Groundwater Mound Research Articles

Artificial aquifer recharge and pumping: transient analytical solutions for hydraulic head and impact on streamflow rate based on the spatial superposition method

The behaviour of transient groundwater mounds in response to infiltration from surface basins has been studied for decades, but some common settings still lack analytical solutions. It has been shown that applying mathematical integration to the line-sink solution developed by Hantush in the 1960s for pumping an unconfined aquifer, considering recharge over the surface of a defined area, is identical to his solution for groundwater mounding below a rectangular basin. This implies that the superposition principle can, generally, be used to directly include pumping wells, as well as aquifer boundaries, to derive a unique solution. Moreover, other line-sink solutions can be used with a spatial superposition method for addressing a variety of hydrogeological settings, provided the behaviour of the relevant partial differential equations is linear. Based on this principle and on existing line-sink solutions, several analytical solutions are proposed that are able to consider a rectangular recharging area and a pumping well in an unconfined aquifer: (1) near a stream, (2) between a stream and a no-flow boundary, with and without the influence of natural recharge, (3) near a stream that partially penetrates an aquifer and (4) for a multi-layer aquifer. For cases including streams, transient solutions of the impact on streamflow rate are also presented. The proposed analytical solutions will be of practical use for managed aquifer recharge, in particular the design of structures for artificially recharging an aquifer, possibly pumped by one or several wells.

Identifying recharge under subtle ephemeral features in a flat-lying semi-arid region using a combined geophysical approach

Abstract. Identifying and quantifying recharge processes linked to ephemeral surface water features is challenging due to their episodic nature. We use a combination of well-established near-surface geophysical methods to provide evidence of a surface and groundwater connection under a small ephemeral recharge feature in a flat, semi-arid region near Adelaide, Australia. We use a seismic survey to obtain P-wave velocity through travel-time tomography and S-wave velocity through the multichannel analysis of surface waves. The ratios between P-wave and S-wave velocities are used to calculate Poisson's ratio, which allow us to infer the position of the water table. Separate geophysical surveys were used to obtain electrical conductivity measurements from time-domain electromagnetics and water contents from downhole nuclear magnetic resonance. The geophysical observations provide evidence to support a groundwater mound underneath a subtle ephemeral surface water feature. Our results suggest that recharge is localized and that small-scale ephemeral features may play an important role in groundwater recharge. Furthermore, we show that a combined geophysical approach can provide a perspective that helps shape the hydrogeological conceptualization of a semi-arid region.

Impact of an artificial lake on the regional groundwater environment in urban area of northwest China

In many densely populated urban areas, artificial lakes have replaced natural river systems as water sources to provide people with water for living and industrial production. However, groundwater mounding due to seepage from water storage facilities will have an impact on the regional groundwater flow regime. This study investigated the effects of the construction and operation of the Doumen Reservoir in northwestern China on the local groundwater flow regime. Groundwater level variations before the construction of the impoundment were analyzed by wavelet analysis and indicated that most of the observed variations in levels were due to changes in groundwater abstraction in the area. Data were collected from drilling and real-time monitoring; a coupled Hydrus-1D and MODFLOW model was developed to simulate the impact of water storage on the surrounding groundwater environment. The simulation results show that compared with the initial flow field, the maximum elevation of the groundwater mound that would develop beneath water storage structured in the area would vary between about 4.9 and 6.9 m. Compared with the impoundment of the test section, other modeled scenarios will cause varying degrees of groundwater mounding around the reservoir area but no flooding of existing infrastructure is predicted to take place. The predicted seepage rate from the facility is 3660 m3/day, and anti-seepage measures are recommended to limit the effects of groundwater mounding.

Water transfer imposes hydrochemical impacts on groundwater by altering the interaction of groundwater and surface water

How the water transfer imposes impacts on the groundwater in the water-receiving area is still unknown. Here, we employed hydrometric observations, hydrogeochemical analyses and data analysis methods to address this issue, taking one of the Yellow River water transfer project of China for example. We found that the interaction between river and groundwater is totally changed by the water transfer and the impact of the water transfer on groundwater is confined by the high stream gradient. With the water transfer, the natural annual pattern of rainy-season high river flow is inversed with low flow in rainy seasons and high flow during the water transfer. Moreover, the river stage is always higher than the groundwater level throughout the year. The usual bidirectional river–groundwater hydraulic gradient is altered to be a gradient always away from the river. The groundwater directly connects with the river water. The regime of groundwater level follows the change of the river stage, even in the valley side. A long-term standing groundwater mound is formed within the riparian zone due to the continuous recharge from the water-receiving river. The transferred Yellow River water is hydrochemically featured by high EC value and concentrations of Na+ and Cl−. The hydrochemical features of the water-receiving river are obviously changed by the transferred Yellow River water. The hydrochemical impact of the river on the groundwater is mainly confined to the riparian zone and some low-reach transections, despite of the dominated control of the river on the valley groundwater table. The high stream gradient trends to produce the downstream groundwater flow and hinder the lateral groundwater flow from river to valley sides in the upper reaches, responsible for the confined impact on groundwater. Our study highlights that the stream gradient could obviously influence the river water and groundwater interaction promoting or confining the impact of river water on groundwater.

Groundwater recharge from an oxbow lake‐wetland system in the Mississippi Alluvial Plain

AbstractThe Mississippi River Valley Alluvial Aquifer ranks among the most overdrafted aquifers in the United States due to intensive irrigation. Concern over declining water levels has increased focus on understanding the sources of recharge. Numerous oxbow lakes overlie the aquifer that are often considered hydraulically disconnected from the groundwater system due to fine‐grained bottom sediments. In the current study, groundwater levels in and around a 445‐ha oxbow lake‐wetland in Mississippi were monitored for a 2‐year period that included an unusually long low‐water condition in the lake (>17 months), followed by a high‐water event lasting over 4 months before returning to earlier low‐water levels. The high‐water pulse (>4 m rise) provided a unique opportunity to track the impact in the underlying alluvial aquifer. During low‐water conditions, groundwater flowed westward beneath the lake. Following the lake rise, groundwater beneath and near the perimeter responded as quickly as the same day, with more delayed responses moving away from the lake. Within 2 months, a groundwater mound formed near the centre of the oxbow (>3 m increase), with a reversal in the local hydraulic gradient towards the east. Flow returned to a westward gradient when the lake level dropped back below 0.3 m. Analysis of precipitation and nearby river stage could not account for the observed behavior. Recharge to the aquifer is attributed to rising water levels spreading over point bar deposits and into the surrounding forested wetlands where preferential flow pathways are likely to exist due to buried and decomposing tree remains. An earlier study in the wetland demonstrated an increasing redox potential in isolated zones, consistent with the existence of preferential flow pathways through the bottom sediments (Lahiri & Davidson, 2020). Retaining high‐water levels in oxbow lakes could be a relatively low‐cost water management practice for enhancing aquifer recharge.

Simulation of Two Dimensional Subsurface Seepage Flow In Isolated Anisotropic Aquifer

Mathematical models have emerged as important tools for prediction of transient and steady state behavior of water table in unconfined aquifer under seepage and recharge condition. This paper presents analytical solution to predict the behavior of water table in anisotropic unconfined aquifer overlaying a leaky base in response to multiple recharge and withdrawal. The hydrological setting consists of rectangular aquifer with all four sides with no flow conditions. The aquifer is subjected to recharge and withdrawal activities through multiple recharge basin and extraction/injection wells. The subsurface seepage flow is approximated by two-dimensional Boussinesq equation and solved using Fourier cosine transform. Application of closed form solution is demonstrated using an illustrative example. Effect of aquifer parameter on formation of ground water mound and cone of depression due to recharge and withdrawal are discussed. The influence of permeability on the mound height and cone depth is also analyzed.

Multi-directional flow dynamics shape groundwater quality in sloping bedrock strata

Multi-directional fluid flow and transport dynamics as intrinsic characteristics of hillslope flow regimes can strongly contribute to the quality evolution of groundwater resources and compartmentalization of subsurface ecosystems. However, their extent and importance in topographic highs (groundwater recharge areas) is typically less investigated, because productive groundwater bodies and thus monitoring activities are frequently lacking. In the Hainich Critical Zone Exploratory, we explored the hydrogeological functioning of the widely distributed setting of thin-bedded, alternating mixed carbonate-/siliciclastic bedrock, by using basic environmental timeseries. Up to 8-year records of weather parameters, ground temperatures, multi-depth hydraulic heads, and non-conservative tracers, were exploited applying a scheme for the exploratory analysis of dis-/continuous groundwater quality data. We identified transient, multi-directional flow dynamics, comprising fluctuating perched groundwater, localized recharge and groundwater mounding in the aeration zone that modify and partly reverse flow patterns in the phreatic zone. This interplay of flow dynamics within the hillslope aeration and phreatic zone causes significant re-distribution (e.g. oxygen, nitrate) that even overcome the “protective cover” of thick argillaceous strata by supplying surface-sourced substances to deep groundwater resources. As a factor for ecosystem compartmentalization, our results further suggest to carefully consider the hillslope multi-directional flow and matter exchange in biogeochemical models, as well as in resource protection and groundwater management practices.

Effective water quantity of multi-source water recharging aquifers in Yufuhe River based on groundwater and surface water semi-coupled modelling

AbstractWith rapid urbanisation, a karst water recharge area of the Jinan spring catchment was damaged. Thus, managed aquifer recharge projects were built in the western Jinan spring catchment to protect the water supply of the spring. Yufuhe River was selected as the study area to compute the effective recharge rate into karst aquifers. This strong seepage zone has a large gradient and undergoes a specific hydrogeological condition in which two strata of a gravel layer and limestone change to three strata of gravel, impermeable clay shale and limestone at the open window of the karst aquifers. A hydraulic model called HEC-RAS was applied to simulate the river stage, and a numerical groundwater model called HYDRUS-3D was adopted to simulate the groundwater mound dynamics and estimate river flow seepage into the aquifers. The effective recharge rates are 64.9%, 65.2% and 68.1% when the buried depths of groundwater are 40, 30 and 25 m. An analysis of the electric conductivity, water table, temperature and water volume data found an effective recharge rate of 68.3%. Results of field monitoring confirmed the accuracy of the numerical simulation and showed that most of the recharged water in the study reach can be effectively recharged into the karst aquifers.

Programming and evaluation of the 2D Hantush analytical solution for artificial recharge of aquifers

The method of artificial recharge of aquifers represents an opportunity to answer many problems related to the management of water resources, especially in arid and semi-arid regions. However, before installation and management of water infiltration, structures require planning in the design through forecasting tools, which can simulate scenarios of evolution of the hydraulic mound formed under this charging device. In this context, several analytical solutions have been developed (Baumann in Trans ASCE 117:1024–1060, 1952; Glover in Mathematical derivations as pertain to ground-water recharge. In: United States Department of Agriculture, western soil and water management research branch, Fort Collins, Colorado, 1960; Hantush in Water Resour Res 3(1):227–234, 1967; Hunt in J Hydraul Div 97(7):1017–1030, 1971; Marino in Growth and decay of ground-water ridges in response to deep percolation. MS thesis, New Mexico Institute of Mining and Technology, Socorro, New Mexico, 1965, J Res Geophys 72:1195, 1967; Rao and Sarma in Groundwater 19(3):270–274, 1981) and others, which predict the evolution of the hydraulic mound and its geometry from input parameters which represent some properties of the aquifer and recharge control data. We have proposed for this work, a detailed study of the analytical solution of Hantush (1967), through a comparison with experimental data of Bianchi and Haskell in (Field observations of transient ground water mounds produced by artificial recharge into an unconfined aquifer. In: USDA Agricultural Research Services Rapport ARS W-27, p 27, 1975), a sensitivity study of the different input parameters of the model such as the geometry of the Infiltration basin, hydraulic conductivity, storage coefficient, recharge rate and saturated aquifer thickness. Finally, we did a case study of this applied model to the Mnasra basin, in the northwest of Morocco. The solution shows reasonable estimates of the evolution of the hydraulic mound, and could be a good forecasting tool; however, the non-respect of the initial assumptions of the model, such as the homogeneity of the soil, can induce some errors in the estimation of the height and shape of the hydraulic mound.

Statistical and geochemical fingerprinting analysis of arsenic mobilization and natural background associated with artificial groundwater recharge

Differentiation of anthropogenic water-quality impacts and natural water quality is an important consideration for water management, and is a common concern in the vicinity of managed aquifer recharge operations. Application of geochemical fingerprinting methods at a mine site in Nevada, USA allowed for groundwater monitoring wells that have been influenced by infiltration operations to be discriminated from groundwater with naturally elevated arsenic. Increasing arsenic concentrations are shown to correlate with the formation of a groundwater mound on the site. Following filtering of human-impacted water-quality records, a natural background concentration of arsenic in groundwater equal to 0.036 mg/L was calculated using statistical methods suited to non-parametric datasets. This calculated background concentration corresponds well to various observations on the site, where arsenic greater than the drinking water standard is ubiquitous. Results of empirical leach testing (Meteoric Water Mobility Procedure) were compared with temporal changes in arsenic concentrations in groundwater, and indicated that arsenic is likely leached from soils in the unsaturated zone and may be conservatively transported to the water table. The applied methodology allows for regulatory compliance to be easily evaluated and can be applied in a number of different environments aside from mining operations.

Groundwater dynamics due to general stream fluctuations in an unconfined single or dual-porosity aquifer subjected to general areal recharge

Analytical solutions of groundwater dynamics in an elongated aquifer subjected to general time-dependent recharge are presented. The lateral boundaries are specified heads with the head variations governed by general time-dependent functions. General recharge function was not considered in previous works of this kind. Both single and double-porosity aquifers are considered. The solution is obtained using Fourier sine and Laplace transformations, followed by an inverse Fourier-Laplace transform involving residue theorem and convolutional integral. For the unconfined single-porosity aquifer case, the exact time-domain solution is obtained using the residue theorem; for the unconfined double-porosity aquifer case, the time-domain head is calculated using the de Hoog inverse Laplace algorithm. The presented solution can be used to estimate the hydraulic parameters of 1) groundwater head variation of a river basin aquifer subjected to general lateral head variation and recharge; 2) groundwater head variation in a double-porosity elongated fractured anticline; 3) groundwater depletion of an elongated fractured anticline subjected to recharge due to rainfall or snowmelt to its adjacent alluvial aquifer. In addition, the presented solution can be utilized to optimize the irrigation pattern in a cropland between two trench drains to control the groundwater mound.

Response of Triangular-Shaped Leaky Aquifers to Rainfall-Induced Groundwater Recharge: an Analytical Study

Different aspects of groundwater mound dynamics in triangular-shaped aquifers are investigated analytically under spatially uniform recharge of time-varying rate. The aquifer response is analyzed relying on 2-D linearized Boussinesq equation, subject to two different configurations of hydrogeological boundary conditions (constant-head streams and no-flow barrier). The aquifer is homogeneous, isotropic and rests over a horizontal, semipervious layer, through which vertical leakage can take place. Point-recharge formula (Green’s function) is first derived for the intended aquifer domain and then properly converted to accommodate the effect of rainfall-induced areal recharge. Components of groundwater budget are evaluated in terms of volumetric rates, taking into account the jointed effects of leakage, mound storage and outflow to adjacent streams. The resulting expressions are then proven to obey the expected mass balance in a rigorous mathematical fashion. Hypothetical examples illustrating main features of flow field are presented, with attention paid on groundwater equpotentials and streamlines. The computed mound profiles appear to agree well with numerical results from finite element method. Further, the most influential parameters affecting each component of groundwater budget are identified with the help of sensitivity analysis. Finally, the combined effects of a pumping well and rainfall-induced mound are discussed. The present solution may serve as a test case for verifying numerical schemes that are being developed for more comprehensive mound analysis.

Resolving Changes to Freshwater Lens Systems in a “Sea of Salinity” using Multi-date Airborne EM

Saline aquifers in the Murray River or SE Australia are traversed by freshwater rivers, with adjoining riparian and floodplain regions containing freshwater lenses. Bore data and more recent Airborne electromagnetic (AEM) surveys have determined that these lenses are spatially extensive, but have widely varying geometries. The maintenance of these lens systems is important as they support ecologically significant riparian vegetation communities such as Red Gum and Black Box. A more complete understanding of their hydrogeology is required to ascertain how they develop and degrade. Limited ground investigations including 14C geochemistry have determined that the lens systems contain recent water, indicating that they are dynamic systems with their development defined by the relative rates of recharge from the river and mixing with groundwater. Changes in groundwater gradients and depth, floodplain extent, and topography are believed to control their initial location. The same controls also govern their stability.The potential of AEM systems for defining the geometry of these lens systems in 3D is considered along with an assessment of their value for monitoring variations associated with these ecosystems. The advent of “calibrated” AEM systems and robust inversion tools have given added impetus to their use for environmental monitoring. Spatio-temporal variations are observed in the near surface (top 20m) from a multi-temporal assessment of Clark’s Floodplain, adjacent to the Bookpurnong irrigation area, with co-incident AEM surveys acquired between 2008 and 2015. Spatial changes in ground conductivity, attributed to changing groundwater quality have been observed. The freshwater lens systems appear to have contracted significantly over the last decade. This is attributed, in part, to land use patterns and the development of an irrigation-related groundwater mound on the highlands adjacent to the floodplain, and an increased hydraulic gradient towards the river. The results indicate the geometry of the hyporheic zone may have also changed along the river.

Interactions between shallow groundwater and low‐impact development underdrain flow at different temporal scales

AbstractLow‐impact development (LID) practices are effective in managing surface run‐off, mitigating nonpoint source pollution, and recharging groundwater. However, they are often less effective in shallow groundwater environments as their surface infiltration and bottom exfiltration rates could be reduced, and the underdrains (i.e., underground drains within the media of the LID practices) may drain groundwater and alter groundwater dynamics. To evaluate and better understand the interactions between shallow groundwater and LID underdrain flow at different temporal scales, multiple statistical analyses were conducted on the monitored groundwater table depth and underdrain flow of porous pavements at the Central Kitsap Community Campus (Washington) and both porous pavements and bioretention cells at the IMAX parking lot (Ontario, Canada). The wavelet power, which represents oscillation behaviour and is obtained by continuous wavelet transform, of underdrain flow was a hybrid of that of rainfall and groundwater table depth. Rainfall variations at finer temporal scales (5 min to 10 days) resulted in finer scale underdrain flow, whereas groundwater fluctuations at coarser temporal scales (10 to 30 days) accounted for coarser scale underdrain flow and extended the duration of it. The impact of groundwater on underdrain flow was greater when the groundwater table was shallower than the underdrain (e.g., <0.6 m), particularly at finer temporal resolutions (5 min to 1 hr), which correspond to short‐term responses of LID practices and their corresponding effects on flooding. It was possible that the local groundwater mounds formed by infiltrated water instead of regional groundwater fluctuations that affected the underdrain flow in these conditions. Finally, through an evolutionary polynomial regression, two simple equations were obtained to predict the total amount and peak of LID underdrain flow using event‐based rainfall, groundwater table depth, and the hydraulic conductivity of media and in situ soils. The results may facilitate LID design and planning, and their implementation in shallow groundwater areas. The derived equations may benefit the real‐time control of LID‐related sewer and treatment systems in shallow groundwater urban catchments.

Web-based tool compilation of analytical equations for groundwater management applications

The INOWAS platform provides a compilation of free web-based tools for groundwater management. All tools are running on a web server and can be accessed via standard web browsers. The implemented analytical equations enable the assessment of saltwater intrusion induced by pumping or sea level rise, the calculation of travel time through unconfined aquifers and the evaluation of pumping-induced river drawdown. The groundwater mounding calculator can be used to estimate the rise of groundwater levels underneath infiltration basins. To determine the contaminant concentration downgradient of a constant source, an analytical tool solving the advection-dispersion equation can be utilized. All tools are incorporated into a decision support environment. The user is provided with detailed online support that contains the theoretical background of the tools, possible applications and examples.

Modelling hydrological response to a fully‐monitored urban bioretention cell

AbstractMunicipalities and agencies use green infrastructure to combat pollution and hydrological impacts (e.g., flooding) related to excess stormwater. Bioretention cells are one type of infiltration green infrastructure intervention that infiltrate and redistribute otherwise uncontrolled stormwater volume. However, the effects of these installations on the rest of the local water cycle is understudied; in particular, impacts on stormwater return flows and groundwater levels are not fully understood. In this study, full water cycle monitoring data were used to construct and calibrate a two‐dimensional Richards equation model (HYDRUS‐2D/3D) detailing hydrological implications of an unlined bioretention cell (Cleveland, Ohio) that accepts direct runoff from surrounding impervious surfaces. Using both preinstallation and postinstallation data, the model was used to (a) establish a mass balance to determine reduction in stormwater return flow, (b) evaluate green infrastructure effects on subsurface water dynamics, and (c) determine model sensitivity to measured soil properties. Comparisons of modelled versus observed data indicated that the model captured many hydrological aspects of the bioretention cell, including subsurface storage and transient groundwater mounding. Model outputs suggested that the bioretention cell reduced stormwater return flows into the local sewer collection system, though the extent of this benefit was attenuated during high inflow events that may have exhausted detention capacity. The model also demonstrated how, prior to bioretention cell installation, surface and subsurface hydrology were largely decoupled, whereas after installation, exfiltration from the bioretention cell activated a new groundwater dynamic. Still, the extent of groundwater mounding from the cell was limited in spatial extent and did not threaten other subsurface infrastructure. Finally, the sensitivity analysis demonstrated that the overall hydrological response was regulated by the hydraulics of the bioretention cell fill material, which controlled water entry into the system, and by the water retention parameters of the native soil, which controlled connectivity between the surface and groundwater.

Feedbacks Between Shallow Groundwater Dynamics and Surface Topography on Runoff Generation in Flat Fields

AbstractIn winter, saturation excess (SE) ponding is observed regularly in temperate lowland regions. Surface runoff dynamics are controlled by small topographical features that are unaccounted for in hydrological models. To better understand storage and routing effects of small‐scale topography and their interaction with shallow groundwater under SE conditions, we developed a model of reduced complexity to investigate SE runoff generation, emphasizing feedbacks between shallow groundwater dynamics and mesotopography. The dynamic specific yield affected unsaturated zone water storage, causing rapid switches between negative and positive head and a flatter groundwater mound than predicted by analytical agrohydrological models. Accordingly, saturated areas were larger and local groundwater fluxes smaller than predicted, leading to surface runoff generation. Mesotopographic features routed water over larger distances, providing a feedback mechanism that amplified changes to the shape of the groundwater mound. This in turn enhanced runoff generation, but whether it also resulted in runoff events depended on the geometry and location of the depressions. Whereas conditions favorable to runoff generation may abound during winter, these feedbacks profoundly reduce the predictability of SE runoff: statistically identical rainfall series may result in completely different runoff generation. The model results indicate that waterlogged areas in any given rainfall event are larger than those predicted by current analytical groundwater models used for drainage design. This change in the groundwater mound extent has implications for crop growth and damage assessments.

Evaluating hydrologic performance of bioretention cells in shallow groundwater

AbstractBioretention cells, which are generally effective in controlling surface runoff and recharging groundwater, have been widely adopted as low impact development practices. However, shallow groundwater has limited their implementation in some locations due to the potential problems of a reduction in surface runoff control, groundwater pollution, and continuous groundwater drainage through the underdrain. Many guidelines have established minimum requirements for the groundwater depth below bioretention cells, but they may not be optimized for certain environmental conditions and bioretention cell designs. This study made use of a variably saturated flow model to examine the hydrologic performance of a single bioretention cell in shallow groundwater with event‐based simulations, considering a wide range of initial groundwater depths, media and in situ soil types, surface runoff loads, and underdrain sizes. Performance indicators (e.g., runoff reduction, time for infiltrated water to reach the bioretention cell bottom and the groundwater table, and height and dissipation time of groundwater mound) were evaluated to examine the processes of runoff generation, the formation and dissipation of groundwater mounds, and the bioretention cell's performance in a shallow groundwater environment. The most influential factors were the initial groundwater depth, the hydraulic conductivity of the media soil, and the rainfall runoff load. With a deeper initial groundwater table, infiltrated water took longer to reach the bioretention cell bottom and groundwater table. Groundwater mounds, however, took longer to dissipate even though they were smaller. The groundwater quality can be better protected if relatively less‐permeable soil types (e.g., sandy loam) are used as the media, although it may compromise the performance in runoff quantity control. However, only very high surface runoff loads would cause concerns regarding a reduction in runoff quantity control and possible groundwater contamination due to the shallow groundwater. A distance of 1.5–3 m between the bioretention cell bottom and the groundwater table is generally sufficient. The results of this study could help to guide the planning and design of bioretention cells in areas of shallow groundwater.

Data-Driven Approach for Analyzing Hydrogeology and Groundwater Quality Across Multiple Scales.

Recent trends of assimilating water well records into statewide databases provide a new opportunity for evaluating spatial dynamics of groundwater quality and quantity. However, these datasets are scarcely rigorously analyzed to address larger scientific problems because they are of lower quality and massive. We develop an approach for utilizing well databases to analyze physical and geochemical aspects of groundwater systems, and apply it to a multiscale investigation of the sources and dynamics of chloride (Cl- ) in the near-surface groundwater of the Lower Peninsula of Michigan. Nearly 500,000 static water levels (SWLs) were critically evaluated, extracted, and analyzed to delineate long-term, average groundwater flow patterns using a nonstationary kriging technique at the basin-scale (i.e., across the entire peninsula). Two regions identified as major basin-scale discharge zones-the Michigan and Saginaw Lowlands-were further analyzed with regional- and local-scale SWL models. Groundwater valleys ("discharge" zones) and mounds ("recharge" zones) were identified for all models, and the proportions of wells with elevated Cl- concentrations in each zone were calculated, visualized, and compared. Concentrations in discharge zones, where groundwater is expected to flow primarily upwards, are consistently and significantly higher than those in recharge zones. A synoptic sampling campaign in the Michigan Lowlands revealed concentrations generally increase with depth, a trend noted in previous studies of the Saginaw Lowlands. These strong, consistent SWL and Cl- distribution patterns across multiple scales suggest that a deep source (i.e., Michigan brines) is the primary cause for the elevated chloride concentrations observed in discharge areas across the peninsula.

Prediction of Groundwater Quality Trends Resulting from Anthropogenic Changes in Southeast Florida.

The effects of surface water flow system changes caused by constructing water-conservation areas and canals in southeast Florida on groundwater quality under the Atlantic Coastal Ridge was investigated with numerical modeling. Water quality data were used to delineate a zone of groundwater with low total dissolved solids (TDS) within the Biscayne aquifer under the ridge. The delineated zone has the following characteristics. Its location generally coincides with an area where the Biscayne aquifer has high transmissivities, corresponds to a high recharge area of the ridge, and underlies a part of the groundwater mound formed under the ridge prior to completion of the canals. This low TDS groundwater appears to be the result of pre-development conditions rather than seepage from the canals constructed after the 1950s. Numerical simulation results indicate that the time for low TDS groundwater under the ridge to reach equilibrium with high TDS surface water in the water-conservation areas and Everglades National Park are approximately 70 and 60 years, respectively. The high TDS groundwater would be restricted to the water-conservation areas and the park due to its slow eastward movement caused by small hydraulic gradients in Rocky Glades and its mixing with the low TDS groundwater under the high-recharge area of the ridge. The flow or physical boundary conditions such as high recharge rates or low hydraulic conductivity layers may affect how the spatial distribution of groundwater quality in an aquifer will change when a groundwater flow system reaches equilibrium with an associated surface water flow system.