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As drinking-water scarcity grows worldwide, we need to improve predictions of the quantity and quality of our water resources. An overarching problem for model improvement is that we do not know the geological structure of aquifers in sufficient detail. In this work, we demonstrate that mineral-water reactions imprint structure in the subsurface that impacts the flow and transport of some chemical species. Specifically, pyrite, a ubiquitous mineral, commonly oxidizes and depletes in the upper layers of the weathering profile in most humid watersheds, only remaining at depths of meters. We hypothesize that variations in concentrations (C) of pyrite-derived sulfate released into rivers as a function of discharge (q) reflect the rate-limiting step and depth of pyrite-oxidizing layers. We found that log C − log q behaviors thus differ in small and large watersheds in the Susquehanna River Basin as well as in selected watersheds in the Western United States. Although coal mining changes pyrite oxidation from closed to open system with respect to O2, patterns in stream chemistry as a function of discharge are consistent with deep and shallow pyrite oxidation zones in small and large watersheds respectively. Therefore, understanding the subsurface patterns of mineral reactions and how they affect the architecture of aquifers will elucidate patterns of changing river chemistry and our ability to manage water resources in the future under accelerated land use and climate change., Water Resources Research, 59 (5), ISSN:0043-1397, ISSN:1944-7973

We present a geostatistics-based stochastic salinity estimation framework for the Montebello Oil Field that capitalizes on available total dissolved solids (TDS) data from groundwater samples as well as electrical resistivity (ER) data from borehole logging. Data from TDS samples (n = 4,924) was coded into an indicator framework based on falling below four selected thresholds (500, 1,000, 3,000, and 10,000 milligrams per liter (mg/L)). Collocated TDS-ER data from the surrounding groundwater basin were then employed to produce a kernel density estimator to establish conditional probabilities for ER data (n = 8 boreholes) falling below the selected TDS thresholds within the Montebello Oil Field area. Directional variograms were estimated from these indicator coded data, and 500 TDS realizations from conditional indicator simulation were generated for the subsurface region above the Montebello Oil Field reservoir. Simulations were summarized as 3D maps of median TDS, most likely salinity class, and probability for exceeding each of the specified TDS thresholds. Results suggested TDS was below 500 mg/L in most of the study area, with a trend toward higher values (500-1,000 mg/L) to the southwest; consistent with the average regional groundwater flow direction. Discrete localized zones of TDS greater than 1,000 mg/L were observed, with one of these zones in the greater than 10,000 mg/L range; however, these areas were not prevalent. The probabilistic approach used here is adaptable and is readily modified to include additional data and types and can be employed in time-lapse salinity modeling through Bayesian updating. This article is protected by copyright. All rights reserved.

The interactions between old abandoned wellbores of suspect well integrity with hydraulic fracturing (HF), enhanced oil recovery (EOR), or salt water disposal (SWD) operations can result in upward leakage of deep aqueous liquids into overlying aquifers. This potential for upward fluid migration is largely unquantified as monitoring abandoned wells is rarely done, and leakage may go unnoticed especially when in deeper aquifers. This study performs a proximity analysis between old abandoned wells and HF, EOR, and SWD wells, and identifies commingled old abandoned wellbores, which are those wells where groundwater may flow from one aquifer to one or more other aquifers, to identify the locations with the greatest potential for upward aqueous fluid migration at three study sites in the Western Canadian Sedimentary Basin. Our analysis indicates that at all three study sites there are several locations where HF, EOR, or SWD operations are located in close proximity to a given old abandoned well. Much of this overlap occurs in formations above typically produced hydrocarbon reservoirs but below exploited potable aquifers, otherwise known as the intermediate zone, which is often connected between abandonment plugs in old abandoned wells. Information on the intermediate zone is often lacking, and this study suggests that unanticipated alterations to groundwater flow systems within the intermediate zone may be occurring. Results indicate the need for more field-based research on the intermediate zone.

About 14.5 months after the 2018 eruption and summit collapse of Kīlauea Volcano, Hawai'i, liquid water started accumulating in the deepened summit crater, forming a lake that attained 51 m depth before rapidly boiling off on December 20, 2020, when an eruption from the crater wall poured lava into the lake. Modeling the growth of the crater lake at Kīlauea summit is important for assessing the potential for explosive volcanism. Our current understanding of the past 2500 years of eruptive activity at Kīlauea suggests a slight dominance of explosive behavior over effusive. The deepened summit crater and presence of the crater lake in 2019 raised renewed concerns about explosive activity. Groundwater models using hydraulic-property data from a nearby drillhole successfully forecast the timing and rate of lake filling. Here we compare the groundwater-model predictions with observational data through the demise of the crater lake, examine the implications for local water-table configuration, consider the potential role of evaporation and recharge (neglected in previous models), and briefly discuss the energetics of the rapid boil-off. This post audit of groundwater-flow models of Kīlauea summit shows that simple models can sometimes be used effectively to simulate complex settings such as volcanoes.

Groundwater flow direction within the critical zone of headwater catchments is often assumed to mimic land surface topographic gradients. However, groundwater hydraulic gradients are also influenced by subsurface permeability contrasts, which can result in variability in flow direction and magnitude. In this study, we investigated the relationship between shallow groundwater flow direction, surface topography, and the subsurface topography of low permeability units in a headwater catchment at the Hubbard Brook Experimental Forest (HBEF), NH. We continuously monitored shallow groundwater levels in the solum throughout several seasons in a well network (20 wells of 0.18–1.1 m depth) within the upper hillslopes of Watershed 3 of the HBEF. Water levels were also monitored in four deeper wells, screened from 2.4 to 6.9 m depth within glacial drift of the C horizon. We conducted slug tests across the well network to determine the saturated hydraulic conductivity (Ksat) of the materials surrounding each well. Results showed that under higher water table regimes, groundwater flow direction mimics surface topography, but under lower water table regimes, flow direction can deviate as much as 56 degrees from surface topography. Under these lower water table conditions, groundwater flow direction instead followed the topography of the top of the C horizon. The interquartile range of Ksat within the C horizon was two orders of magnitude lower than within the solum. Overall, our results suggest that the land surface topography and the top of the C horizon acted as end members defining the upper and lower bounds of flow direction variability. This suggests that temporal dynamics of groundwater flow direction should be considered when calculating hydrologic fluxes in critical zone and runoff generation studies of headwater catchments that are underlain by glacial drift. Published version

Arsenic in private drinking water wells is a significant problem across much of eastern Wisconsin, USA. The release mechanism and stratigraphic distribution of sulfide and iron (hydr)oxide sources of arsenic in bedrock aquifers are well understood for northeastern Wisconsin. However, recent geologic mapping has identified numerous small bedrock folds to the south, and the impact of these geologic structures on local groundwater flow and well contamination has been little studied. This paper examines the hydrologic and structural effects of the Beaver Dam anticline, southeast Wisconsin, on arsenic in groundwater in the region. Multivariate logistic regression shows wells near the Beaver Dam anticline are statistically more likely to detect arsenic in groundwater compared to wells farther away. Structural and hydrologic changes related to folding are interpreted to be the cause. Core drilled near the fold axis is heavily fractured, and many fractures are filled with sulfides. Elevated hydraulic conductivity estimates are also recorded near the fold axis, which may reflect a higher concentration of vertical fractures. These structural and hydrologic changes may have led to systematic changes in the distribution and concentration of arsenic-bearing mineral hosts, resulting in the observed detection pattern. For areas with similar underlying geology, this approach may improve prediction of arsenic risk down to the local level.

Mass balance calculations and hydrodynamics of groundwater flow suggest that the solutes in brines of the coastal sabkha aquifer from the Emirate of Abu Dhabi are derived largely from ascending geologic brines into the sabkha from the underlying formations. Solute interpretation for the ascending brine model (ABM) was based on two independent but secondary lines of evidence (solute ratios and solute fluxes). In the current study, direct primary evidence for this ABM was provided through analyses of δ81 Br, δ37 Cl, and 87 Sr/86 Sr. Different solute histories of geologic brine and sea water provide an "isotopic fingerprint" that can uniquely distinguish between the two possible sources. Samples from the coastal sabkha aquifer of Abu Dhabi were determined to have a mean δ81 Br of 1.17‰ that is statistically equal, at the 95% confidence level, to the mean of 1.11‰ observed in the underlying geologic brine and statistically different than sea water. Similarly, the δ37 Cl in sabkha brine has a mean of 0.25‰ and is statistically equal to a mean of 0.21‰ in the underlying geologic brines at the 95% confidence level and statistically different from sea water. Also, dissolved strontium isotope data are consistent with the ABM and even with the complex set of processes in the sabkha, the variance in strontium isotope results is similar to the geologic brine. These observations provide primary direct evidence consistent that the major source of these solutes (and presumably others in the aquifer) is from discharging geologic brines, not from adjacent sea water.

Global demands for energy efficient heating and cooling systems coupled with rising commitments toward net zero emissions is resulting in wide deployment of shallow geothermal systems, typically installed to a depth of 100 to 200m, and in the continued growth of the global ground source heat pump (GSHP) market. Ground coupled heat pump (GCHP) systems take up to 85% of the global GSHP market. With increasing deployment of GCHP systems in urban areas coping with limited regulations, there is growing potential and risk for these systems to impact the subsurface thermal regime and to interact with each other or with nearby heat‐sensitive subsurface infrastructures. In this paper, we present three numerical modelling case studies, from the UK and Canada, which examine GCHP systems’ response to perturbation of the wider hydrogeological and thermal regimes. The studies demonstrate how GCHP systems can be impacted by external influences and perturbations arising from subsurface activities that change the thermal and hydraulic regimes in the area surrounding these systems. Additional subsurface heat loads near existing schemes are found to have varied impacts on system efficiency with reduction ranging from

High transmissivity aquifers typically have low hydraulic gradients (i.e., a flat water table). Measuring low gradients using water levels can be problematic because measurement error may be greater than the true difference in water levels (i.e., a low signal-to-noise ratio). In this study, the feasibility of measuring a hydraulic gradient in the range of 10-6 to 10-5 m/m was demonstrated. The study was performed at a site where the depth to water from land surface ranged from 40.1 to 94.2 m and the aquifer transmissivity was estimated at 41,300 m2 /d (hydraulic conductivity of 18,800 m/d). The goals of the study were to reduce measurement error as much as practicable and assess the importance of factors affecting water level measurement accuracy. Well verticality was the largest source of error (0.000 to 0.168 m; median of 0.014 m), and geodetic survey of casing elevations was the next most important source of error (0.002 to 0.013 m; median of 0.005 m). Variability due to barometric pressure fluctuations was not an important factor at the site. Hydraulic heads were measured to an accuracy of ±0.0065 m, and the average hydraulic gradient was estimated to be 8.0 × 10-6 (±0.9 × 10-6 ) m/m. The improvement in accuracy allowed for two reversals in the groundwater flow direction to be identified, after which the gradient averaged 2.5 × 10-5 (±0.4 × 10-5 ) m/m. This study showed it is possible to sufficiently control sources of error to measure hydraulic gradients in the 10-6 to 10-5 m/m range.

The main goal of this study is to investigate the effect of soil properties such as permeability on the dispersion and movement of a water dissolved contaminant in three types of soil in saturated and 2D conditions. The experimental modeling was conducted using a constructed sand box. In order to evaluate the effect of soils particle size on the distribution and self-remediation of the contaminant, three types of soil, as coarse, medium, and fine-grained sand were used. Results of experiments showed that, at the first 25% of the test time, the contaminated area reduction rate in all three specimens varies significantly, so that for the medium and coarse sand, it is 2.2 and 3 times that of fine sand, respectively. The contaminant width reduction at the first 25% of the test time was 5%, 6%, and 35% for the fine, medium, and coarse sand, respectively, while the contaminant length reduction was 13%, 18%, and 37% for the fine, medium, and coarse sand, respectively. In addition, by comparing the contaminant movement in the saturated and semi-saturated areas, it was observed that the longitudinal and transverse movement of the contaminant under the water level are almost 2.5 times of the semi-saturated area. PRACTITIONER POINTS: Reduction rate of solution area in fine, medium and coarse-grained sample are nearly convex, linear and concave-shaped, respectively. The remediation process in saturated zones is implemented in both directions with higher intensity in a shorter time than unsaturated zones. In the strip formed plumes, the volume of the self-remediation is proportional to the time intervals during the test. In the elliptic masses the self-purification amount is lower at the beginning, due to the small cross-section ending of the contamination mass.

This review focuses on investigations of groundwater flow and solute transport in karst aquifers through laboratory scale models (LSMs). In particular, LSMs have been used to generate new data under different hydraulic and contaminant transport conditions, testing of new approaches for site characterization, and providing new insights into flow and transport processes through complex karst aquifers. Due to the increasing need for LSMs to investigate a wide range of issues, associated with flow and solute migration karst aquifers this review attempts to classify, and introduce a framework for constructing a karst aquifer physical model that is more representative of field conditions. The LSMs are categorized into four groups: sand box, rock block, pipe/fracture network, and pipe-matrix coupling. These groups are compared and their advantages and disadvantages highlighted. The capabilities of such models have been extensively improved by new developments in experimental methods and measurement devices. Newer technologies such as 3D printing, computed tomography scanning, X-rays, nuclear magnetic resonance, novel geophysical techniques, and use of nanomaterials allow for greater flexibilities in conducting experiments. In order for LSMs to be representative of karst aquifers, a few requirements are introduced: (1) the ability to simulate heterogeneous distributions of karst hydraulic parameters, (2) establish Darcian and non-Darcian flow regimes and exchange between the matrix and conduits, (3) placement of adequate sampling points and intervals, and (4) achieving some degree of geometric, kinematic, and dynamic similitude to represent field conditions.

The fractional Brownian motion (fBm) and fractional Levy motion (fLm) can easily describe the geometry and the statistical structure of hydraulic conductivity (K) for real‐world. However, the fBm and fLm models have not been systematically evaluated when building the K field for a low‐permeability site. In this study, both the fBm and fLm are used to simulate the low‐K field at NingCheGu (NCG), Tianjin, China. Groundwater flow and solute transport are then computed using MODFLOW and MT3DMS, respectively, and the influence of the fBm/fLm models for K on groundwater flow and solute transport is discussed. Results show that the fLm fits better the statistics of the low‐K medium than fBm, and the random logarithmic K (LnK) field generated by fLm is more stable because the resultant LnK field captures more of the measured properties at the field site than that generated by fBm. In contrast, the LnK generated by fBm is more likely to form both high‐K channels and low‐K barriers. The fBm therefore predicts more extreme behaviours in flow and transport, including the preferential flow, low‐concentration blocks and solute retention. The overall groundwater renewal period and solute travel time for the fLm simulation are slightly shorter than those for fBm. The impacts of the fLm and fBm models on the statistics of the resultant LnK fields and the dynamics of groundwater flow and solute transport revealed by this study shed light on the selection and evaluation of the fractional probability distribution models in capturing the K fields for low‐K media.

In Belgium, IWVA uses managed aquifer recharge (MAR) to recharge the aquifer with treated wastewater generated from the communities to sustain the potable water supply on the Belgian coast. This MAR facility is faced with a challenge of reduced infiltration rates during the winter season when pond water temperatures near 4°C. This study involves the identification of the predominant factor influencing the rate of infiltration through the pond bed. Several factors, including pumping rates, natural recharge, tidal influences of the North Sea and pond‐water temperature, were identified as potential causes for variation of the recharge rate. Correlation statistics and linear regression analysis were used to determine the sensitivity of the infiltration rate to the aforementioned factors. Two groundwater flow models were developed in visual MODFLOW to simulate the water movement under the pond bed and to obtain the differences in flux to track the effects of variation of hydraulic conductivity during the two seasons. A 32% reduction in vertical hydraulic gradient in the top portion of the aquifer was observed in winter, causing the recharge rates to fluctuate. Results showed that water temperature caused a 30% increase in hydraulic conductivity in summer as compared with winter and has the maximum impact on infiltration rate. Cyclic variations in water viscosity, occurring because of seasonal temperature changes, influence the saturated hydraulic conductivity of the pond bed. Results from the models confirm the impact on infiltration rate by temperature‐influenced hydraulic conductivity.

Geophysical well logging has been applied for fracture characterization in crystalline terrains by physical properties measurements and borehole wall imaging. Some of these methods can be applied to monitor pumping tests to identify fractures contributing to groundwater flow and, with this, determine hydraulic conductivity and transmissivity along the well. We present a procedure to identify fractures contributing to groundwater flow using spontaneous potential measurements generated by electrokinetic processes when the borehole water head is lowered and then monitored while recovering. The electrokinetic model for flow through a tabular gap is used to interpret the measured data and determine the water head difference that drives the flow through the fracture. We present preliminary results at a test site in crystalline rocks on the campus of the University of São Paulo.

Taliks are bodies or layers of unfrozen ground in permafrost areas. Recent research on taliks has been driven largely by the potential for release of greenhouse gases as taliks expand, and engineering challenges associated with thawing permafrost. Observations of talik configuration and development have been assisted by advances in geophysical techniques that complement mechanical and thermal measurements. Suprapermafrost taliks have been observed in a range of settings associated with disturbance from wildfire and infrastructure. These features are included in a revised talik classification scheme presented in this paper. Observations of methane release have renewed interest in lake talik initiation and development, resulting in substantial efforts to model thaw lake expansion. Hotspots of methane release have also been identified at saline springs. Recent simulations indicate that groundwater flow can significantly accelerate talik expansion and that incorporating heat advection may be required for accurate transient simulations. Ongoing global warming is expected to exacerbate the effects of surface disturbances on talik development and limit the ability of permafrost to recover in marginal permafrost areas.

International audience; Large proportions of rain water and snowmelt infiltrate into the subsurface before contributing to stream flow and stream water quality. Subsurface flow dynamics steer the transport and transformation of contaminants, carbon, weathering products and other biogeochemistry. The distribution of groundwater ages with depth is a key feature of these flow dynamics. Predicting these ages are a strong test of hypotheses about subsurface structures and time varying processes. CFC‐based groundwater ages revealed an unexpected groundwater age stratification in a 0.47 km2 forested catchment called Svartberget in northern Sweden. An overall groundwater age stratification, representative for the Svartberget site, was derived by measuring CFCs from 9 different wells with depths of 2 m to 18 m close to the stream network. Immediately below the water table, CFC‐based groundwater ages of already 30 years that increased with depth were found. Using complementary groundwater flow models, we could reproduce the observed groundwater age stratification and show that the 30 year lag in rejuvenation comes from return flow of groundwater at a subsurface discharge zone that evolves along the interface between two soil types. By comparing the observed groundwater age stratification with a simple analytical approximation, we show that the observed lag in rejuvenation can be a powerful indicator of the extent and structure of the subsurface discharge zone, while the vertical gradient of the age‐depth relationship can still be used as a proxy of the overall aquifer recharge even when sampled in the discharge zone. The single age stratification profile measured in the discharge zone, close to the aquifer outlet, can reveal the main structure of the groundwater flow pattern from recharge to discharge. This groundwater flow pattern provides information on the participation of groundwater in the hydrological cycle and indicates the lower boundary of hydrological connectivity.

This study investigated collaborative groundwater-flow modeling and scenario analysis in the Little Plover River basin, Wisconsin, USA where an unconfined aquifer supplies groundwater for agricultural irrigation, industrial processing, municipal water supply, and stream baseflow. We recruited stakeholders with diverse interests to identify, prioritize, and evaluate scenarios defined as management changes to the landscape. Using a groundwater flow model, we simulated the top 10 stakeholder-ranked scenarios under historically informed dry, average, and wet weather conditions and evaluated the ability of scenarios to meet government-defined stream flow performance measures. Results show that multiple changes to the landscape are necessary to maintain optimum stream flow, particularly during dry years. Yet, when landscape changes from three scenarios-transferring water from the local waste water treatment plant to basin headwaters, moving municipal wells further from the river and downstream, and converting 240 acre (97 ha) of irrigated land to unirrigated land-were simulated in combination, the probability of meeting or exceeding optimum flows rose to 75, 65, and 34% at upper, mid, and lower stream gages, respectively, in dry climate conditions. Discussions with stakeholders reveal that the collaborative model and scenario analysis process resulted in social learning that built upon the existing complex and dynamic institutional landscape. The approach provided a forum for solution-based discussions, and the model served as an important mediation tool for the development and evaluation of community-defined scenarios in a high conflict environment. Today, stakeholders continue to work collaboratively to overcome challenges and implement voluntary solutions in the basin.

Groundwater temperature is a useful hydrogeological parameter that is easy to measure and can provide much insight into groundwater flow systems, but can be difficult to interpret. For measuring temperature directly in the ground, dedicated specifically designed monitoring wells are recommended since conventional groundwater wells are not optimal for temperature monitoring. Multilevel monitoring of groundwater temperature is required to identify contributions of different possible heat inputs (sources) on measured temperature signals. Interpreting temperature data as a cosine function, including period, average temperature, amplitude, and phase offset, is helpful. Amplitude dampening and increasing phase shift with distance from a boundary can be used for estimation of transport parameters. Temperature measurements at different depths can be used for evaluation of unknown parameters of analytical functions by optimization of regression fits in Python. These estimated parameters can be used to calculate temperatures at known water table depths which can be applied as a fixed transient boundary condition in MT3DMS to overcome the limitations of MT3DMS heat transport modeling in the unsaturated zone. In this study, temperature monitoring and modeling was used to evaluate the influence of a department store's heated basement foundation on groundwater temperature within a green space (city park), with the main outcome that 17 years after construction, the department store foundation has increased the mean groundwater temperature by 3.2 °C. Heat input evaluated by the MT3DMS model varied from 0.1 W/m2 at a distance of 100 m up to 12 W/m2 next to the building.

In karst environments, typically characterized by peculiar hydrogeological features and high heterogeneity and anisotropy, the connection between the recharge areas and the springs is often not str...

Rivers have been subject to the construction of numerous small-scale anthropogenic structures, causing the alteration and fragmentation of habitats. Despite their impact on fish habitat selection, migration, and swimming performance, more hydraulic structures are being added to riverine systems. These mainly have the purpose of harnessing renewable energy or mitigating the impact of flooding, as in the case of leaky barriers that are widely used for natural flood management. By providing a sustainable and cost-effective supplement to traditional hard engineering flood risk management methods, these channel-spanning wooden barriers are constructed using sustainable, local materials, intended to slow down surface water and groundwater flow, reduce flood peaks, and attenuate the flow reaching downstream communities. Despite their increasing popularity, little is known about the design implications on fish movement or hydrodynamics. Using scaled laboratory flume experiments we investigate how the physical design of four leaky barriers varying in porosity, length, provision of overhead cover, and color, impact on fish movement and spatial usage, and the channel hydrodynamics. Our fish behavioral analysis reveals that juvenile rainbow trout (Oncorhynchus mykiss) movement reduces with barrier presence. Upstream passage increases with barrier color but not cover, for shorter rather than longer leaky barriers, and for a non-porous barrier compared to its porous counterpart. Barrier-specific flow alterations appear to play a secondary role compared to barrier color. Our study showed that physical barrier design and leaky barrier presence alter fish movement, and therefore care needs to be taken during the design of such natural flood management structures.

Faults can act as flow barriers or conduits to groundwater flow by introducing heterogeneity in permeability. We examine the hydrogeology of the Sandwich Fault Zone, a 137 km long zone of high-angle faults in northern Illinois, using a large-scale historic aquifer test. The fault zone is poorly understood at depth due to the majority of the faults being buried by glacial deposits and its near-vertical orientation which limits geologic sampling across faults. The aquifer test-perhaps one of the largest in terms of overall withdrawal in North American history-was conducted in 1942 at a facility adjacent to the fault zone. More than 34,000 m3 /day was pumped for 37 days from nine multiaquifer wells open to the stratified Cambrian-Ordovician sandstone aquifer system. We modeled the aquifer test using a transient MODFLOW-USG model and simulated pumping wells with the CLN package. We tested numerous fault core/damage zone conceptualizations and calibrated to drawdown values recorded at production and observation wells. Our analysis indicates that the fault zone is a low-permeability feature that inhibits lateral movement of groundwater and that there is at least an order of magnitude decrease in horizontal hydraulic conductivity in the fault core compared to the undeformed sandstone. Large head declines have occurred north of the fault zone (over 300 m since predevelopment conditions) and modifying fault zone parameters significantly affects calibration to regional drawdown on a decadal scale. The flow-barrier behavior of the fault zone has important implications for future groundwater availability in this highly stressed region.

In west-central Lower Peninsula of Michigan, population growth and expanded agricultural activities over recent decades have resulted in significant increases in distributed groundwater withdrawals. The growth of the extensive well network and anecdotes of water shortages (dry wells) have raised concerns over the region's groundwater sustainability. We developed an unsteady, three-dimensional (3D) groundwater flow model to describe system dynamics over the last 50 years and evaluate long-term impacts of groundwater use. Simulating this large aquifer system was challenging; the site is characterized by strong, spatially distributed, and statistically nonstationary heterogeneity, making it difficult to avoid over-parameterization using traditional approaches for conceptualizing and calibrating a flow model. Moreover, traditional pumping and water level data were lacking and prohibitively expensive to collect given the large-scale and long-term nature of this study. An integrated, stochastic-deterministic approach was developed to characterize the system and calibrate the flow model through innovative use of high-density water well datasets. This approached allowed (1) implementation of a "zone-based," nonstationary stochastic approach to conceptualize complex spatial variability using a small set of geologic material types; (2) modeling the spatiotemporal evolution of many water well withdrawals across several decades using sector-based parameterization; and (3) critical analysis of long-term water level changes at different locations in the aquifer system for characterizing the system dynamics and calibrating the model. Results show the approach is reasonably successful in calibrating a complex model for a highly complex site in a way that honors complex distributed heterogeneity and stress configurations.

The Guarani Aquifer System (GAS) has been studied since the 1970s, a time frame that coincides with the advent of isotopic techniques in Brazil. The GAS isotope data from many studies are organized in different phases: (a) the advent of isotope techniques, (b) consolidation and new applications, (c) isotope assessments and hydrochemistry evolution, and (d) a roadmap to a new conceptual model. The reasons behind the phases, their methodological approaches, and impacts on the regional flow conceptual models are examined. Starting with local δ²H and δ¹⁸O assessments of values for water fingerprinting and estimates of recharge palaeoclimate scenarios, studies evolved to more integrated approaches based on multiple tracers. Stable isotope application techniques were consolidated during the 1980s, when new dating approaches dealing with radiogenic and heavy isotopes were introduced. Through the execution of an international transboundary project, the GAS was studied and extensively sampled for isotopes. These results have triggered wider application of isotope techniques, reflecting also world research trends. Presently, hydrochemical evolution models along flow lines from recharge to discharge areas, across large‐scale tectonic features within the entire sedimentary basin, are being combined with residence time estimates at GAS outcrop areas and deep confined units. In a complex system, it is normal that many, and even contradictory hypotheses are proposed, but isotope techniques provide a unique chance to test them. Stable isotope assessments are still needed near recharge areas, and they can be combined with groundwater classical dating procedures, complemented by newer techniques (³H‐³He, CFCs, and SF₆). Recent noble gas sampling and world pioneer analytical efforts focused on the confined units in the GAS will certainly led to new findings on the overall GAS circulation. The objective of this article is to discuss how isotope information can contribute to the evolution of conceptual groundwater flow models for regional continental aquifers, such as the GAS.

It becomes increasingly important and challenging for nitrogen pollution prevention to identify key controls for spatial variability of nitrogen in groundwater that could be affected by multiple factors, including anthropogenic input, groundwater flow, and local geochemistry. This study characterized spatial variability of both nitrate and ammonium in the Pleistocene aquifer of central Yangtze River Basin and assessed the effect of various factors in controlling nitrate and ammonium levels based on multiple statistical approaches (correlation, geostatistics, multiple liner regression). The results indicate that nitrate is mostly influenced by Cl- that represents anthropogenic input, while Eh representing local redox state is a secondary variable influencing nitrate concentrations. The groundwater with elevated nitrate concentrations are estimated to occur mainly in areas with higher-permeability near-surface sediments which can facilitate more anthropogenic nitrate transport and less nitrate removal owing to more oxidized state. Ammonium is mostly correlated to Eh, followed by dissolved organic carbon (DOC), but only DOC improves significantly the accuracy of co-kriging prediction model. The groundwater with elevated ammonium concentrations are estimated to occur mainly in areas with more organic-rich sediments within or around the aquifer which can facilitate more ammonium release owing to natural organic matter consumption accompanying strong reducing conditions. The regional groundwater flow is not a factor significantly controlling nitrate or ammonium levels owing to flat topography and sluggish lateral flow.

Modern microbialites are often located within groundwater discharge zones, yet the role of groundwater in microbialite accretion has yet to be resolved. To understand relationships between groundwater, microbialites, and associated microbial communities, we quantified and characterized groundwater flow and chemistry in active thrombolitic microbialites in Lake Clifton, Western Australia, and compared these observations to inactive thrombolites and lakebed sediments. Groundwater flows upward through an interconnected network of pores within the microstructure of active thrombolites, discharging directly from thrombolite heads into the lake. This upwelling groundwater is fresher than lake water and is hypothesized to support microbial mat growth by reducing salinity and providing limiting nutrients in an osmotically stressful and oligotrophic habitat. This is in contrast to inactive thrombolites that show no evidence of microbial mat colonization and are infiltrated by hypersaline lake water. Groundwater discharge through active thrombolites contrasts with the surrounding lakebed, where hypersaline lake water flows downward through sandy sediments at very low rates. Based on an appreciation for the role of microorganisms in thrombolite accretion, our findings suggest conditions favorable to thrombolite formation still exist in certain locations of Lake Clifton despite increasing lake water salinity. This study is the first to characterize groundwater flow rates, paths, and chemistry within a microbialite-forming environment and provides new insight into how groundwater can support microbial mats believed to contribute to microbialite formation in modern and ancient environments.

Locally collected precipitation water can be actively used as a groundwater tracer solution based on four inherent tracer signals: electrical conductivity, stable isotopic signatures of deuterium [δ2 H], oxygen-18 [δ18 O], and heat, which all may strongly differ from the corresponding background values in the tested groundwater. In hydrogeological practice, a tracer test is one of the most important methods for determining subsurface connections or field parameters, such as porosity, dispersivity, diffusion coefficient, groundwater flow velocity, or flow direction. A common problem is the choice of tracer and the corresponding permission by the appropriate authorities. This problem intensifies where tracer tests are conducted in vulnerable conservation or water protection areas (e.g., around drinking water wells). The use of (if required treated) precipitation as an elemental groundwater tracer is a practical solution for this problem, as it does not introduce foreign matters into the aquifer system, which may contribute positively to the permission delivery. Before tracer application, the natural variations of the participating end members' tracer signals have to be evaluated locally. To obtain a sufficient volume of tracer solution, precipitation can be collected as rain using a detached, large-scale rain collector, which will be independent from possibly existing surfaces like roofs or drained areas. The collected precipitation is then stored prior to a tracer experiment.

The Arctic is warming rapidly. Changing seasonal freezing and thawing cycles of the soil are expected to affect river run-off substantially, but how soil frost influences river run-off at catchment scales is still largely unknown. We hypothesize that soil frost alters flow paths and therefore affects storage–discharge relations in subarctic catchments. To test this hypothesis, we used an approach that combines meteorological records and recession analysis. We studied streamflow data (1986–2015) of Abiskojokka, a river that drains a mountainous catchment (560 km 2 ) in the north of Sweden (68° latitude). Recessions were separated into frost periods (spring) and no-frost periods (summer) and then compared. We observed a significant difference between recessions of the two periods: During spring, discharge was linearly related to storage, whereas storage–discharge relationships in summer were less linear. An analysis of explanatory factors showed that after winters with cold soil temperatures and low snowpack, storage–discharge relations approached linearity. On the other hand, relatively warm winter soil conditions resulted in storage–discharge relationships that were less linear. Even in summer, relatively cold antecedent winter soils and low snowpack levels had a propagating effect on streamflow. This could be an indication that soil frost controls recharge of deep groundwater flow paths, which affects storage–discharge relationships in summer. We interpret these findings as evidence for soil frost to have an important control over river run-off dynamics. To our knowledge, this is the first study showing significant catchment-integrated effects of soil frost on this spatiotemporal scale.

Fractures in porous media have been documented extensively. However, they are often omitted from groundwater flow and mass transport models due to a lack of data on fracture hydraulic properties and the computational burden of simulating fractures explicitly in large model domains. We present a MATLAB toolbox, FracKfinder, that automates HydroGeoSphere (HGS), a variably saturated, control volume finite-element model, to simulate an ensemble of discrete fracture network (DFN) flow experiments on a single cubic model mesh containing a stochastically generated fracture network. Because DFN simulations in HGS can simulate flow in both a porous media and a fracture domain, this toolbox computes tensors for both the matrix and fractures of a porous medium. Each model in the ensemble represents a different orientation of the hydraulic gradient, thus minimizing the likelihood that a single hydraulic gradient orientation will dominate the tensor computation. Linear regression on matrices containing the computed three-dimensional hydraulic conductivity (K) values from each rotation of the hydraulic gradient is used to compute the K tensors. This approach shows that the hydraulic behavior of fracture networks can be simulated where fracture hydraulic data are limited. Simulation of a bromide tracer experiment using K tensors computed with FracKfinder in HGS demonstrates good agreement with a previous large-column, laboratory study. The toolbox provides a potential pathway to upscale groundwater flow and mass transport processes in fractured media to larger scales.

Groundwater flow through coastal wetlands plays an important role in the maintenance of productivity of intertidal ecosystems. Groundwater can reduce salinity and increase nutrient availability which can enhance plant growth and alter plant biomass allocation patterns. Here, we used stable isotopes of oxygen and hydrogen to assess how groundwater influences below-ground and above-ground growth in the widespread mangrove species Avicennia marina. We found source water within tree stems varied seasonally, with non-saline water use higher in the wet season when rainwater availability was highest compared to the dry season. Stems with higher proportional contribution of non-saline water had increased above-ground growth but no effect on below-ground growth. Below-ground growth was however influenced by nutrient availability across the intertidal zone which was higher in the high- compared to the low-intertidal zone. Synthesis. This study shows that mangroves use non-saline groundwater and rainwater when available rather than saline water sources. Groundwater flows into the intertidal stimulates organic matter accumulation in above-ground biomass suggesting the availability of non-saline water sources, such as groundwater and rainfall, are important for the growth and productivity of mangrove forests.

There is an extensive literature dealing with the interaction between groundwater and surface water, and this includes major reviews on baseflow, transmission losses, baseflow recession analysis, and broader aspects of low flows. Although these are mature topics in hydrology, they continue to attract strong interest, with hundreds of papers published in leading journals over the past 20 years. Our comprehensive review of the relevant literature focusses in detail on the use of linear and nonlinear models of recession discharge, and on transmission loss. From this review we identify three approaches to understanding recession, namely (a) theoretical approaches based on the Boussinesq equation, (b) applications of recession slope analysis, and (c) estimates of transmission loss by input–output water balance. We review the application of these techniques to ephemeral and humid areas and note the wide adoption of linear models despite most researchers accepting that the processes are nonlinear. Few papers address the characteristics of groundwater flow to ephemeral streams from unconfined aquifers. Although initially considered an ephemeral arid/semi-arid stream feature, transmission loss occurs in streams in humid areas. Interestingly, we identify a significant omission in the literature, in that while it is common to observe nonlinear behavior in streamflow recessions, little explicit account has been given to the role of transmission loss which can be an important factor in identifying the structure of nonlinear recession models. This article is categorized under: Science of Water > Hydrological Processes.

The role of faults in controlling groundwater flow in the Sahara and most of the hyper‐arid deserts is poorly understood due to scarcity of hydrological data. The Wadi Araba Basin (WAB), in the Eastern Sahara, is highly affected by folds and faults associated with Senonian tectonics and Paleogene rifting. Using the WAB as a test site, satellite imagery, aeromagnetic maps, field observations, isotopic and geochemical data were examined to unravel the structural control on groundwater flow dynamics in the Sahara. Analysis of satellite imagery indicated that springs occur along structurally controlled scarps. Isotopic data suggested that cold springs in the WAB showed a striking similarity with the Sinai Nubian aquifer system (NAS) water and the thermal springs along the Gulf of Suez (e.g., δ¹⁸O = −8.01‰ to −5.24‰ and δD = −53.09‰ to −31.12‰) demonstrating similar recharge sources. The findings advocated that cold springs in the WAB represent a natural discharge from a previously undefined aquifer in the Eastern Desert of Egypt rather than infiltrated precipitation over the plateaus surrounding the WAB or through hydrologic windows from deep crystalline basement flow. A complex role of the geological structures was inferred including: (1) channelling of the groundwater flow along low‐angle faults, (2) compartmentalization of the groundwater flow upslope from high‐angle faults, and (3) reduction of the depth to the main aquifer in a breached anticline setting, which resulted in cold spring discharge temperatures (13–22°C). Our findings emphasize on the complex role of faults and folds in controlling groundwater flow, which should be taken into consideration in future examination of aquifer response to climate variability in the Sahara and similar deserts worldwide.

In continuous permafrost regions, pathways for transport of sub-permafrost groundwater to the surface sometimes perforate the frozen ground and result in the formation of a pingo. Explanations offered for the locations of such pathways have so far included hydraulically conductive geological units and faults. On Svalbard, several pingos locate at valley flanks where these controls are apparently lacking. Intrigued by this observation, we elucidated the geological setting around such a pingo with electrical resistivity tomography. The inverted resistivity models showed a considerable contrast between the uphill and valley-sides of the pingo. We conclude that this contrast reflects a geological boundary between low-permeable marine sediments and consolidated strata. Groundwater presumably flows toward the pingo spring through glacially induced fractures in the strata immediately below the marine sediments. Our finding suggests that flanks of uplifted Arctic valleys deserve attention as possible discharge locations for deep groundwater and greenhouse gases to the surface.

The growth of bioflm in porous media causes disruption in the groundwater flow patterns. In conservative tracer tests performed in columns, this translates in significant changes in the observed br...

This study proposes the utilization of municipal well records as an alternative dataset for large-scale heterogeneity characterization of hydraulic conductivity (K) and specific storage (Ss) using hydraulic tomography (HT). To investigate the performance of HT and the feasibility of utilizing municipal well records, a three-dimensional aquifer/aquitard system is constructed and synthetic groundwater flow and solute transport experiments are conducted to generate data for inverse modeling and validation of results. In particular, we simultaneously calibrate four groundwater models with varying parameterization complexity using five datasets consisting of different time durations and periods. Calibration and validation results are qualitatively and quantitatively assessed to evaluate the performance of investigated models. The estimated K and Ss tomograms from different model cases are also validated through the simulation of independently conducted pumping tests and conservative solute transport. Our study reveals that: 1) the HT analysis of municipal well records is feasible and yields reliable heterogeneous K and Ss distributions where drawdown records are available; 2) accurate geological information is of critical importance when data density is low and should be incorporated for geostatistical inversions; 3) the estimated K and Ss tomograms from the geostatistical model with geological information are capable in providing robust predictions of both groundwater flow and solute transport. Overall, this synthetic study provides a general framework for large-scale heterogeneity characterization using HT through the interpretation of municipal well records, and provides guidance for applying this concept to field problems.

Sulfur hexafluoride (SF6) is an established tracer for use in managed aquifer recharge projects. SF6exsolves from groundwater when it encounters trapped air according to Henry's law. This results in its retardation relative to groundwater flow, which can help determine porous media saturation and flow dynamics. SF6and the conservative, nonpartitioning tracer, bromide (Br−added as KBr), were introduced to recharge water infiltrated into stacked glacial aquifers in Thurston County, Washington, providing the opportunity to observe SF6partitioning. Br−, which is assumed to travel at the same velocity as the groundwater, precedes SF6at most monitoring wells (MWs). Average groundwater velocity in the unconfined aquifer in the study area ranges from 3.9 to 40 m/d, except in the southwestern corner where it is slower. SF6in the shallow aquifer exhibits an average retardation factor of 2.5 ± 3.8, suggesting an air-to-water ratio on the order of 10−3to 10−2in the pore space. Notable differences in tracer arrival times at adjacent wells indicate very heterogeneous conductivity. One MW exhibits double peaks in concentrations of both tracers with different degrees of retardation for the first and second peaks. This suggests multiple flowpaths to the well with variable saturation. The confining layer between the upper two aquifers appears to allow intermittent connection between aquifers but serves as an aquitard in most areas. This study demonstrates the utility of SF6partitioning for evaluating hydrologic conditions at prospective recharge sites.

Both shallow and deep groundwater flow mediates a variety of geologic processes. In the discharge zones of the nested groundwater flow systems, saltwater often emerges due to high evaporation (in e...