Groundwater Flux Research Articles

Lithium isotope fractionation in the Ganges–Brahmaputra floodplain and implications for groundwater impact on seawater isotopic composition

Lithium isotopes are a promising proxy for reconstructing past weathering processes, but unraveling the seawater record requires a comprehensive understanding of the magnitude and isotopic composition of Li fluxes to the oceans, and of how these change over time. Little information is available on the role of floodplain sediments and groundwater systems in setting the Li isotope signature of the dissolved flux delivered from the continents to the oceans. Here we investigate the Li dissolved fluxes of river waters and groundwaters in the Ganges–Brahmaputra floodplain. The data suggest that a maximum of 3.1×108 and 1.5×108 moles Li/yr are carried to the Bay of Bengal by Ganges–Brahmaputra rivers and groundwaters, respectively. The riverine flux has a significantly heavier Li isotope composition (average δ7Li: 26‰) than the groundwater flux (average δ7Li: 16‰) and increases downstream across the floodplain. δ7Li in both river waters and shallow groundwater can be explained by Li scavenging by Quaternary floodplain sediments following a Rayleigh fractionation process, with preferential removal of 6Li. On the other hand, deep groundwaters (>40 m) contributing to submarine groundwater discharge to the Bay of Bengal are enriched in 6Li at depth, likely due to the dissolution of floodplain sediments releasing Li with a light isotope composition. Similarly low δ7Li has been reported in other large sedimentary aquifers. The deep groundwater values are close to the average isotope composition of the global Li inputs to the ocean (∼15‰), so groundwater submarine discharge has only a minor influence on the assessment of the modern Li isotope budget of the ocean. Our analysis further suggests that groundwater discharge of Li has probably played at most a small and secondary role in past changes in the isotope composition of the total continental flux of Li to the ocean.

Sub-surface water contribution to recession flow in a mountain headwater stream system based on single monitoring campaign

Discharge in mountain streams may be a mixture of snowmelt, water from surface runoff, and deep return flow through valley bottom alluvia. We used δ18O and δ2H, solute concentrations, and 222Rn to determine water sources of a headwater stream located at the McDonald Creek watershed, Glacier National Park, USA, during summer recession flow period. We analysed minimal water isotope ranges of −17.6‰ to −16.5‰ and −133‰ to −121‰ for δ18O and δ2H, respectively, potentially due to dominance of snow-derived water in the stream. Likewise, solute concentrations measured in the stream through the watershed showed minimal variation with little indication of subsurface water input into the stream. However, we observed 222Rn activities in the stream that ranged from 39 to 2646 Bq/m3 with the highest value measured in middle of the watershed associated with channel constriction corresponding to changes in local orientation of underlying rocks. Downstream from this point, 222Rn activity decreased from 581 to 117 Bq/m3 in a series of punctuated steps associated with small rapids and waterfalls that we hypothesized to cause radon degassing with a maximum predicted loss of 427 Bq/m3 along a 400 m distance. Based on mass balance calculations using 222Rn activity values, streamflow, and channel characteristics, we estimated that groundwater contributed between 0.3% and 29% of total flow. Overall, we estimated a 5.9% of groundwater contribution integrated for stream reach measured at McDonald Creek during recession flow period. Finally, a lower mean hyporheic flux of 14 m3/day was estimated compared to the groundwater flux of 70 710 m3/day. These assessments highlight the potential for radon as a conservative tracer that can be used to estimate subsurface water contribution in mountain streams within a complex geologic setting. Copyright © 2015 John Wiley & Sons, Ltd.

Groundwater fluxes and flow paths within coastal barriers: Observations from a large-scale laboratory experiment (BARDEX II)

The dynamics of groundwater at the beach face land–ocean boundary have important implications to the exchange of water, nutrients, and pollutants between the ocean and coastal aquifers, and more subtly, varying groundwater levels may induce differing morphological response at the beach face. As a component of the multi-institution Barrier Dynamics Experiment (BARDEX II), groundwater fluxes and flow paths within a prototype-scale sandy barrier are quantified and reported at the three fundamental spatio-temporal scales (individual waves, the beach face, and total barrier), under controlled wave and water level conditions. A particular feature of the experimental programme was the inclusion of a back-barrier ‘lagoon’, that via a pump system and an intermediate water reservoir enabled the forcing of contrasting hydraulic gradients across the barrier. It was observed that the groundwater level, flow paths, and fluxes within the beach face region of the sand barrier were predominantly controlled by the action of waves at the beach face, regardless of the overall seaward- or landward-directed barrier-scale hydraulic gradients. In the presence of waves, all tests undertaken to complete this study developed a seaward gradient in this zone under the influence of waves. As a further result of wave forcing at the beach face boundary, localised groundwater flow divides were observed to develop, further partitioning the circulation and flow paths of groundwater within the prototype-scale sand barrier.

Ostreopsis cf. ovata dynamics in the NW Mediterranean Sea in relation to biotic and abiotic factors

An expansion of the distribution of Ostreopsis cf. ovata, a dinoflagellate which produces palytoxin-like compounds, has been reported in recent years. Economical and social interests are affected by blooms, as they are responsible for respiratory and skin problems in humans and may cause damage to marine organisms. In order to identify the most influential environmental factors that trigger proliferations of O. cf. ovata in the area of the adjacent shallow rocky coast of the Ebro Delta (NW Mediterranean Sea) a three-year survey was performed on the metaphytic microalgae community growing on the macrophytes Jania rubens and Corallina elongata. Small-size diatoms were more abundant than dinoflagellates; O. cf. ovata was identified as the only species present from the genus. Seawater temperature was the primary driver defining the ecological niche of O. cf. ovata. Freshwater and groundwater fluxes were more pronounced in southern than in northern sites, which may have resulted in a distinct O. cf. ovata spatial distribution, with the highest records of abundance and more frequent blooms in the north. In consequence, negative correlations between the abundance of O. cf. ovata and nitrate concentrations and significant positive correlation with salinity were observed. The temporal pattern of O. cf. ovata dynamics from mid-July to early-November is probably due to the fact that this species is observed only above a certain threshold temperature of seawater. Metaphytic cells of O. cf. ovata were smaller in the northern site than in the south, possibly as a result of an increase in cell division, coinciding with higher abundance, and this could be an indicator of favorable conditions. Toxicity in planktonic cells was negatively correlated with cell abundance in the water column, achieving maximum concentrations of 25pg. PLTX eqcell−1.

Contribution of the finite volume point dilution method for measurement of groundwater fluxes in a fractured aquifer

Measurement of groundwater fluxes is the basis of all hydrogeological study, from hydraulic characterization to the most advanced reactive transport modeling. Usual groundwater flux estimation with Darcy's law may lead to cumulated errors on spatial variability, especially in fractured aquifers where local direct measurement of groundwater fluxes becomes necessary.In the present study, both classical point dilution method (PDM) and finite volume point dilution method (FVPDM) are compared on the fractured crystalline aquifer of Ploemeur, France. The manipulation includes the first use of the FVPDM in a fractured aquifer using a double packer. This configuration limits the vertical extent of the tested zone to target a precise fracture zone of the aquifer. The result of this experiment is a continuous monitoring of groundwater fluxes that lasted for more than 4days.Measurements of groundwater flow rate in the fracture (Qt) by PDM provide good estimates only if the mixing volume (Vw) (volume of water in which the tracer is mixed) is precisely known. Conversely, the FVPDM allows for an independent estimation of Vw and Qt, leading to better precision in case of complex experimental setup such as the one used. The precision of a PDM does not rely on the duration of the experiment while a FVPDM may require long experimental duration to guarantees a good precision.Classical PDM should then be used for rapid estimation of groundwater flux using simple experimental setup. On the other hand, the FVPDM is a more precise method that has a great potential for development but may require longer duration experiment to achieve a good precision if the groundwater fluxes investigated are low and/or the mixing volume is large.

Global separation of plant transpiration from groundwater and streamflow.

Current land surface models assume that groundwater, streamflow and plant transpiration are all sourced and mediated by the same well mixed water reservoir--the soil. However, recent work in Oregon and Mexico has shown evidence of ecohydrological separation, whereby different subsurface compartmentalized pools of water supply either plant transpiration fluxes or the combined fluxes of groundwater and streamflow. These findings have not yet been widely tested. Here we use hydrogen and oxygen isotopic data ((2)H/(1)H (δ(2)H) and (18)O/(16)O (δ(18)O)) from 47 globally distributed sites to show that ecohydrological separation is widespread across different biomes. Precipitation, stream water and groundwater from each site plot approximately along the δ(2)H/δ(18)O slope of local precipitation inputs. But soil and plant xylem waters extracted from the 47 sites all plot below the local stream water and groundwater on the meteoric water line, suggesting that plants use soil water that does not itself contribute to groundwater recharge or streamflow. Our results further show that, at 80% of the sites, the precipitation that supplies groundwater recharge and streamflow is different from the water that supplies parts of soil water recharge and plant transpiration. The ubiquity of subsurface water compartmentalization found here, and the segregation of storm types relative to hydrological and ecological fluxes, may be used to improve numerical simulations of runoff generation, stream water transit time and evaporation-transpiration partitioning. Future land surface model parameterizations should be closely examined for how vegetation, groundwater recharge and streamflow are assumed to be coupled.

Simulating the effects of a beaver dam on regional groundwater flow through a wetland

Abstract Study Focus This research examines a wetland environment before and after the construction of a beaver dam to determine the hydrologic impacts on regional groundwater flow and quantify changes to the capture zone of a wetland pond. Increased hydraulic head behind a newly built beaver dam can cause shifts in the capture zone of a wetland pond. Changes in groundwater flux, and the extent of both the capture and discharge zones of this wetland were examined with the use of a groundwater flow model, created using MODFLOW. New hydrological insights for the region The construction of a beaver dam resulted in minimal changes to regional groundwater flow paths at this site, which is attributed to a clay unit underlying the peat, disconnecting this wetland from regional groundwater flow. However, groundwater discharge from the wetland pond increased by 90%. Simulating a scenario with the numerical model in which the wetland is connected to regional groundwater flow results in a much larger impact on flow paths. In the absence of the clay layer, the simulated construction of a beaver dam causes a 70% increase in groundwater discharge from the wetland pond and increases the surface area of both the capture zone and the discharge zone by 30% and 80%, respectively.

Comparing the groundwater contribution in two groundwater-fed streams using a combination of methods

Groundwater contributions to streams often vary both spatially and temporally. In this study, the relative contributions of groundwater to Fishtrap and Bertrand Creeks in the Lower Fraser Valley are compared during the summer and annually using a combination of methods. The streams drain watersheds that have a similar climate, area and topographic relief. However, the geological substrate differs, with finer grained material underlying Bertrand Creek. A simplified heat budget model is first used to estimate temperature at the air–water interface. Then, the correlation between the air–water and bed–water (interface) temperatures is compared for both annual and summer periods in each stream. The air temperature explains ~30% of the variance in the summer interface temperature in Fishtrap Creek, but more than 75% in Bertrand Creek. This difference suggests that factors at the bed–water interface influence the heat budgets, and that these factors differ between the two sites. The measured groundwater flux during the summer periods was higher in Fishtrap Creek than in Bertrand Creek, due to a higher vertical hydraulic gradient into the stream and higher aquifer sediment permeability. Thus, the larger groundwater contribution to Fishtrap Creek moderates stream temperature and stream discharge during the summer periods, whereas Bertrand Creek has greater potential for elevated stream temperatures and lower flows during the critical summer period The combination of methods used in this study strengthens the interpretations given the wide range of uncertainty in measurements.

Evaluating the relationship between topography and groundwater using outputs from a continental‐scale integrated hydrology model

AbstractWe study the influence of topography on groundwater fluxes and water table depths across the contiguous United States (CONUS). Groundwater tables are often conceptualized as subdued replicas of topography. While it is well known that groundwater configuration is also controlled by geology and climate, nonlinear interactions between these drivers within large real‐world systems are not well understood and are difficult to characterize given sparse groundwater observations. We address this limitation using the fully integrated physical hydrology model ParFlow to directly simulate groundwater fluxes and water table depths within a complex heterogeneous domain that incorporates all three primary groundwater drivers. Analysis is based on a first of its kind, continental‐scale, high‐resolution (1 km), groundwater‐surface water simulation spanning more than 6.3 million km2. Results show that groundwater fluxes are most strongly driven by topographic gradients (as opposed to gradients in pressure head) in humid regions with small topographic gradients or low conductivity. These regions are generally consistent with the topographically controlled groundwater regions identified in previous studies. However, we also show that areas where topographic slopes drive groundwater flux do not generally have strong correlations between water table depth and elevation. Nonlinear relationships between topography and water table depth are consistent with groundwater flow systems that are dominated by local convergence and could also be influenced by local variability in geology and climate. One of the strengths of the numerical modeling approach is its ability to evaluate continental‐scale groundwater behavior at a high resolution not possible with other techniques.

FINIFLUX: An implicit finite element model for quantification of groundwater fluxes and hyporheic exchange in streams and rivers using radon

AbstractA quantitative understanding of groundwater‐surface water interactions is vital for sustainable management of water quantity and quality. The noble gas radon‐222 (Rn) is becoming increasingly used as a sensitive tracer to quantify groundwater discharge to wetlands, lakes, and rivers: a development driven by technical and methodological advances in Rn measurement. However, quantitative interpretation of these data is not trivial, and the methods used to date are based on the simplest solutions to the mass balance equation (e.g., first‐order finite difference and inversion). Here we present a new implicit numerical model (FINIFLUX) based on finite elements for quantifying groundwater discharge to streams and rivers using Rn surveys at the reach scale (1–50 km). The model is coupled to a state‐of‐the‐art parameter optimization code Parallel‐PEST to iteratively solve the mass balance equation for groundwater discharge and hyporheic exchange. The major benefit of this model is that it is programed to be very simple to use, reduces nonuniqueness, and provides numerically stable estimates of groundwater fluxes and hyporheic residence times from field data. FINIFLUX was tested against an analytical solution and then implemented on two German rivers of differing magnitude, the Salzach (∼112 m3 s−1) and the Rote Main (∼4 m3 s−1). We show that using previous inversion techniques numerical instability can lead to physically impossible negative values, whereas the new model provides stable positive values for all scenarios. We hope that by making FINIFLUX freely available to the community that Rn might find wider application in quantifying groundwater discharge to streams and rivers and thus assist in a combined management of surface and groundwater systems.

Groundwater discharge into an estuary using spatially distributed radon time series and radium isotopes

Summary Quantifying groundwater discharge remains a challenge due to its large temporal and spatial variability. Here, we quantify groundwater discharge into a small estuary using radon (222Rn) and radium isotopes (223Ra and 224Ra). High temporal resolution (30 min time steps) radon observations at 4 time series stations were used to determine where groundwater discharge is prevalent in the estuary, and to reduce mass balance model uncertainties. A three-endmember mixing model was developed based on short-lived radium isotopes (sampled at a single location) to separate the shallow saline and deep fresh sources of the discharging groundwater. The results show that using multiple 222Rn time series stations decreased the overall uncertainty of groundwater discharge estimates from about 41% to 23%. The radon derived groundwater flux was 56 ± 13 and 35 ± 12 cm d−1 in wet and dry conditions, respectively. The spatially distributed stations detected a well-defined small area located four kilometers upstream from the mouth of the estuary as a groundwater discharging hotspot. Estimates based on a 223Ra and 224Ra mass balance resulted in groundwater discharge estimates of 65 ± 18 and 84 ± 48 cm d−1 in the wet and 18 ± 5 and 20 ± 6 cm d−1 in the dry. The mixing model revealed contrasting results for deep vs. fresh groundwater contribution in wet and dry conditions. In wet conditions, deep fresh groundwater discharging into the estuary contributed 65% compared to the shallow saline groundwater (35%), while during dry conditions a larger contribution (80%) was related to shallow groundwater. A comprehensive spatial and temporal sampling strategy can produce groundwater discharge estimates with lower uncertainty and provides additional insight on where groundwater enters surface waters.

Calculating discharge of phosphorus and nitrogen with groundwater base flow to a small urban stream reach

Summary Elevated levels of nutrients, especially phosphorus, in urban streams can lead to eutrophication and general degradation of stream water quality. Contributions of phosphorus from groundwater have typically been assumed minor, though elevated concentrations have been associated with riparian areas and urban settings. The objective of this study was to investigate the importance of groundwater as a pathway for phosphorus and nitrogen input to a gaining urban stream. The stream at the 28-m study reach was 3–5 m wide and straight, flowing generally eastward, with a relatively smooth bottom of predominantly sand, with some areas of finer sediments and a few boulders. Temperature-based methods were used to estimate the groundwater flux distribution. Detailed concentration distributions in discharging groundwater were mapped using in-stream piezometers and diffusion-based peepers, and showed elevated levels of soluble reactive phosphorus (SRP) and ammonium compared to the stream (while nitrate levels were lower), especially along the south bank, where groundwater fluxes were lower and geochemically reducing conditions dominated. Field evidence suggests the ammonium may originate from nearby landfills, but that local sediments likely contribute the SRP. Ammonium and SRP mass discharges with groundwater were then estimated as the product of the respective concentration distributions and the groundwater flux distribution. These were determined as approximately 9 and 200 g d−1 for SRP and ammonium, respectively, which compares to stream mass discharges over the observed range of base flows of 20–1100 and 270–7600 g d−1, respectively. This suggests that groundwater from this small reach, and any similar areas along Dyment’s Creek, has the potential to contribute substantially to the stream nutrient concentrations.

Dynamic river–groundwater exchange in the presence of a saline, semi‐confined aquifer

AbstractUnderstanding groundwater–surface water exchange in river banks is crucial for effective water management and a range of scientific disciplines. While there has been much research on bank storage, many studies assume idealized aquifer systems. This paper presents a field‐based study of the Tambo Catchment (southeast Australia) where the Tambo River interacts with both an unconfined aquifer containing relatively young and fresh groundwater (<500 μS/cm and <100 years old) and a semi‐confined artesian aquifer containing old and saline groundwater (electrical conductivity > 2500 μS/cm and >10 000 years old). Continuous groundwater elevation and electrical conductivity monitoring within the different aquifers and the river suggest that the degree of mixing between the two aquifers and the river varies significantly in response to changing hydrological conditions. Numerical modelling using MODFLOW and the solute transport package MT3DMS indicates that saline water in the river bank moves away from the river during flooding as hydraulic gradients reverse. This water then returns during flood recession as baseflow hydraulic gradients are re‐established. Modelling also indicates that the concentration of a simulated conservative groundwater solute can increase for up to ~34 days at distances of 20 and 40 m from the river in response to flood events approximately 10 m in height. For the same flood event, simulated solute concentrations within 10 m of the river increase for only ~15 days as the infiltrating low‐salinity river water drives groundwater dilution. Average groundwater fluxes to the river stretch estimated using Darcy's law were 7 m3/m/day compared with 26 and 3 m3/m/day for the same periods via mass balance using Radon (222Rn) and chloride (Cl), respectively. The study shows that by coupling numerical modelling with continuous groundwater–surface water monitoring, the transient nature of bank storage can be evaluated, leading to a better understanding of the hydrological system and better interpretation of hydrochemical data. Copyright © 2015 John Wiley & Sons, Ltd.

Groundwater discharge to wetlands driven by storm and flood events: Quantification using continuous Radon-222 and electrical conductivity measurements and dynamic mass-balance modelling

The dynamic response of groundwater discharge to external influences such as rainfall is an often neglected part of water and solute balances in wetlands. Here we develop a new field platform for long-term continuous 222Rn and electrical conductivity (EC) measurements at Sale Wetland, Australia to study the response of groundwater discharge to storm and flood events. The field measurements, combined with dynamic mass-balance modelling, demonstrate that the groundwater flux can increase from 3 to ∼20mmd−1 following storms and up to 5mmd−1 on the receding limb of floods. The groundwater pulses are likely produced by activation of local groundwater flow paths by water ponding on the surrounding flood plains. While 222Rn is a sensitive tracer for quantifying transient groundwater discharge, the mass-balance used to estimate fluxes is sensitive to parameterisation of gas exchange (k) with the atmosphere. Comparison of six equations for calculating k showed that, based on parameterisation of k alone, the groundwater flux estimate could vary by 58%. This work shows that neglecting transient processes will lead to errors in water and solute flux estimates based on infrequent point measurements. This could be particularly important for surface waters connected to contaminated or saline groundwater systems.

Performance of different assessment methods to evaluate contaminant sources and fate in a coastal aquifer.

The present study deals with the application of different monitoring techniques and numerical models to characterize coastal aquifers affected by multiple sources of contamination. Specifically, equivalent freshwater heads in 243 monitoring wells were used to reconstruct the piezometric map of the studied aquifer; flow meter tests were carried out to infer vertical groundwater fluxes at selected wells; deuterium and oxygen isotopes were used to identify the groundwater origin, and tritium was analyzed to estimate the residence time; compound-specific isotope analyses and microbial analyses were employed to track different sources of contamination and their degradation; numerical modelling was used to estimate and verify groundwater flow direction and magnitude throughout the aquifer. The comparison of the information level for each technique allowed determining which of the applied approaches showed the best results to locate the possible sources and better understanding of the fate of the contaminants. This study reports a detailed site characterization process and outcomes for a coastal industrial site, where a comprehensive conceptual model of pollution and seawater intrusion has been built using different assessment methods. Information and results from this study encourages combining different methods for the design and implementation of the monitoring activities in real-life coastal contaminated sites in order to develop an appropriate strategy for control and remediation of the contamination.

Seasonal Changes in Submarine Groundwater Discharge and Associated Nutrient Transport into a Tideless Semi-enclosed Embayment (Obama Bay, Japan)

We carried out a seasonal study of fresh submarine groundwater discharge (SGD) and associated nutrient fluxes in a semi-enclosed bay along a tideless coastal zone using a 222Rn and salinity mass balance model for a whole bay scale. The resulting SGD rates showed large intra-annual variability from 0.05 × 106 to 0.77 × 106 m3 day−1, which were controlled by seasonal changes in the interaction of multiple driving forces, including water table height and seawater level. The highest SGD rate in early spring was induced by heavy snow and low sea level, whereas the seasonal increase in sea level gradually suppressed fresh SGD rates. In summer, an elevated water table may induce higher SGD rates (approximately 0.4 × 106 m3 day−1) regardless of high sea levels. The highest SGD fraction in total terrestrial freshwater fluxes also occurred in summer (>40 %), due to the decreasing rate of surface river discharge. The seasonally averaged SGD rate was 0.36 × 106 m3 day−1. This value was similar to the annual groundwater recharge rate (0.33 × 106 m3 day−1) estimated by the water balance method in the basin. Nutrient fluxes from SGD were approximately 42, 65, and 33 % of all terrestrial fluxes of dissolved inorganic nitrogen, phosphorous, and silicate, respectively. The average fraction of SGD in the water fluxes including terrestrial and oceanic water was low (0.3 %), but that of nutrient fluxes increased to 20–38 %. Higher nutrient concentrations in groundwater compensated for the lower volumetric flux of groundwater. Because primary production was mostly restricted by phosphorous throughout the year, phosphorous-enriched nutrient transport via SGD would play an important role in biological production.

Hydrologic and Biogeochemical Drivers of Riparian Denitrification in an Agricultural Watershed

This study investigated drivers of denitrification and overall NO3 − removal in an agricultural riparian area in central New York. Denitrification was measured using an in situ “push-pull” method with 15N–NO3 − as a tracer during summer and fall 2011 at a pair of riparian sites characterized by different hydrologic regimes. Median denitrification rates were 1347 and 703 μg N kg soil−1 day−1 for the two study sites. These rates are higher than those reported for other riparian areas, emphasizing the role of some riparian areas as hotspots of NO3 − removal. N2O production was significantly higher at one site, demonstrating that riparian areas can be a greenhouse gas source under certain conditions. Denitrification was negatively correlated with groundwater flux, suggesting that slower flushing of water, and thus longer residence time, promotes denitrification. A mass balance of NO3 − loss revealed that denitrification only accounted for 5–12 % of total NO3 − loss, and production of NH4 + indicated that dissimilatory NO3 − reduction to NH4 + (DNRA) may be occurring at both sites. While both sites were characterized by high NO3 − removal, differences in denitrification rates and NO3 − removal processes demonstrate the need to improve our ability to capture spatial and process heterogeneity in landscape biogeochemical models.

Quantitative assessment of groundwater controls across major US river basins using a multi-model regression algorithm

Spatial patterns in the physical controls of groundwater depth and flux are assessed quantitatively using results from a first of its kind, integrated groundwater surface water simulation over the majority of the contiguous US. We apply a novel, k-regression algorithm to the simulated system to simultaneously identify spatial subsets of grid cells with similar relationships between explanatory variables and groundwater metrics while quantifying behavior using multiple linear regression. The combination of this statistical approach with the results of a large-scale, high-resolution groundwater simulation allows us to evaluate the ability to represent complex groundwater behavior with simple linear models across an unprecedented range of climates and physical settings. In almost all of the eight major basins considered, we identify at least some areas where the coefficient of determination for the linear regression model is larger than 0.7, and in many cases this is achieved for more than 50% of the total basin area. In general, we show that water table depth is most strongly related to location within a basin and slope, while conductivity and recharge are more important predictors for groundwater flux metrics. Results also illustrate spatial variability in these relationships; further demonstrating the historic difficulty in developing spatially contiguous classifications of groundwater behavior. This work highlights the potential to combine new statistical techniques with integrated hydrologic models to help improve our understanding of complex heterogeneous systems.

Detecting groundwater discharge dynamics from point-to-catchment scale in a lowland stream: combining hydraulic and tracer methods

Abstract. Detecting, quantifying and understanding groundwater discharge to streams are crucial for the assessment of water, nutrient and contaminant exchange at the groundwater–surface water interface. In lowland agricultural catchments with significant groundwater discharge this is of particular importance because of the risk of excess leaching of nutrients to streams. Here we aim to combine hydraulic and tracer methods from point-to-catchment scale to assess the temporal and spatial variability of groundwater discharge in a lowland, groundwater gaining stream in Denmark. At the point-scale, groundwater fluxes to the stream were quantified based on vertical streambed temperature profiles (VTPs). At the reach scale (0.15–2 km), the spatial distribution of zones of focused groundwater discharge was investigated by the use of distributed temperature sensing (DTS). Groundwater discharge to the stream was quantified using differential gauging with an acoustic Doppler current profiler (ADCP). At the catchment scale (26–114 km2), runoff sources during main rain events were investigated by hydrograph separations based on electrical conductivity (EC) and stable isotopes 2H/1H. Clear differences in runoff sources between catchments were detected, ranging from approximately 65% event water for the most responsive sub-catchment to less than 10% event water for the least responsive sub-catchment. This was supported by the groundwater head gradients, where the location of weaker gradients correlated with a stronger response to precipitation events. This shows a large variability in groundwater discharge to the stream, despite the similar lowland characteristics of sub-catchments indicating the usefulness of environmental tracers for obtaining information about integrated catchment functioning during precipitation events. There were also clear spatial patterns of focused groundwater discharge detected by the DTS and ADCP measurements at the reach scale indicating high spatial variability, where a significant part of groundwater discharge was concentrated in few zones indicating the possibility of concentrated nutrient or pollutant transport zones from nearby agricultural fields. VTP measurements confirmed high groundwater fluxes in discharge areas indicated by DTS and ADCP, and this coupling of ADCP, DTS and VTP proposes a novel field methodology to detect areas of concentrated groundwater discharge with higher resolution.

Methodology for the Estimation of Groundwater Flux across Simplified Boundary Using GIS and Groundwater Levels

Accurate estimation of the groundwater budget requires good estimate of flux across the boundary of a region. Different methods and techniques are available to estimate recharge to groundwater however methods for estimation of flux across boundaries are not available. The present article represents a GISbased methodology to estimate the groundwater flow rate and volume of water flux across the boundary. Groundwater level data (pre-monsoon and postmonsoon) are used to derive detailed maps like flow direction, hydraulic gradient and velocity component perpendicular to the boundary. For the study area, the calculated flux through the simplified boundary average 6.63 × 10 5 m 3 for monsoon period, 13.75 × 10 5 m 3 for non-monsoon period and 20.38 × 10 5 m 3 annually. Flux of monsoon period varies from maximum of 11.41 × 10 5 m 3 in 2003 to minimum of 0.83 × 10 5 m 3 in the year 2013 and for annual period it varies from maximum 38.8 × 10 5 m 3 in 2003-04 to minimum -7.94 × 10 5 m 3 in 2012-13.