, 1-, 2-, 3-hydroxy- and carboxy-ibuprofen were the primary transformation products. Quantitative real-time PCR analysis revealed a significantly higher 16S rRNA abundance in ibuprofen-amended relative to un-amended incubations. Time-resolved microbial community dynamics evaluated by 16S rRNA gene and 16S rRNA analyses revealed many new ibuprofen responsive taxa of the Acidobacteria, Actinobacteria, Bacteroidetes, Gemmatimonadetes, Latescibacteria, and Proteobacteria. Two ibuprofen-degrading strains belonging to the genera Novosphingobium and Pseudomonas were isolated from the ibuprofen-enriched sediments, consuming 400 and 300 µM ibuprofen within three and eight days, respectively. The collective results indicated that the hyporheic zone sediments sustain an efficient biotic (micro-)pollutant degradation potential, and hitherto unknown microbial diversity associated with such (micro)pollutant removal.

Hyporheic zones (HZs) are reactive transition regions between rivers and aquifers which are thought to play an important role in the attenuation of micropollutants. Micropollutants are chemical substances such as pharmaceuticals, industrial chemicals or personal care products that are found in trace concentrations in the environment and that can be harmful to organisms. This thesis aimed to narrow the knowledge gap on the environmental fate of wastewater-derived polar organic micropollutants in the aquatic environment, with a specific emphasis on the hyporheic zone. In Paper I an efficient workflow was developed for the in-situ characterization of polar organic micropollutants and their transformation products (TPs) in the hyporheic zone at high spatial and temporal resolution and with minimal disturbance of natural flow paths. A low volume sampling device was combined with a newly developed high throughput-direct injection-UHPLC-MS/MS method. Application in the field revealed significant differences in micropollutant concentrations that varied over small time- and spatial scales. In Paper II the results of a comprehensive field study performed in the urban lowland river Erpe in Berlin, Germany, are presented. The work provided data on in-situ attenuation behavior of 24 micropollutants and TPs, along with novel insights into the spatially- and temporally varying environmental factors which play a major role in controlling in-stream attenuation of micropollutants. Paper III describes a novel, multi-flume experiment designed to investigate the influence of hyporheic exchange flow and sediment bacterial diversity on dissipation half-lives of 31 micropollutants and associated TPs. Attenuation and transformation of most substances increased significantly with bacterial diversity; fewer compounds responded to both bacterial diversity and hyporheic exchange flow. In addition to the discovery of several novel TPs, a number of bacterial strains were identified that might be, At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.

The interactions of groundwater with surface waters such as streams, lakes, wetlands, or oceans are relevant for a wide range of reasons—for example, drinking water resources may rely on hydrologic fluxes between groundwater and surface water. However, nutrients and pollutants can also be transported across the interface and experience transformation, enrichment, or retention along the flow paths and cause impacts on the interconnected receptor systems. To maintain drinking water resources and ecosystem health, a mechanistic understanding of the underlying processes controlling the spatial patterns and temporal dynamics of groundwater–surface water interactions is crucial. This Special Issue provides an overview of current research advances and innovative approaches in the broad field of groundwater–surface water interactions. The 20 research articles and 1 communication of this Special Issue cover a wide range of thematic scopes, scales, and experimental and modelling methods across different disciplines (hydrology, aquatic ecology, biogeochemistry, environmental pollution) collaborating in research on groundwater–surface water interactions. The collection of research papers in this Special Issue also allows the identification of current knowledge gaps and reveals the challenges in establishing standardized measurement, observation, and assessment approaches. With regards to its relevance for environmental and water management and protection, the impact of groundwater–surface water interactions is still not fully understood and is often underestimated, which is not only due to a lack of awareness but also a lack of knowledge and experience regarding appropriate measurement and analysis approaches. This lack of knowledge exchange from research into management practice suggests that more efforts are needed to disseminate scientific results and methods to practitioners and policy makers., Horizon 2020, Deutsche Forschungsgemeinschaft, Peer Reviewed

Hyporheic zones (HZs) are reactive transition regions between rivers and aquifers which are thought to play an important role in the attenuation of micropollutants. Micropollutants are chemical substances such as pharmaceuticals, industrial chemicals or personal care products that are found in trace concentrations in the environment and that can be harmful to organisms. This thesis aimed to narrow the knowledge gap on the environmental fate of wastewater-derived polar organic micropollutants in the aquatic environment, with a specific emphasis on the hyporheic zone. In Paper I an efficient workflow was developed for the in-situ characterization of polar organic micropollutants and their transformation products (TPs) in the hyporheic zone at high spatial and temporal resolution and with minimal disturbance of natural flow paths. A low volume sampling device was combined with a newly developed high throughput-direct injection-UHPLC-MS/MS method. Application in the field revealed significant differences in micropollutant concentrations that varied over small time- and spatial scales. In Paper II the results of a comprehensive field study performed in the urban lowland river Erpe in Berlin, Germany, are presented. The work provided data on in-situ attenuation behavior of 24 micropollutants and TPs, along with novel insights into the spatially- and temporally varying environmental factors which play a major role in controlling in-stream attenuation of micropollutants. Paper III describes a novel, multi-flume experiment designed to investigate the influence of hyporheic exchange flow and sediment bacterial diversity on dissipation half-lives of 31 micropollutants and associated TPs. Attenuation and transformation of most substances increased significantly with bacterial diversity; fewer compounds responded to both bacterial diversity and hyporheic exchange flow. In addition to the discovery of several novel TPs, a number of bacterial strains were identified that might be, At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.

The organic carbon in streambed sediments drives multiple biogeochemical reactions, including the attenuation of organic micropollutants. An attenuation assay using sediment microcosms differing in the initial total organic carbon (TOC) revealed higher microbiome and sorption associated removal efficiencies of trace organic compounds (TrOCs) in the high-TOC compared to the low-TOC sediments. Overall, the combined microbial and sorption associated removal efficiencies of the micropollutants were generally higher than by sorption alone for all compounds tested except propranolol whose removal efficiency was similar via both mechanisms. Quantitative real-time PCR and time-resolved 16S rRNA gene amplicon sequencing revealed that higher bacterial abundance and diversity in the high-TOC sediments correlated with higher microbial removal efficiencies of most TrOCs. The bacterial community in the high-TOC sediment samples remained relatively stable against the stressor effects of TrOC amendment compared to the low-TOC sediment community that was characterized by a decline in the relative abundance of most phyla except Proteobacteria. Bacterial genera that were significantly more abundant in amended relative to unamended sediment samples and thus associated with biodegradation of the TrOCs included Xanthobacter, Hyphomicrobium, Novosphingobium, Reyranella and Terrimonas. The collective results indicated that the TOC content influences the microbial community dynamics and associated biotransformation of TrOCs as well as the sorption potential of the hyporheic zone sediments.

The organic carbon in streambed sediments drives multiple biogeochemical reactions, including the attenuation of organic micropollutants. An attenuation assay using sediment microcosms differing in the initial total organic carbon (TOC) revealed higher microbiome and sorption associated removal efficiencies of trace organic compounds (TrOCs) in the high-TOC compared to the low-TOC sediments. Overall, the combined microbial and sorption associated removal efficiencies of the micropollutants were generally higher than by sorption alone for all compounds tested except propranolol whose removal efficiency was similar via both mechanisms. Quantitative real-time PCR and time-resolved 16S rRNA gene amplicon sequencing revealed that higher bacterial abundance and diversity in the high-TOC sediments correlated with higher microbial removal efficiencies of most TrOCs. The bacterial community in the high-TOC sediment samples remained relatively stable against the stressor effects of TrOC amendment compared to the low-TOC sediment community that was characterized by a decline in the relative abundance of most phyla except Proteobacteria. Bacterial genera that were significantly more abundant in amended relative to unamended sediment samples and thus associated with biodegradation of the TrOCs included Xanthobacter, Hyphomicrobium, Novosphingobium, Reyranella and Terrimonas. The collective results indicated that the TOC content influences the microbial community dynamics and associated biotransformation of TrOCs as well as the sorption potential of the hyporheic zone sediments.

Ibuprofen, a non-steroidal anti-inflammatory pain reliever, is among pharmaceutical residues of environmental concern ubiquitously detected in wastewater effluents and receiving rivers. Thus, ibuprofen removal potentials and associated bacteria in the hyporheic zone sediments of an impacted river were investigated. Microbially mediated ibuprofen degradation was determined in oxic sediment microcosms amended with ibuprofen (5, 40, 200, and 400 µM), or ibuprofen and acetate, relative to an un-amended control. Ibuprofen was removed by the original sediment microbial community as well as in ibuprofen-enrichments obtained by re-feeding of ibuprofen. Here, 1-, 2-, 3-hydroxy- and carboxy-ibuprofen were the primary transformation products. Quantitative real-time PCR analysis revealed a significantly higher 16S rRNA abundance in ibuprofen-amended relative to un-amended incubations. Time-resolved microbial community dynamics evaluated by 16S rRNA gene and 16S rRNA analyses revealed many new ibuprofen responsive taxa of the Acidobacteria, Actinobacteria, Bacteroidetes, Gemmatimonadetes, Latescibacteria, and Proteobacteria. Two ibuprofen-degrading strains belonging to the genera Novosphingobium and Pseudomonas were isolated from the ibuprofen-enriched sediments, consuming 400 and 300 µM ibuprofen within three and eight days, respectively. The collective results indicated that the hyporheic zone sediments sustain an efficient biotic (micro-)pollutant degradation potential, and hitherto unknown microbial diversity associated with such (micro)pollutant removal.

Gravel bars (GBs) are hotspots of biogeochemical activity, likely impacting carbon dynamics in streams and rivers. However, it remains unclear how GBs process dissolved organic matter (DOM) received from streamwater and groundwater. Here we investigate the spatial and temporal variability of DOM concentration and composition within a GB using DOM absorbance and fluorescence measurements. We found clear seasonal and diurnal patterns in DOM composition within the GB, indicating changing contributions of DOM sources and transformation processes. While DOM composition was characterized by more protein‐like DOM in summer, with increasing contributions of humic‐like DOM toward winter within the streamwater and GB, groundwater DOM exhibited a strong aromatic and humic‐like character year‐round. DOM composition and concentration varied diurnally during autumn and winter within the GB and streamwater, while seasonally higher groundwater inputs in summer likely muted the diurnal pattern, pointing to the importance of seasonal hydrological controls on DOM dynamics. Mixing model analysis showed GB DOM characteristics to differ from that predicted by streamwater‐groundwater mixing within the GB, particularly in summer. Dissolved organic carbon (DOC) concentration decreased along flowpaths through the GB with increasing residence time, likely pointing to microbial uptake and/or adsorption to sediment surfaces within the GB, with concurrent clear shifts in DOM composition. While freshly produced and more humified DOM increased along GB flowpaths, protein‐like fluorescence (C3) was removed, indicating the simultaneous production and removal of DOM. Together our findings highlight the role of GBs in DOM removal and transformation, influenced by seasonal shifts in temperature and hydrodynamics.

The interactions of groundwater with surface waters such as streams, lakes, wetlands, or oceans are relevant for a wide range of reasons—for example, drinking water resources may rely on hydrologic fluxes between groundwater and surface water. However, nutrients and pollutants can also be transported across the interface and experience transformation, enrichment, or retention along the flow paths and cause impacts on the interconnected receptor systems. To maintain drinking water resources and ecosystem health, a mechanistic understanding of the underlying processes controlling the spatial patterns and temporal dynamics of groundwater–surface water interactions is crucial. This Special Issue provides an overview of current research advances and innovative approaches in the broad field of groundwater–surface water interactions. The 20 research articles and 1 communication of this Special Issue cover a wide range of thematic scopes, scales, and experimental and modelling methods across different disciplines (hydrology, aquatic ecology, biogeochemistry, environmental pollution) collaborating in research on groundwater–surface water interactions. The collection of research papers in this Special Issue also allows the identification of current knowledge gaps and reveals the challenges in establishing standardized measurement, observation, and assessment approaches. With regards to its relevance for environmental and water management and protection, the impact of groundwater–surface water interactions is still not fully understood and is often underestimated, which is not only due to a lack of awareness but also a lack of knowledge and experience regarding appropriate measurement and analysis approaches. This lack of knowledge exchange from research into management practice suggests that more efforts are needed to disseminate scientific results and methods to practitioners and policy makers., Horizon 2020, Deutsche Forschungsgemeinschaft, Peer Reviewed

Hyporheic zones (HZs) are reactive transition regions between rivers and aquifers which are thought to play an important role in the attenuation of micropollutants. Micropollutants are chemical substances such as pharmaceuticals, industrial chemicals or personal care products that are found in trace concentrations in the environment and that can be harmful to organisms. This thesis aimed to narrow the knowledge gap on the environmental fate of wastewater-derived polar organic micropollutants in the aquatic environment, with a specific emphasis on the hyporheic zone. In Paper I an efficient workflow was developed for the in-situ characterization of polar organic micropollutants and their transformation products (TPs) in the hyporheic zone at high spatial and temporal resolution and with minimal disturbance of natural flow paths. A low volume sampling device was combined with a newly developed high throughput-direct injection-UHPLC-MS/MS method. Application in the field revealed significant differences in micropollutant concentrations that varied over small time- and spatial scales. In Paper II the results of a comprehensive field study performed in the urban lowland river Erpe in Berlin, Germany, are presented. The work provided data on in-situ attenuation behavior of 24 micropollutants and TPs, along with novel insights into the spatially- and temporally varying environmental factors which play a major role in controlling in-stream attenuation of micropollutants. Paper III describes a novel, multi-flume experiment designed to investigate the influence of hyporheic exchange flow and sediment bacterial diversity on dissipation half-lives of 31 micropollutants and associated TPs. Attenuation and transformation of most substances increased significantly with bacterial diversity; fewer compounds responded to both bacterial diversity and hyporheic exchange flow. In addition to the discovery of several novel TPs, a number of bacterial strains were identified that might be, At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.

Hyporheic zones (HZs) are reactive transition regions between rivers and aquifers which are thought to play an important role in the attenuation of micropollutants. Micropollutants are chemical substances such as pharmaceuticals, industrial chemicals or personal care products that are found in trace concentrations in the environment and that can be harmful to organisms. This thesis aimed to narrow the knowledge gap on the environmental fate of wastewater-derived polar organic micropollutants in the aquatic environment, with a specific emphasis on the hyporheic zone. In Paper I an efficient workflow was developed for the in-situ characterization of polar organic micropollutants and their transformation products (TPs) in the hyporheic zone at high spatial and temporal resolution and with minimal disturbance of natural flow paths. A low volume sampling device was combined with a newly developed high throughput-direct injection-UHPLC-MS/MS method. Application in the field revealed significant differences in micropollutant concentrations that varied over small time- and spatial scales. In Paper II the results of a comprehensive field study performed in the urban lowland river Erpe in Berlin, Germany, are presented. The work provided data on in-situ attenuation behavior of 24 micropollutants and TPs, along with novel insights into the spatially- and temporally varying environmental factors which play a major role in controlling in-stream attenuation of micropollutants. Paper III describes a novel, multi-flume experiment designed to investigate the influence of hyporheic exchange flow and sediment bacterial diversity on dissipation half-lives of 31 micropollutants and associated TPs. Attenuation and transformation of most substances increased significantly with bacterial diversity; fewer compounds responded to both bacterial diversity and hyporheic exchange flow. In addition to the discovery of several novel TPs, a number of bacterial strains were identified that might be, At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.

Hyporheic zones (HZs) are reactive transition regions between rivers and aquifers which are thought to play an important role in the attenuation of micropollutants. Micropollutants are chemical substances such as pharmaceuticals, industrial chemicals or personal care products that are found in trace concentrations in the environment and that can be harmful to organisms. This thesis aimed to narrow the knowledge gap on the environmental fate of wastewater-derived polar organic micropollutants in the aquatic environment, with a specific emphasis on the hyporheic zone. In Paper I an efficient workflow was developed for the in-situ characterization of polar organic micropollutants and their transformation products (TPs) in the hyporheic zone at high spatial and temporal resolution and with minimal disturbance of natural flow paths. A low volume sampling device was combined with a newly developed high throughput-direct injection-UHPLC-MS/MS method. Application in the field revealed significant differences in micropollutant concentrations that varied over small time- and spatial scales. In Paper II the results of a comprehensive field study performed in the urban lowland river Erpe in Berlin, Germany, are presented. The work provided data on in-situ attenuation behavior of 24 micropollutants and TPs, along with novel insights into the spatially- and temporally varying environmental factors which play a major role in controlling in-stream attenuation of micropollutants. Paper III describes a novel, multi-flume experiment designed to investigate the influence of hyporheic exchange flow and sediment bacterial diversity on dissipation half-lives of 31 micropollutants and associated TPs. Attenuation and transformation of most substances increased significantly with bacterial diversity; fewer compounds responded to both bacterial diversity and hyporheic exchange flow. In addition to the discovery of several novel TPs, a number of bacterial strains were identified that might be, At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.

Rivers are important ecosystems under continuous anthropogenic stresses. The hyporheic zone is a ubiquitous, reactive interface between the main channel and its surrounding sediments along the river network. We elaborate on the main physical, biological, and biogeochemical drivers and processes within the hyporheic zone that have been studied by multiple scientific disciplines for almost half a century. These previous efforts have shown that the hyporheic zone is a modulator for most metabolic stream processes and serves as a refuge and habitat for a diverse range of aquatic organisms. It also exerts a major control on river water quality by increasing the contact time with reactive environments, which in turn results in retention and transformation of nutrients, trace organic compounds, fine suspended particles, and microplastics, among others. The paper showcases the critical importance of hyporheic zones, both from a scientific and an applied perspective, and their role in ecosystem services to answer the question of the manuscript title. It identifies major research gaps in our understanding of hyporheic processes. In conclusion, we highlight the potential of hyporheic restoration to efficiently manage and reactivate ecosystem functions and services in river corridors., QC 20200103

Streambed morphology, streamflow dynamics, and the heterogeneity of streambed sediments critically controls the interaction between surface water and groundwater. The present study investigated the impact of different flow regimes on hyporheic exchange in a boreal stream in northern Sweden using experimental and numerical approaches. Low-, base-, and high-flow discharges were simulated by regulating the streamflow upstream in the study area, and temperature was used as the natural tracer to monitor the impact of the different flow discharges on hyporheic exchange fluxes in stretches of stream featuring gaining and losing conditions. A numerical model was developed using geomorphological and hydrological properties of the stream and was then used to perform a detailed analysis of the subsurface water flow. Additionally, the impact of heterogeneity in sediment permeability on hyporheic exchange fluxes was investigated. Both the experimental and modelling results show that temporally increasing flow resulted in a larger (deeper) extent of the hyporheic zone as well as longer hyporheic flow residence times. However, the result of the numerical analysis is strongly controlled by heterogeneity in sediment permeability. In particular, for homogeneous sediments, the fragmentation of upwelling length substantially varies with streamflow dynamics due to the contribution of deeper fluxes., QC 20190830

Streambed morphology, streamflow dynamics, and the heterogeneity of streambed sediments critically controls the interaction between surface water and groundwater. The present study investigated the impact of different flow regimes on hyporheic exchange in a boreal stream in northern Sweden using experimental and numerical approaches. Low-, base-, and high-flow discharges were simulated by regulating the streamflow upstream in the study area, and temperature was used as the natural tracer to monitor the impact of the different flow discharges on hyporheic exchange fluxes in stretches of stream featuring gaining and losing conditions. A numerical model was developed using geomorphological and hydrological properties of the stream and was then used to perform a detailed analysis of the subsurface water flow. Additionally, the impact of heterogeneity in sediment permeability on hyporheic exchange fluxes was investigated. Both the experimental and modelling results show that temporally increasing flow resulted in a larger (deeper) extent of the hyporheic zone as well as longer hyporheic flow residence times. However, the result of the numerical analysis is strongly controlled by heterogeneity in sediment permeability. In particular, for homogeneous sediments, the fragmentation of upwelling length substantially varies with streamflow dynamics due to the contribution of deeper fluxes.

Streambed morphology, streamflow dynamics, and the heterogeneity of streambed sediments critically controls the interaction between surface water and groundwater. The present study investigated the impact of different flow regimes on hyporheic exchange in a boreal stream in northern Sweden using experimental and numerical approaches. Low-, base-, and high-flow discharges were simulated by regulating the streamflow upstream in the study area, and temperature was used as the natural tracer to monitor the impact of the different flow discharges on hyporheic exchange fluxes in stretches of stream featuring gaining and losing conditions. A numerical model was developed using geomorphological and hydrological properties of the stream and was then used to perform a detailed analysis of the subsurface water flow. Additionally, the impact of heterogeneity in sediment permeability on hyporheic exchange fluxes was investigated. Both the experimental and modelling results show that temporally increasing flow resulted in a larger (deeper) extent of the hyporheic zone as well as longer hyporheic flow residence times. However, the result of the numerical analysis is strongly controlled by heterogeneity in sediment permeability. In particular, for homogeneous sediments, the fragmentation of upwelling length substantially varies with streamflow dynamics due to the contribution of deeper fluxes.

Streambed morphology, streamflow dynamics, and the heterogeneity of streambed sediments critically controls the interaction between surface water and groundwater. The present study investigated the impact of different flow regimes on hyporheic exchange in a boreal stream in northern Sweden using experimental and numerical approaches. Low-, base-, and high-flow discharges were simulated by regulating the streamflow upstream in the study area, and temperature was used as the natural tracer to monitor the impact of the different flow discharges on hyporheic exchange fluxes in stretches of stream featuring gaining and losing conditions. A numerical model was developed using geomorphological and hydrological properties of the stream and was then used to perform a detailed analysis of the subsurface water flow. Additionally, the impact of heterogeneity in sediment permeability on hyporheic exchange fluxes was investigated. Both the experimental and modelling results show that temporally increasing flow resulted in a larger (deeper) extent of the hyporheic zone as well as longer hyporheic flow residence times. However, the result of the numerical analysis is strongly controlled by heterogeneity in sediment permeability. In particular, for homogeneous sediments, the fragmentation of upwelling length substantially varies with streamflow dynamics due to the contribution of deeper fluxes.

Rivers are important ecosystems under continuous anthropogenic stresses. The hyporheic zone is a ubiquitous, reactive interface between the main channel and its surrounding sediments along the river network. We elaborate on the main physical, biological, and biogeochemical drivers and processes within the hyporheic zone that have been studied by multiple scientific disciplines for almost half a century. These previous efforts have shown that the hyporheic zone is a modulator for most metabolic stream processes and serves as a refuge and habitat for a diverse range of aquatic organisms. It also exerts a major control on river water quality by increasing the contact time with reactive environments, which in turn results in retention and transformation of nutrients, trace organic compounds, fine suspended particles, and microplastics, among others. The paper showcases the critical importance of hyporheic zones, both from a scientific and an applied perspective, and their role in ecosystem services to answer the question of the manuscript title. It identifies major research gaps in our understanding of hyporheic processes. In conclusion, we highlight the potential of hyporheic restoration to efficiently manage and reactivate ecosystem functions and services in river corridors.

Streams and rivers have been modified in the past centuries for agricultural purposes. The Baltic Sea suffers from problems regarding eutrophication. Regulations of point-sources have decreased nutrient levels, but for a scattered source of nutrient pollution, streams are important. One way of mitigating nitrogen is with coupled denitrification and nitrification processes when stream water is transported through flow paths in the hyporheic zone, an area in the stream sediments where groundwater and stream water mix. Tullstorps brook is an agricultural stream that flows into the Baltic Sea. It has had problems with high nutrient loads and poor water quality and has therefore been restored. The fieldwork in this project was conducted in Tullstorps brook in May 2019, where Rhodamine WT (RWT) tracer test and Hydraulic Conductivity (HC) measurements were done in 3 reaches, and compared to similar fieldwork since before restorations, during the summer of 2015. Two reaches in an agricultural setting that have been restored, Reach 4 and Reach 6, were measured, as well as a control reach, Reach 5, which is in a natural setting. The tracer tests indicated a significant decrease in the velocity in remediated reaches. The results of exchange velocity between the stream flow and the hyporheic zone suggest an increase after remediation of the reaches and the residence time seems to be decreasing simultaneously. When comparing the hydraulic characteristics, different stream flow during measurements was considered in a qualitative manner. The results of HC measurements show a decrease from 2015 to 2019 in the remediated reaches. In Reach 4 it decreased from 1.20E-03 m/s to 5.0E-4 m/s and in Reach 6, HC decreased from 7.70E-04 m/s before remediations to 5.6E-04 m/s after remediation actions. All the measurements have uncertainties, especially since homogeneity is assumed to some extent and the natural environment will always be heterogeneous.

Streambed morphology, streamflow dynamics, and the heterogeneity of streambed sediments critically controls the interaction between surface water and groundwater. The present study investigated the impact of different flow regimes on hyporheic exchange in a boreal stream in northern Sweden using experimental and numerical approaches. Low-, base-, and high-flow discharges were simulated by regulating the streamflow upstream in the study area, and temperature was used as the natural tracer to monitor the impact of the different flow discharges on hyporheic exchange fluxes in stretches of stream featuring gaining and losing conditions. A numerical model was developed using geomorphological and hydrological properties of the stream and was then used to perform a detailed analysis of the subsurface water flow. Additionally, the impact of heterogeneity in sediment permeability on hyporheic exchange fluxes was investigated. Both the experimental and modelling results show that temporally increasing flow resulted in a larger (deeper) extent of the hyporheic zone as well as longer hyporheic flow residence times. However, the result of the numerical analysis is strongly controlled by heterogeneity in sediment permeability. In particular, for homogeneous sediments, the fragmentation of upwelling length substantially varies with streamflow dynamics due to the contribution of deeper fluxes.

Increased nitrogen loads have significant negative consequences in streams. The hyporheic zone of streams, where surface-subsurface water exchange happens, provides significant attenuation for nitrogen compounds from the fluvial system. From many geomorphic features that contribute to hyporheic flow, meanders are particularly important for these biogeochemical reactions. Meanders provide longer residence times to allow complete denitrification of various nitrate to nitrogen gas, which can support more significant improvements in water quality compared to other channel features that only allow for partial denitrification due to short residence time. From the published literature, it is evident that there is not enough understanding about the extent of temporal variation of meander driven hyporheic flow except for few theoretical studies that have been performed to investigate the physical behavior of water and solute movement. Therefore, this research aims to investigate how meandering morphology impacts on transport characteristics of water and dissolved solutes in the hyporheic zone and to identify primary mechanisms controlling the flow. This was achieved through a set of laboratory experiments and a three-dimensional numerical model developed for meandering stream. Laboratory experiments were conducted in a recirculating flume with a set of river discharges and meandering morphologies. Residence time distribution and downwelling water flux were determined by continuously monitoring the exchange of a conservative tracer (Rhodamine WT) introduced into the surface flow. The experimental results show that both; stream discharge and meander wavelength affect the residence time and water flux into the sediment bed, hence affecting the rates of biogeochemical reactions. Most importantly, change in stream discharge affects residence times in the hyporheic zone, but the importance of considering river flow conditions in modeling hyporheic flow through meanders has been ne

Rivers are important ecosystems under continuous anthropogenic stresses. The hyporheic zone is a ubiquitous, reactive interface between the main channel and its surrounding sediments along the river network. We elaborate on the main physical, biological, and biogeochemical drivers and processes within the hyporheic zone that have been studied by multiple scientific disciplines for almost half a century. These previous efforts have shown that the hyporheic zone is a modulator for most metabolic stream processes and serves as a refuge and habitat for a diverse range of aquatic organisms. It also exerts a major control on river water quality by increasing the contact time with reactive environments, which in turn results in retention and transformation of nutrients, trace organic compounds, fine suspended particles, and microplastics, among others. The paper showcases the critical importance of hyporheic zones, both from a scientific and an applied perspective, and their role in ecosystem services to answer the question of the manuscript title. It identifies major research gaps in our understanding of hyporheic processes. In conclusion, we highlight the potential of hyporheic restoration to efficiently manage and reactivate ecosystem functions and services in river corridors.

Streams and rivers have been modified in the past centuries for agricultural purposes. The Baltic Sea suffers from problems regarding eutrophication. Regulations of point-sources have decreased nutrient levels, but for a scattered source of nutrient pollution, streams are important. One way of mitigating nitrogen is with coupled denitrification and nitrification processes when stream water is transported through flow paths in the hyporheic zone, an area in the stream sediments where groundwater and stream water mix. Tullstorps brook is an agricultural stream that flows into the Baltic Sea. It has had problems with high nutrient loads and poor water quality and has therefore been restored. The fieldwork in this project was conducted in Tullstorps brook in May 2019, where Rhodamine WT (RWT) tracer test and Hydraulic Conductivity (HC) measurements were done in 3 reaches, and compared to similar fieldwork since before restorations, during the summer of 2015. Two reaches in an agricultural setting that have been restored, Reach 4 and Reach 6, were measured, as well as a control reach, Reach 5, which is in a natural setting. The tracer tests indicated a significant decrease in the velocity in remediated reaches. The results of exchange velocity between the stream flow and the hyporheic zone suggest an increase after remediation of the reaches and the residence time seems to be decreasing simultaneously. When comparing the hydraulic characteristics, different stream flow during measurements was considered in a qualitative manner. The results of HC measurements show a decrease from 2015 to 2019 in the remediated reaches. In Reach 4 it decreased from 1.20E-03 m/s to 5.0E-4 m/s and in Reach 6, HC decreased from 7.70E-04 m/s before remediations to 5.6E-04 m/s after remediation actions. All the measurements have uncertainties, especially since homogeneity is assumed to some extent and the natural environment will always be heterogeneous.

Novel observation techniques (e.g., “smart” tracers) for characterizing coupled hydrological and biogeochemical processes are improving understanding of stream network transport and transformation dynamics. In turn, these observations are thought to enable increasingly sophisticated representations within transient storage models (TSM). However, TSM parameter estimation is prone to issues with insensitivity and equifinality, which grow as parameters are added to model formulations. Currently, it is unclear whether (or not) observations from different tracers may lead to greater process inference and reduced parameter uncertainty in the context of TSM. Herein, we aim to unravel the role of in‐stream processes alongside metabolically active (MATS) and inactive storage zones (MITS) using variable TSM formulations. Models with one (1SZ) and two storage zones (2SZ) and with and without reactivity were applied to simulate conservative and “smart” tracer observations obtained experimentally for two reaches with differing morphologies. As we show, “smart” tracers are unsurprisingly superior to conservative tracers when it comes to partitioning MITS and MATS. However, when transient storage is lumped within a 1SZ formulation, little improvement in parameter uncertainty is gained by using a “smart” tracer, suggesting the addition of observations should scale with model complexity. Importantly, our work identifies several inconsistencies and open questions related to reconciling timescales of tracer observation with conceptual processes (“parameters”) estimated within TSM. Approaching TSM with multiple models and tracer observations may be key to gaining improved insight into transient storage simulation as well as advancing feedback loops between models and observations within hydrologic science.

Streambed morphology, streamflow dynamics, and the heterogeneity of streambed sediments critically controls the interaction between surface water and groundwater. The present study investigated the impact of different flow regimes on hyporheic exchange in a boreal stream in northern Sweden using experimental and numerical approaches. Low-, base-, and high-flow discharges were simulated by regulating the streamflow upstream in the study area, and temperature was used as the natural tracer to monitor the impact of the different flow discharges on hyporheic exchange fluxes in stretches of stream featuring gaining and losing conditions. A numerical model was developed using geomorphological and hydrological properties of the stream and was then used to perform a detailed analysis of the subsurface water flow. Additionally, the impact of heterogeneity in sediment permeability on hyporheic exchange fluxes was investigated. Both the experimental and modelling results show that temporally increasing flow resulted in a larger (deeper) extent of the hyporheic zone as well as longer hyporheic flow residence times. However, the result of the numerical analysis is strongly controlled by heterogeneity in sediment permeability. In particular, for homogeneous sediments, the fragmentation of upwelling length substantially varies with streamflow dynamics due to the contribution of deeper fluxes.

Streambed morphology, streamflow dynamics, and the heterogeneity of streambed sediments critically controls the interaction between surface water and groundwater. The present study investigated the impact of different flow regimes on hyporheic exchange in a boreal stream in northern Sweden using experimental and numerical approaches. Low-, base-, and high-flow discharges were simulated by regulating the streamflow upstream in the study area, and temperature was used as the natural tracer to monitor the impact of the different flow discharges on hyporheic exchange fluxes in stretches of stream featuring gaining and losing conditions. A numerical model was developed using geomorphological and hydrological properties of the stream and was then used to perform a detailed analysis of the subsurface water flow. Additionally, the impact of heterogeneity in sediment permeability on hyporheic exchange fluxes was investigated. Both the experimental and modelling results show that temporally increasing flow resulted in a larger (deeper) extent of the hyporheic zone as well as longer hyporheic flow residence times. However, the result of the numerical analysis is strongly controlled by heterogeneity in sediment permeability. In particular, for homogeneous sediments, the fragmentation of upwelling length substantially varies with streamflow dynamics due to the contribution of deeper fluxes.

While most ecohydrological studies focus on either the hyporheic zone or aquifers, no previous studies have sought to compare microbial activity in these two different ecosystems within the same study. By comparing microbial activity in the hyporheic zone and groundwater, it is possible to examine the assumption that the hyporheic zone is a site of concentrated biogeochemical processing, while the groundwater is a lower microbial activity environment. It was expected that there would be higher rates of microbial activity in the hyporheic zone compared to the groundwater, and further, that environmental factors might influence differences in microbial activity of each environment. This research examines whether there are differences in microbial activity between the hyporheic zone and groundwater in the Maules Creek subcatchment (New South Wales, Australia). It further examines the environmental factors controlling these differences within each ecosystem. For two 4-6 week sampling campaigns (Spring and Autumn), microbial activity was measured using the cotton strip assay method - where loss of fabric tensile strength is correlated with microbial degradation of the cotton - in both the hyporheic zone and groundwater boreholes. In the hyporheic zone, unprimed cotton canvas was placed in pairs at the streambed surface and at ~15 cm in the subsurface, in three different habitats (gravel bar, riffle and pools) at three different water regimes (perennial-gaining, intermittent, and perennial-losing). Cotton strips were deployed in 13 boreholes located adjacent to the creek. Microbial activity was calculated as a cotton tensile force ratio (CTFR) by dividing the cotton tensile strength of incubated strips with that of procedural controls. Water quality parameters were also measured during the deployment of the cotton strips. The results of the study showed that microbial activity in the hyporheic zone was significantly higher compared to the aquifer environment, which confir

Bioturbation activity by macroinvertebrates in freshwaters may influence the hyporheic exchange processes in river systems. In this study, I focus on understanding the complex interaction of bioturbation by upward conveyor macroinvertebrates Lumbriculus variegatus and clogging processes in the hyporheic zone through novel methodological approaches. Upward conveyors are bottom feeders which align themselves vertically (heads buried in the sediment with their posterior ends at the surface water interface) and ingest and egest in a way that sediments move vertically upwards. The study aims at investigating (i) the effects of bioturbation on hyporheic flowpath patterns in clogged and unclogged sandy bedforms and (ii) the vertical spatial distribution of bioturbators in clogged sandy bedforms using an experimental setup. There is a paucity of agreed methodology to study the effects of bioturbation on hyporheic exchange processes in flowing waters. Therefore, this study is focused on developing the methods to understand these interactions in the laboratory flume. The above-mentioned objectives were addressed by running a series of dye tracing experiments in a recirculating perspex flume of dimensions 2.5m (L) X 0.2m (W) X 0.3m (H) filled with triple washed sand of 0.2mm mean particle size. The sand bed was later clogged with clay particles of 0.002mm to compare the flowpaths in clogged and unclogged bed conditions. Using image analysis of the injected dye flowpaths and X-ray technology for understanding the galleried structures created by the bioturbators we are able to better understand the role of these organisms in clogged river beds and their importance in the hyporheic exchange processes. Galleries were imaged using the micro-CT scans by taking core samples along the flume and scanning the top 5 cm of the core. This study can help us understand the need and importance to protect the bioturbator communities for maintaining ecosystem services and healthy functioning of