Coupled Groundwater Research Articles

Modeling surface water-groundwater interaction in arid and semi-arid regions with intensive agriculture

In semi-arid and arid areas with intensive agriculture, surface water-groundwater (SW-GW) interaction and agricultural water use are two critical and closely interrelated hydrological processes. However, the impact of agricultural water use on the hydrologic cycle has been rarely explored by integrated SW-GW modeling, especially in large basins. This study coupled the Storm Water Management Model (SWMM), which is able to simulate highly engineered flow systems, with the Coupled Ground-Water and Surface-Water Flow Model (GSFLOW). The new model was applied to study the hydrologic cycle of the Zhangye Basin, northwest China, a typical arid to semi-arid area with significant irrigation. After the successful calibration, the model produced a holistic view of the hydrological cycle impact by the agricultural water use, and generated insights into the spatial and temporal patterns of the SW-GW interaction in the study area. Different water resources management scenarios were also evaluated via the modeling. The results showed that if the irrigation demand continuous to increase, the current management strategy would lead to acceleration of the groundwater depletion, and therefore introduce ecological problems to this basin. Overall, this study demonstrated the applicability of the new model and its value to the water resources management in arid and semi-arid areas.

Hydrology of the Forest City basin, Mid-Continent, USA: implications for CO2 sequestration in the St. Peter Sandstone

The central United States contain numerous coal-burning electric power plants that emit large amounts of CO2 for which a nearby suitable disposal site would be desirable. The results of a reconnaissance study reported here indicate that the St. Peter Sandstone in the Forest City basin has potential large-scale CO2 sequestration capacity, despite the basin’s overall relatively shallow depth. A three-dimensional stratigraphic model of the basin constructed from published well logs reveals that about 59 % of the St. Peter in the basin lies below a depth of 750 m, where CO2 would exist in a supercritical state for increased storage efficiency. Based on a porosity of 15 % and a storage efficiency factor of 5.4 %, the St. Peter in the basin could potentially sequester upto ~830 M tonnes of CO2, equating to upto 23,500 tonnes of CO2 per km2. A three-dimensional hydrologic model coupling groundwater flow, heat transport, and salinity transport provided insights into how CO2 might be transported in the basin. The results show a consistent pattern of predominantly southwestward groundwater flow in the northern part of the basin and predominantly eastward flow in the southern part of the basin. Groundwater average linear velocities are known with much less confidence, but in the St. Peter are mostly under 10 m/year and lack a strong vertical component. Mineralogical analysis shows the St. Peter to be composed almost entirely of quartz, suggesting that the St. Peter would be resistant to dissolution by CO2-enriched fluids and accompanying increases in porosity and permeability.

Advance in Coupled Surface Water and Groundwater Models

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Coupling groundwater modeling and biological indicators for identifying river/aquifer exchanges

Future climate changes and the resulting modifications in anthropogenic activities will alter the interactions between rivers and groundwater. The quantification of these hydraulic interactions is absolutely necessary for achieving sustainable water use and requires accurate analytical methodologies. This report proposes an interdisciplinary approach to the quantitative and qualitative characterization of hydraulic interactions between rivers and shallow aquifers, wherein it outlines the advantages of coupling groundwater modeling with biological markers. As a first step, we built independent diagnostic maps of hydrological exchanges at the sector scale on the basis of hydrogeological modeling and biological indicators. In a second step, these maps were compared to provide a quantitative and qualitative understanding of exchanges between groundwater and surface water. This comparison significantly improved the calibration of groundwater models through a better assessment of boundary zones. Our approach enabled us to identify the conditions under which it could be possible to use biological indicators instead of a large set of piezometric measures. The integration of such combined tools in a future decision support system will assist governmental authorities in proposing appropriate long-term water policies for the preservation of groundwater resources, such as for supplying potable water and/or mitigating pollution risks.Electronic supplementary materialThe online version of this article (doi:10.1186/2193-1801-3-68) contains supplementary material, which is available to authorized users.

Channel Representation in Physically Based Models Coupling Groundwater and Surface Water: Pitfalls and How to Avoid Them

Recent models that couple three-dimensional subsurface flow with two-dimensional overland flow are valuable tools for quantifying complex groundwater/stream interactions and for evaluating their influence on watershed processes. For the modeler who is used to defining streams as a boundary condition, the representation of channels in integrated models raises a number of conceptual and technical issues. These models are far more sensitive to channel topography than conventional groundwater models. On all spatial scales, both the topography of a channel and its connection with the floodplain are important. For example, the geometry of river banks influences bank storage and overbank flooding; the slope of the river is a primary control on the behavior of a catchment; and at the finer scale bedform characteristics affect hyporheic exchange. Accurate data on streambed topography, however, are seldom available, and the spatial resolution of digital elevation models is typically too coarse in river environments, resulting in unrealistic or undulating streambeds. Modelers therefore perform some kind of manual yet often cumbersome correction to the available topography. In this context, the paper identifies some common pitfalls, and provides guidance to overcome these. Both aspects of topographic representation and mesh discretization are addressed. Additionally, two tutorials are provided to illustrate: (1) the interpolation of channel cross-sectional data and (2) the refinement of a mesh along a stream in areas of high topographic variability.

Sequential and joint hydrogeophysical inversion using a field-scale groundwater model with ERT and TDEM data

Abstract. Increasingly, ground-based and airborne geophysical data sets are used to inform groundwater models. Recent research focuses on establishing coupling relationships between geophysical and groundwater parameters. To fully exploit such information, this paper presents and compares different hydrogeophysical inversion approaches to inform a field-scale groundwater model with time domain electromagnetic (TDEM) and electrical resistivity tomography (ERT) data. In a sequential hydrogeophysical inversion (SHI) a groundwater model is calibrated with geophysical data by coupling groundwater model parameters with the inverted geophysical models. We subsequently compare the SHI with a joint hydrogeophysical inversion (JHI). In the JHI, a geophysical model is simultaneously inverted with a groundwater model by coupling the groundwater and geophysical parameters to explicitly account for an established petrophysical relationship and its accuracy. Simulations for a synthetic groundwater model and TDEM data showed improved estimates for groundwater model parameters that were coupled to relatively well-resolved geophysical parameters when employing a high-quality petrophysical relationship. Compared to a SHI these improvements were insignificant and geophysical parameter estimates became slightly worse. When employing a low-quality petrophysical relationship, groundwater model parameters improved less for both the SHI and JHI, where the SHI performed relatively better. When comparing a SHI and JHI for a real-world groundwater model and ERT data, differences in parameter estimates were small. For both cases investigated in this paper, the SHI seems favorable, taking into account parameter error, data fit and the complexity of implementing a JHI in combination with its larger computational burden.

Coupled groundwater flow and heat transport simulation for estimating transient aquifer–stream exchange at the lowland River Spree (Germany)

AbstractSubsurface flow and heat transport near Freienbrink, NE Germany, was simulated in order to study groundwater–surface water exchange between a floodplains aquifer and a section of the lowland River Spree and an adjacent oxbow. Groundwater exfiltration was the dominant process, and only fast surface water level rises resulted in temporary infiltration into the aquifer. The main groundwater flow paths are identified based on a 3D groundwater flow model. To estimate mass fluxes across the aquifer–surface water interfaces, a 2D flow and heat transport modelling approach along a transect of 12 piezometers was performed. Results of steady‐state and transient water level simulations show an overall high accuracy with a Spearman coefficient ρ = 0.9996 and root mean square error (RMSE) = 0.008 m. Based on small groundwater flow velocities of about 10−7 to 10−6 ms−1, mean groundwater exfiltration rates of 233 l m−2 d−1 are calculated. Short periods of surface water infiltration into the aquifer do not exceed 10 days, and the infiltration rates are in the same range. The heat transport was modelled with slightly less accuracy (ρ = 0.8359 and RMSE = 0.34 °C). In contrast to the predominant groundwater exfiltration, surface water temperatures determine the calculated temperatures in the upper aquifer below both surface water bodies down to 10 m during the whole simulation period. These findings emphasize prevailing of heat conduction over advection in the upper aquifer zones, which seems to be typical for lowland streams with sandy aquifer materials and low hydraulic gradients. Moreover, this study shows the potential of coupled numerical flow and heat transport modelling to understand groundwater–surface water exchange processes in detail. Copyright © 2013 John Wiley & Sons, Ltd.

Modelling the interaction of surface and subsurface water flow by linking TOPMODEL and MODFLOW

A coupled ground water–surface water model was used to quantify the hydrological processes in a lowland watershed. The model was constructed by coupling TOPMODEL and MODFLOW without writing new code for the integrated model. Meteorological, land use and soil type data for the Tenmile watershed, USA, were used to run TOPMODEL, which partitions precipitation into evapotranspiration, runoff and recharge. The ground water model, MODFLOW, is driven by recharge derived from TOPMODEL and provides water table elevation and baseflow. The coupled model was calibrated and validated with stream discharge and ground water head data for the period 1968 to 1972. Nash-Sutcliffe model efficiency (E) and correlation coefficient (r) were used to evaluate model behaviour. The model demonstrated good performance, with average E and r values greater than 0.7 for stream flow and ground water head during calibration and validation. Despite obvious challenges in harmonising the different spatial and temporal discretisation in surface water and ground water models, the results from this study demonstrate that the coupled model is capable of simulating hydrological processes in the study area and that with further improvements, it can be a useful tool for integrated management of surface water and ground water resources.

Analyzing Low Impact Development Strategies Using Continuous Fully Distributed Coupled Groundwater and Surface Water Models

In recent years, Low-Impact Development (LID) strategies have received more attention due to the requirement to reduce the adverse hydrologic and water quality…

Ecohydrological evolution model on riparian vegetation in hyperarid regions and its validation in the lower reach of Tarim River

AbstractThe evolution of the ecohydrological system driven by external climatic forcing and internal feedbacks between vegetation and hydrology, which is more remarkable in arid and semiarid regions, has attracted substantial research attention in recent years. To examine critically the state‐of‐the‐art assumptions and dynamic equations used in the evolution study of an ecohydrological system, the rule of proceeding from simplicity to complexity should be followed. The riparian vegetation ecohydrological system in hyperarid regions (e.g. the lower Tarim River) can serve as a starting point given its simplicity, which has been seldom examined before in terms of system evolution. Further, the water transfer practice from 2000 to 2006 in the lower Tarim River serves as a valuable prototype experiment for model validation. This is because the remarkable changes in groundwater and vegetation in the area have taken place within a shorter period and thus can be easily observed. In the present study, the ecohydrological evolution model on riparian vegetation (ERV model) in hyperarid regions was proposed by coupling groundwater movement and vegetation dynamics. In the ERV model, the groundwater table serves as a critical feedback variable that determines the vegetation dynamics (colonization and mortality) and is determined by vegetation transpiration other than groundwater movement. The monitored groundwater table by wells and satellite‐observed vegetation coverage from the Moderate Resolution Imaging Spectroradiometer are used for model validation. The simulation results show the good performance of the ERV model with uncalibrated parameters. It was also calibrated manually using a multiobjective method, and the fine‐tuned parameters are close to the uncalibrated ones, indicating the robustness of the model. The analysis shows further that the increased evapotranspiration is substantially due to the water transfer and thus the vegetation growth, which implies the importance of ecohydrological coupling for long‐term hydrological modelling. Copyright © 2011 John Wiley & Sons, Ltd.

Development of a Coupled Groundwater–Atmosphere Model

Abstract Complete models of the hydrologic cycle have gained recent attention as research has shown interdependence between the coupled land and energy balance of the subsurface, land surface, and lower atmosphere. PF.WRF is a new model that is a combination of the Weather Research and Forecasting (WRF) atmospheric model and a parallel hydrology model (ParFlow) that fully integrates three-dimensional, variably saturated subsurface flow with overland flow. These models are coupled in an explicit, operator-splitting manner via the Noah land surface model (LSM). Here, the coupled model formulation and equations are presented and a balance of water between the subsurface, land surface, and atmosphere is verified. The improvement in important physical processes afforded by the coupled model using a number of semi-idealized simulations over the Little Washita watershed in the southern Great Plains is demonstrated. These simulations are initialized with a set of offline spinups to achieve a balanced state of initial conditions. To quantify the significance of subsurface physics, compared with other physical processes calculated in WRF, these simulations are carried out with two different surface spinups and three different microphysics parameterizations in WRF. These simulations illustrate enhancements to coupled model physics for two applications: water resources and wind-energy forecasting. For the water resources example, it is demonstrated how PF.WRF simulates explicit rainfall and water storage within the basin and runoff. Then the hydrographs predicted by different microphysics schemes within WRF are compared. Because soil moisture is expected to impact boundary layer winds, the applicability of the model to wind-energy applications is demonstrated by using PF.WRF and WRF simulations to provide estimates of wind and wind shear that are useful indicators of wind-power output.

Coupling groundwater and land surface processes: Idealized simulations to identify effects of terrain and subsurface heterogeneity on land surface energy fluxes

This work investigates the role of terrain and subsurface heterogeneity on the interactions between groundwater dynamics and land surface energy fluxes using idealized simulations. A three‐dimensional variably saturated groundwater code (ParFlow) coupled to a land surface model (Common Land Model) is used to account for both vertical and lateral water and pressure movement. This creates a fully integrated approach, coupling overland and subsurface flow while having an explicit representation of the water table and all land surface processes forced by atmospheric data. Because the water table is explicitly represented in these simulations, regions with stronger interaction between water table depth and the land surface energy balance (known as critical zones) can be identified. This study uses simple terrain and geologic configurations to demonstrate the importance of lateral surface and subsurface flows in determining land surface heat and moisture fluxes. Strong correlations are found between the land surface fluxes and water table depth across all cases, including terrain shape, subsurface heterogeneity, vegetation type, and climatological region. Results show that different land forms and subsurface heterogeneities produce very different water table dynamics and land surface flux responses to atmospheric forcing. Subsurface formation and properties have the greatest effect on the coupling between the water table and surface heat and moisture fluxes. Changes in landform and land surface slope also have an effect on these interactions by influencing the fraction of rainfall contributing to overland flow versus infiltration. This directly affects the extent of the critical zone with highest coupling strength along the hillside. Vegetative land cover, as seen in these simulations, has a large effect on the energy balance at the land surface but a small effect on streamflow and water table dynamics and thus a limited impact on the land surface‐subsurface interactions. Although climate forcing has a direct effect on water table dynamics and feedbacks to the land surface, in this study it does not overcome that of subsurface heterogeneity and terrain.

书评:“地下水与地质过程; 第二版” by Steven E. Ingebritsen, Ward E. Sanford, and Christopher E. Neuzil

Although the role of groundwater in various disparate geological fields has been studied for at least a century, it is only more recently that a more unified approach to the study of hydrogeology in large-scale geologic systems has emerged. Reasons for this shift include the advent of more transdisciplinary and interconnected theories within the earth sciences, the development of improved numerical models (and computational power) for studying larger scale threedimensional problems, and advances in our theoretical and measurement abilities. The book Groundwater in Geologic Processes by Ingebritsen and Sanford (1998) played a key role in creating a unified view of groundwater in earth systems by developing a rigorous technical and theoretical framework for studying groundwater processes at large scales. The recently published Groundwater in Geologic Processes, 2nd edn. by Ingebritsen et al. (2006) further solidifies this book as an important teaching and reference tool in the research field. The book contains substantial new material and is much more than simply an update to the first edition. There is a new coauthor, Christopher Neuzil, and completely new chapters on compaction and diagenesis, metamorphism, and sub-sea hydrogeology. Additionally there are expanded discussions of the coupling of groundwater flow and deformation/compaction (i.e., hydromechanics). The underlying theme of the book is the concept of coupled-flow, or to quote the authors, “everything is coupled.” For most shallow or low-temperature applications one assumes that groundwater has a constant density and viscosity, thereby simplifying much of the mathematics used to describe groundwater flow. In contrast, the flow of groundwater in a larger geologic setting requires a more comprehensive mathematical treatment as the movement of pore water is inextricably linked, or coupled, to properties of the water itself such as density and viscosity, and flow is driven not only by hydraulic pressure but also potentially by density variations resulting from thermal or solute gradients and/or by compaction. Complicating this situation is the fact that although these gradients drive groundwater flow, their distribution is also controlled somewhat by groundwater flow, hence the concept of coupled flow. This situation is, in many ways, the primary driver and underpinning of the theory presented in the book. The book is divided into two sections with a total of 13 chapters. The first section of the book focuses on the theory and development of mathematical formulations of coupled groundwater flow, starting with a primer on more basic hydrogeology, though cast at a crustal scale. The authors then develop mathematical formulations for coupling groundwater flow with individual ‘couplings’, including solute transport, heat transport, regional-scale flow and transport, and hydromechanical stresses. Each coupling is treated individually, although obviously in real-world settings these couplings can occur simultaneously (e.g., heat and solute transport). The second section of the book presents the application of coupled flow in specific geologic settings. The settings include ore deposits (with a focus primarily on Mississippi Valley Type deposits), hydrocarbon maturation (i.e., optimal thermal conditions) and migration, geothermal processes (crustal heat flux, hydrogeology and magmatic bodies, and geysers), groundwater and earthquakes, and sub-sea hydrogeology. The sub-sea chapter was very interesting, and a nice addition to the 2nd edn of the book. The authors also discuss the role of groundwater in the formation and dissolution of evaporites and in metamorphism and diagenesis in sedimentary-basin environments. The book is ideal for an upper-level or graduate course in hydrogeology. For an undergraduate or first course in hydrogeology the book likely would be too advanced, though some instructors have used the book as supplemental class reading. The book includes four or five questions at the end of each chapter, and answer keys (restricted to instructors) for all of the questions are Received: 15 February 2008 /Accepted: 30 April 2008 Published online: 15 July 2008

Atacamite formation by deep saline waters in copper deposits from the Atacama Desert, Chile: evidence from fluid inclusions, groundwater geochemistry, TEM, and 36Cl data

The presence of large amounts of atacamite in oxide zones from ore deposits in the Atacama Desert of northern Chile requires saline solutions for its formation and hyperarid climate conditions for its preservation. We investigated the nature and origin of atacamite-forming solutions by means of coupling groundwater geochemical analyses with fluid inclusion data, high-resolution mineralogical observations, and chlorine-36 (36Cl) data in atacamite from the Mantos Blancos and Spence Cu deposits. In both deposits, the salinities of fluid inclusions in atacamite are comparable to those measured in saline groundwaters sampled from drill holes. The average salinity of fluid inclusions in atacamite for the Mantos Blancos and Spence deposits (~7–9 and 2–3 wt.% NaCleq, respectively) are strongly correlated to the salinities at which gypsum supersaturates from groundwaters in both deposits (total dissolved solids ~5–9 and 1–3 wt.% NaCleq, respectively). This correlation is confirmed by transmission electron microscopy observations of atacamite-bearing samples, revealing an intimate association between atacamite and gypsum that can be traced down to the nanometer scale. 36Cl data in atacamite provide new lines of evidence concerning the origin and age of the saline waters that formed atacamite in various stratabound and porphyry Cu deposits from the Atacama Desert. All atacamite samples show very low 36Cl-to-Cl ratios (11 × 10−15 to 28 × 10−15 at at−1), comparable to previously reported 36Cl-to-Cl ratios of deep formation waters and old groundwaters. In addition, 36Cl-to-Cl ratios in atacamite correlate with U and Th concentration in the host rocks but are independent from distance to the ocean. This trend supports an interpretation of the low 36Cl-to-Cl ratios in atacamite as representing subsurface production of fissiogenic 36Cl in secular equilibrium with the solutions involved in atacamite formation. Therefore, 36Cl in atacamite strongly suggest that the chlorine in saline waters related to atacamite formation is old (>1.5 Ma) but that atacamite formation occurred more recently (<1.5 Ma) than suggested in previous interpretations. Our data provide new constraints on the origin of atacamite in Cu deposits from the Atacama Desert and support the recent notion that the formation of atacamite in hyperarid climates such as the Atacama Desert is an ongoing process that has occurred intermittently since the onset of hyperaridity.

Common problematic aspects of coupling hydrological models with groundwater flow models on the river catchment scale

Abstract. Model coupling requires a thorough conceptualisation of the coupling strategy, including an exact definition of the individual model domains, the "transboundary" processes and the exchange parameters. It is shown here that in the case of coupling groundwater flow and hydrological models – in particular on the regional scale – it is very important to find a common definition and scale-appropriate process description of groundwater recharge and baseflow (or "groundwater runoff/discharge") in order to achieve a meaningful representation of the processes that link the unsaturated and saturated zones and the river network. As such, integration by means of coupling established disciplinary models is problematic given that in such models, processes are defined from a purpose-oriented, disciplinary perspective and are therefore not necessarily consistent with definitions of the same process in the model concepts of other disciplines. This article contains a general introduction to the requirements and challenges of model coupling in Integrated Water Resources Management including a definition of the most relevant technical terms, a short description of the commonly used approach of model coupling and finally a detailed consideration of the role of groundwater recharge and baseflow in coupling groundwater models with hydrological models. The conclusions summarize the most relevant problems rather than giving practical solutions. This paper aims to point out that working on a large scale in an integrated context requires rethinking traditional disciplinary workflows and encouraging communication between the different disciplines involved. It is worth noting that the aspects discussed here are mainly viewed from a groundwater perspective, which reflects the author's background.

Heat as a Ground Water Tracer

Heat carried by ground water serves as a tracer to identify surface water infiltration, flow through fractures, and flow patterns in ground water basins. Temperature measurements can be analyzed for recharge and discharge rates, the effects of surface warming, interchange with surface water, hydraulic conductivity of streambed sediments, and basin-scale permeability. Temperature data are also used in formal solutions of the inverse problem to estimate ground water flow and hydraulic conductivity. The fundamentals of using heat as a ground water tracer were published in the 1960s, but recent work has significantly expanded the application to a variety of hydrogeological settings. In recent work, temperature is used to delineate flows in the hyporheic zone, estimate submarine ground water discharge and depth to the salt-water interface, and in parameter estimation with coupled ground water and heat-flow models. While short reviews of selected work on heat as a ground water tracer can be found in a number of research papers, there is no critical synthesis of the larger body of work found in the hydrogeological literature. The purpose of this review paper is to fill that void and to show that ground water temperature data and associated analytical tools are currently underused and have not yet realized their full potential.

TRANSPORT ANALYSIS OF MAJOR IONS OF SEAWATER IN HUMIC SOIL WITH CATION EXCHANGE AND BIOCHEMICAL REDUCTION REACTIONS

Groundwater quality is affected by the various reactions such as precipitation of dissolved ions, solid-liquid phase interaction, and the bacteria mediated reduction-oxidation process. The geochemical properties in coastal aquifers are unique and important for the management of groundwater environment. Although the studies analyzing the hydrological behaviors have been intensively done up to now, a study of coupling both groundwater flow and geochemical analysis is limited. In this study, the one-dimensional multicomponent solute transport model which accounts for the cation exchange reaction and the reduction reaction induced by anaerobic degradation is studied. It is found out through both the experiment and numerical simulation that the cation exchange process is significant at the frontal part of the seawater infiltration, while reducing process by the bacteria mediation takes places at the entire soil column except for the top ofthe soil where dissolved oxygen infiltrates.

Groundwater flow and solute transport in fracture zones: an improved model for a large-scale field experiment at Äspö (Sweden)

157-172 Groundwater flow and solute transport in fracture zones: an improved model for a large-scale field experiment at Äspö (Sweden) Jorge Molinero E.T.S. Ingenieros de Caminos Canales y Puertos, Campus de Elvina Universidadde A Coruña 15192 A Coruña Spain E-mail: molinero@iccp.udc.es Javier Samper E.T.S. Ingenieros de Caminos Canales y Puertos, Campus de Elvina Universidad de A Coruna, 15192 A Coruna, Spain. E-mail: jsc@iccp.udc.es Several countries around the world are considering the final disposal of high-level radioactive waste in deep repositories located in fractured granite formations. Evaluating the long term safety of such repositories requires sound conceptual and numerical models of groundwater flow, solute transport and chemical and radiological processes. These models are being developed from data and knowledge gained from in situ experiments carried out at deep underground laboratories such as that of Äspö, Sweden, constructed in fractured granite. The Redox Zone Experiment is one of such experiments performed at Aspö in order to evaluate the effects of the construction of the access tunnel on the hydrogeological and hydrochemical conditions of a fracture zone intersected by the tunnel. Hydrochemical and isotopic data of this experiment were interpreted by Banwart et al. (Appl. Geochem., 14, 1999, 873) using a mass-balance approach based on a qualitative description of groundwater flow conditions. Such an interpretation, however, is subject to uncertainties related to an oversimplified conceptualization of groundwater flow. Here we present finite element numerical models of groundwater flow and solute transport for this fracture zone. The first model is based on Banwart's conceptual model. It presents noticeable inconsistencies and fails to match simultaneously observed drawdowns and chloride breakthrough curves. To overcome its limitations, a revised flow and transport model is presented which relies directly on available hydrodynamic and transport parameters, is based on the identification of appropriate flow and transport boundary conditions and uses, when needed, solute data extrapolated from nearby fracture zones. A significant quantitative improvement is achieved with the revised model because its results match simultaneously drawdown and chloride data. Other improvements are qualitative and include: ensuring consistency of hydrodynamic and hydrochemical data and avoiding inconsistencies in the hydrodynamic model. These results enable us to conclude that quantitative analyses of hydrochemical data should rely on sound conceptual and numerical flow and transport models which provide an appropriate framework for checking the consistency of hydrodynamic and hydrochemical data. This framework can be subsequently used for coupling groundwater flow, solute transport and chemical processes in the future.

Coupled Groundwater Flow and Transport in Porous Media. A Conservative or Non-conservative Form?

A new formulation for the modeling of density coupled flow and transport in porous media is presented. This formulation is based on the development of the mass balance equation by using the conservative form. The system of equations obtained by coupling the flow and transport equations using a state equation is solved by a combination of the mixed hybrid finite element method (MHFEM) and the discontinuous finite element method (DFEM). The former is applied in order to solve the flow equation and the dispersive part of the transport equation, whilst the latter is used to solve the advective part of the transport equation. Although the advantages of the MHFEM are known (efficiency calculation of velocity field and continuity of fluxes from one element to an adjacent one), its application in a classical development form (volumetric fluxes as unknowns) leads to the non-conservative version of the mass balance equation. The associated matrix of the system of equations obtained by hybridization is positive definite but non-symmetrical. By using a new approach (mass fluxes as unknowns) the conservative form of the continuity equation is preserved and the associated matrix of the system of equations obtained by hybridization becomes symmetrical. When applied to Elder's problem involving a strong density contrast, this new approach, with a lower calculation cost, leads to similar or identical results to those found in the specialized literature. The comparison between the conservative and non-conservative formulations solved with the same MHFEM and DFEM combination emphasizes the rigor and the pertinence of this new approach. Furthermore, we show the existence of a limit refinement defining the stability of the numerical solution for Elder's problem.

Modeling the influence of variable pH on the transport of zinc in a contaminated aquifer using semiempirical surface complexation models

Land disposal of sewage effluent resulted in contamination of a sand and gravel aquifer (Cape Cod, Massachusetts) with zinc (Zn). The distribution of Zn was controlled by pH‐dependent adsorption; the Zn extended 15 m into the 30‐m‐thick sewage plume within approximately 100 m of the source but only 2–4 m into the plume between 100 and 400 m downgradient. A two‐dimensional vertical cross section model coupling groundwater flow with solute transport and equilibrium adsorption is used to simulate the influence of pH on Zn transport. Adsorption is described using semiempirical surface complexation models (SCM) by writing chemical reactions between dissolved Zn and mineral surface sites. SCM parameters were determined in independent laboratory experiments. A 59‐year simulation with a one‐site SCM describes the influence of pH on Zn transport well, with greater mobility at the low pH values near the upper sewage plume boundary than at the higher pH values deeper in the sewage‐contaminated zone. Simulation with a two‐site SCM describes both the sharpness and approximate location of the leading edge of the Zn‐contaminated region. Temporal variations in pH of incoming groundwater can result in large increases in Zn concentration and mobility. The influence of spatial and temporal variability in pH on adsorption and transport of Zn was accomplished much more easily with the semiempirical SCM approach than could be achieved with distribution coefficients or adsorption isotherms.