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Living in an urban groundwater ecosystem comes with many challenges for its inhabitants. In Central Europe, groundwater fauna has adapted to a dark, cold, and typically energy-poor habitat for thousands of years, making these highly specialised animals susceptible to short-term (years to decades) changes in environmental conditions. In urban areas, many anthropogenic pressures like surface sealing, subsurface infrastructures, organic pollution, or accumulation of toxicants impact the groundwater ecosystem. On top, the urban subsurface is heating up with an enormous speed, characterized by the formation of subsurface urban heat islands. With urban groundwater temperatures continuously increasing way beyond natural background values, a loss of biodiversity and ecosystem functioning with ultimately a deterioration of water quality is predicted. In the current research project ‘Heat below the City’, we target the urban groundwater ecosystems of the city of Vienna, Austria. 150 groundwater wells distributed all over the city have been sampled twice, in autumn 2021 and spring 2022. Groundwater samples are analysed for a large set of physical-chemical, microbiological and faunistic variables, aiming to identify main drivers of groundwater ecosystem biodiversity and functioning. The focus lies on temperature induced cascading effects that may lead from ecologically intact oxic habitats to anoxic zones lacking fauna with deteriorated water quality. First findings show mean groundwater temperatures in Vienna to be at 14°C, about 3°C above the natural background, with anthropogenic heat sources having a main impact on the distribution and degree of warming. The absence of dissolved oxygen (DO) and NO3- as well as the presence of Fe2+, S2- and CH4 hint at zones of reduced groundwater below the city. First data on physical-chemical conditions, microbiological and faunal communities will be introduced. more...

Proper knowledge of potential groundwater recharge (PGR) and its spatiotemporal distribution are essential for sustainable groundwater management, especially within the context of climate change. Here, a robust GIS-based water budget framework was developed to estimate PGR at a regional scale and map its spatial distribution. This framework is demonstrated over the Saguenay-Lac-Saint-Jean region (13 200 km2) of Quebec (Canada). The PGR mapping process was based on a model incorporating water budget components. The vertical inflows (VI) include water amounts from rainfall and snowmelt, whereby the latter was assessed using HYDROTEL model. VI were combined with the maximum and minimum temperatures to estimate actual evapotranspiration (AET), while the surface runoff (RuS) was assessed using the curve number method. Field observations of annual variation in temperatures and the water budget components, over a period of 100 years (1910–2009), were used to provide a comprehensive overview of the effects of climate change on PGR. The last 10 years of the observation period (i.e., 2000–2009) indicate that 6% of the study area has PGR rates of 35%–50%. PGR rates of 20%–35% occur in 58% of the study area, while 36% have PGR of 5%–20%. The trend analysis of temperature time series reveals an average of 1.1 ± 0.6°C increase over 100 years. Also, an increase in the water budget components is observed. Despite the increasing trends of RuS and AET, PGR still showed an increasing trend with an average increase of 0.7 ± 0.4 mm/year over the past 100 years. This observation indicates that the increase in VI was enough to compensate for the increases in AET and RuS. This finding of an increasing PGR in the study area provides useful information for future studies focusing on predicting long-term PGR evolution and for the development of efficient long-term groundwater management strategies. more...

Faidherbia albida is an emblematic species of agro-sylvo-pastoralism in African semi-arid areas. It combines inverted phenology (strong growth, N-fixation and production of highly palatable fodder during the dry season, ideal for livestock), defoliation during the rainy season (ideal for minimizing competition with crops) and use of deep resources mainly (riparian in its natural habitat, phreatophyte in parklands, deeply rooted, avoiding drought stress, using mostly groundwater (isotopic evidence), ideal for recycling). What could drive the inverted phenology then? Past research most often sought to correlate its peculiar phenology with climate variables, but hardly considered its deep roots and phreatophyte behavior. We set up a collaborative observatory (Faidherbia-Flux ) in a Senegal parkland in 2018 and monitored the foliar phenology of 15 adult trees (LAI2000), radial growth, sap flow and wood water content (capacitive probes). We also monitored the dynamics of soil humidity (TDR profiles) and water table fluctuations (5-6 m, piezometers). Drainage did reach the water table, but its maximum level was delayed till the end of the wet season, corresponding to the time when Faidherbia emitted new leaves. 100% foliage was maintained until the end of December, concurrently with a maximum growth, sap flow and water table level. From January to July (driest period), we observed a slow decrease in the water table level, foliage and transpiration, all reaching minima by the end of July (start of the defoliated phase), but no drought stress. Interestingly, wood rehydrated till end of the rainy season (September-October). Considering such coincidences between deep hydrological (delayed rewatering), wood rehydratation and phenological phases (inverted phenology), we suggest that this deeply rooted and phreatophyte species adjusts its phenology according to the water table and wood water content, shedding leaves when those levels reached minimum and bursting only when they resumed to maximum. more...

Groundwater pollution threatens human and ecosystem health in many areas around the globe. Shortcuts to the groundwater through concentrated recharge are known to transmit short-lived pollutants into carbonate aquifers endangering water quality of around a quarter of the world population. However, the large-scale impact of such concentrated recharge on water quality remains poorly understood. Here we apply a continental-scale model to quantify the danger of groundwater contamination by degradable pollutants through concentrated recharge in carbonate rock regions. We show that in regions where concentrated recharge takes place, the percentage of non-degraded pollutants in groundwater recharge increases from more...

The time that water takes to travel through the terrestrial hydrological cycle and the critical zone is of great interest in Earth system sciences with broad implications for water quality and quantity. Most water age studies to date have focused on individual compartments (or subdisciplines) of the hydrological cycle such as the unsaturated or saturated zone, vegetation, atmosphere, or rivers. However, recent studies have shown that processes at the interfaces between the hydrological compartments (e.g., soil‐atmosphere or soil‐groundwater) govern the age distribution of the water fluxes between these compartments and thus can greatly affect water travel times. The broad variation from complete to nearly absent mixing of water at these interfaces affects the water ages in the compartments. This is especially the case for the highly heterogeneous critical zone between the top of the vegetation and the bottom of the groundwater storage. Here, we review a wide variety of studies about water ages in the critical zone and provide (1) an overview of new prospects and challenges in the use of hydrological tracers to study water ages, (2) a discussion of the limiting assumptions linked to our lack of process understanding and methodological transfer of water age estimations to individual disciplines or compartments, and (3) a vision for how to improve future interdisciplinary efforts to better understand the feedbacks between the atmosphere, vegetation, soil, groundwater, and surface water that control water ages in the critical zone., Reviews of Geophysics, 57 (3), ISSN:8755-1209, ISSN:0096-1043, ISSN:1944-9208 more...

This paper is the outcome of a community initiative to identify major unsolved scientific problems in hydrology motivated by a need for stronger harmonisation of research efforts. The procedure involved a public consultation through online media, followed by two workshops through which a large number of potential science questions were collated, prioritised, and synthesised. In spite of the diversity of the participants (230 scientists in total), the process revealed much about community priorities and the state of our science: a preference for continuity in research questions rather than radical departures or redirections from past and current work. Questions remain focused on the process-based understanding of hydrological variability and causality at all space and time scales. Increased attention to environmental change drives a new emphasis on understanding how change propagates across interfaces within the hydrological system and across disciplinary boundaries. In particular, the expansion of the human footprint raises a new set of questions related to human interactions with nature and water cycle feedbacks in the context of complex water management problems. We hope that this reflection and synthesis of the 23 unsolved problems in hydrology will help guide research efforts for some years to come. more...

The original version of this Article contained an error in Affiliation 7, which was incorrectly given as ‘Research Unit Environmental Simulation, Helmholtz Centre Munich, Neuherberg, 85764, Germany’. The correct affiliation is listed below: Research Unit Analytical BioGeoChemistry, Helmholtz Centre Munich, Neuherberg, 85764, Germany This error has now been corrected in the HTML and PDF versions of this Article. more...

The time that water takes to travel through the terrestrial hydrological cycle and the critical zone is of great interest in Earth system sciences with broad implications for water quality and quantity. Most water age studies to date have focused on individual compartments (or subdisciplines) of the hydrological cycle such as the unsaturated or saturated zone, vegetation, atmosphere, or rivers. However, recent studies have shown that processes at the interfaces between the hydrological compartments (e.g., soil-atmosphere or soil-groundwater) govern the age distribution of the water fluxes between these compartments and thus can greatly affect water travel times. The broad variation from complete to nearly absent mixing of water at these interfaces affects the water ages in the compartments. This is especially the case for the highly heterogeneous critical zone between the top of the vegetation and the bottom of the groundwater storage. Here, we review a wide variety of studies about water ages in the critical zone and provide (1) an overview of new prospects and challenges in the use of hydrological tracers to study water ages, (2) a discussion of the limiting assumptions linked to our lack of process understanding and methodological transfer of water age estimations to individual disciplines or compartments, and (3) a vision for how to improve future interdisciplinary efforts to better understand the feedbacks between the atmosphere, vegetation, soil, groundwater, and surface water that control water ages in the critical zone. more...

Accurate estimates of soil hydraulic parameters and dispersivities are crucial to simulate water flow and solute transport in terrestrial systems, particularly in the vadose zone. However, parameters obtained from inverse modeling can be ambiguous when identifying multiple parameters simultaneously and when boundary conditions are not well known. Here, we performed an inverse modeling study in which we estimated soil hydraulic parameters and dispersivities of layered soils from soil water content, matric potential, and stable water isotope ( d 18O) measurements in weighable lysimeter systems. We used different optimization strategies to investigate which observation types are necessary for simultaneously estimating soil hydraulic and solute transport parameters. Combining water content, matric potential, and tracer (e.g., d 18O) data in one objective function (OF) was found to be the best strategy for estimating parameters that can simulate all observed water flow and solute transport variables. A sequential optimization, in which first an OF with only water flow variables and subsequently an OF with transport variables was optimized, performed slightly worse indicating that transport variables contained additional information for estimating soil hydraulic parameters. Hydraulic parameters that were obtained from optimizing OFs that used either water contents or matric potential could not predict non-measured water flow variables. When a bromide (Br−) tracer experiment was simulated using the optimized parameters, the arrival time of the bromide pulse was underestimated. This suggested that Br− sorbed onto clay minerals and amorphous oxides under the prevailing geochemical conditions with low pH values. When accounting for anion adsorption in the simulation, Br− concentrations were well predicted, which validated the dispersivity parameterization. more...

Veneto, Friuli-Venezia Giulia, karst aquifer, water budget., Flowpath 2017, National meeting on hydrogeology 2017

Aim of this work is to present conceptual framework and operational tools for characterisation of flow in groundwater systems as related to surface waters and dependent ecosystems within the context of assessing and preventing threats to groundwater quantity and quality. The coherence of this integrative, multidisciplinary effort is achieved by placing different processes and parts of groundwater systems in the framework of conceptual models development and putting emphasis on the tracer-based techniques. These Guidelines are meant to be an aid to groundwater managers, researchers and educators but also to policy makers and stakeholders concerned with understanding and quantitative characterisation of groundwater systems. After presenting an overview of conceptual models the Guidelines consider issues that need to be addressed for a comprehensive characterization of groundwater flow system. They are related to spatially complex and transient patterns of water flow and contaminant transport that are expressed at a wide range of scales. Environmental and artificial tracers are presented as tools in integrated characterization of groundwater systems. Tracers methods provide information integrated over different spatial and temporal scales and are well suited for development of the conceptual models. Use of tracers in quantification of timescales of contaminant transport is demonstrated. Following chapters provide conceptual framework and present methods for characterization of groundwater flow through the linked compartments of groundwater systems, from infiltration to surface waters. Groundwater renewal and its importance for sustainability of groundwater resources is treated in a separate chapter. Finally, impacts of climate and land-use changes on flowpath structure and transport are considered. An example of artificial tracer application in the unconfined, highly heterogeneous alluvial Zagreb aquifer performed in the GENESIS project is presented in an Annex. more...

Aim of this work is to present conceptual framework and operational tools for characterisation of flow in groundwater systems as related to surface waters and dependent ecosystems within the context of assessing and preventing threats to groundwater quantity and quality. The coherence of this integrative, multidisciplinary effort is achieved by placing different processes and parts of groundwater systems in the framework of conceptual models development and putting emphasis on the tracer - based techniques. These Guidelines are meant to be an aid to groundwater managers, researchers and educators but also to policy makers and stakeholders concerned with understanding and quantitative characterisation of groundwater systems. After presenting an overview of conceptual models the Guidelines consider issues that need to be addressed for a comprehensive characterization of groundwater flow system. They are related to spatially complex and transient patterns of water flow and contaminant transport that are expressed at a wide range of scales. Environmental and artificial tracers are presented as tools in integrated characterization of groundwater systems. Tracers methods provide information integrated over different spatial and temporal scales and are well suited for development of the conceptual models. Use of tracers in quantification of timescales of contaminant transport is demonstrated. Following chapters provide conceptual framework and present methods for characterization of groundwater flow through the linked compartments of groundwater systems, from infiltration to surface waters. Groundwater renewal and its importance for sustainability of groundwater resources is treated in a separate chapter. Finally, impacts of climate and land - use changes on flowpath structure and transport are considered. An example of artificial tracer application in the unconfined, highly heterogeneous alluvial Zagreb aquifer performed in the GENESIS project is presented in an Annex. more...

This paper is the outcome of a community initiative to identify major unsolved scientific problems in hydrology motivated by a need for stronger harmonisation of research efforts. The procedure involved a public consultation through online media, followed by two workshops through which a large number of potential science questions were collated, prioritised, and synthesised. In spite of the diversity of the participants (230 scientists in total), the process revealed much about community priorities and the state of our science: a preference for continuity in research questions rather than radical departures or redirections from past and current work. Questions remain focused on the process-based understanding of hydrological variability and causality at all space and time scales. Increased attention to environmental change drives a new emphasis on understanding how change propagates across interfaces within the hydrological system and across disciplinary boundaries. In particular, the expansion of the human footprint raises a new set of questions related to human interactions with nature and water cycle feedbacks in the context of complex water management problems. We hope that this reflection and synthesis of the 23 unsolved problems in hydrology will help guide research efforts for some years to come. more...

Hydrological sciences journal, 64 (10), ISSN:0262-6667, ISSN:2150-3435

This paper is the outcome of a community initiative to identify major unsolved scientific problems in hydrology motivated by a need for stronger harmonisation of research efforts. The procedure involved a public consultation through online media, followed by two workshops through which a large number of potential science questions were collated, prioritised, and synthesised. In spite of the diversity of the participants (230 scientists in total), the process revealed much about community priorities and the state of our science: a preference for continuity in research questions rather than radical departures or redirections from past and current work. Questions remain focused on the process-based understanding of hydrological variability and causality at all space and time scales. Increased attention to environmental change drives a new emphasis on understanding how change propagates across interfaces within the hydrological system and across disciplinary boundaries. In particular, the expansion of the human footprint raises a new set of questions related to human interactions with nature and water cycle feedbacks in the context of complex water management problems. We hope that this reflection and synthesis of the 23 unsolved problems in hydrology will help guide research efforts for some years to come. more...

The Water Framework Directive and Groundwater Directive aim at preserving and improving the groundwater status. Groundwater bodies are classified as being or not being at risk of failing to meet these objectives. Those at risk are subject to more precise risk assessment where the concept of vulnerability is considered in the pathway part of the source–pathway–receptor scheme. However, no further details on implementation strategies are provided. In order to support groundwater managers and decision-makers in implementation of programs protecting groundwater, a systematic operational approach based on a decision tree is proposed, which leads the user through the stages of vulnerability assessment. First, a problem has to be formulated related to a threatening of the quantitative and/or qualitative status of a groundwater body. Next, the stated problem needs to be related to the intrinsic or specific vulnerability. Methods used for the intrinsic vulnerability assessment belong to two categories: subjective rating and objective methods. Method selection depends primarily on: data availability, knowledge and available resources. A key issue is the lag time associated with transport between a source/event of contamination and the water body. This lag time is primarily controlled by the temporal scale of water flow. It provides information about flow processes and at the same time also about timescales required for the implementation of strategies. Effects of any measures taken cannot be observed immediately but at the earliest after these estimated lag times emphasizing the need to also proactively safeguard groundwater resources and preserve their good status. more...

Assessing the vulnerability of groundwater to adverse effects of human impacts is one of the most important problems in applied hydrogeology. At the same time, many of the widespread vulnerability assessment methods do not provide physically meaningful and operational indicators of vulnerability. Therefore, this review summarizes (i) different methods used for intrinsic vulnerability assessment and (ii) methods for different groundwater systems. It particularly focuses on (iii) timescale methods of water flow as an appropriate tool and (iv) provides a discussion on the challenges in applying these methods. The use of such physically meaningful indices based on timescales is indispensable for groundwater resources management. more...

Summary A multi-layered aquifer system (eastern Po plain, northern Italy) was investigated by means of isotopic data, with the goal of quantifying groundwater recharge from different sources and assessing the intrinsic vulnerability of aquifers to surface sources of contamination. The geology of the area is based on a stratigraphic alternation of several sandy aquifers and silty–clayey aquitards, down to a maximum depth of 200 m b.g.s. Water isotopes (δ 18 O, δ 2 H), and hydrochemical analyses were performed on groundwater samples collected from a regional network of 70 boreholes distributed on an area of almost 2000 km 2 . In addition to the regional sampling, detailed vertical isotopic profiling was performed in one location by means of groundwater and sediment samples collected through the whole sequence of aquifers and aquitards. Water isotopes indicated mixing from different sources of recharge (i.e., vertical recharge, Po river, deeper aquifers). Mixing calculations were used to quantify the contributions to the aquifers from the different sources. The vertical profiling allowed for integrating and validating the interpretations at a regional scale. The recharge pattern defined for the different aquifers was translated into an index of hydrogeologic interconnections with the surface, which represents a physically based proxy of the intrinsic vulnerability of the aquifers to surficial sources of contamination. The investigated setting can be considered to be representative of many other anthropized and groundwater demanding plain settings around the world. Thus, the proposed method represents a valuable approach for such settings both for recharge quantification (e.g., to be used as input for numerical modeling) and for a physically based assessment of the intrinsic vulnerability. more...