Numerical Groundwater Flow Model Research Articles

A Bayesian maximum entropy-based methodology for optimal spatiotemporal design of groundwater monitoring networks.

This paper presents a new methodology for analyzing the spatiotemporal variability of water table levels and redesigning a groundwater level monitoring network (GLMN) using the Bayesian Maximum Entropy (BME) technique and a multi-criteria decision-making approach based on ordered weighted averaging (OWA). The spatial sampling is determined using a hexagonal gridding pattern and a new method, which is proposed to assign a removal priority number to each pre-existing station. To design temporal sampling, a new approach is also applied to consider uncertainty caused by lack of information. In this approach, different time lag values are tested by regarding another source of information, which is simulation result of a numerical groundwater flow model. Furthermore, to incorporate the existing uncertainties in available monitoring data, the flexibility of the BME interpolation technique is taken into account in applying soft data and improving the accuracy of the calculations. To examine the methodology, it is applied to the Dehgolan plain in northwestern Iran. Based on the results, a configuration of 33 monitoring stations for a regular hexagonal grid of side length 3600m is proposed, in which the time lag between samples is equal to 5weeks. Since the variance estimation errors of the BME method are almost identical for redesigned and existing networks, the redesigned monitoring network is more cost-effective and efficient than the existing monitoring network with 52 stations and monthly sampling frequency.

Source partitioning of anthropogenic groundwater nitrogen in a mixed-use landscape, Tutuila, American Samoa

This study presents a modeling framework for quantifying human impacts and for partitioning the sources of contamination related to water quality in the mixed-use landscape of a small tropical volcanic island. On Tutuila, the main island of American Samoa, production wells in the most populated region (the Tafuna-Leone Plain) produce most of the island’s drinking water. However, much of this water has been deemed unsafe to drink since 2009. Tutuila has three predominant anthropogenic non-point-groundwater-pollution sources of concern: on-site disposal systems (OSDS), agricultural chemicals, and pig manure. These sources are broadly distributed throughout the landscape and are located near many drinking-water wells. Water quality analyses show a link between elevated levels of total dissolved groundwater nitrogen (TN) and areas with high non-point-source pollution density, suggesting that TN can be used as a tracer of groundwater contamination from these sources. The modeling framework used in this study integrates land-use information, hydrological data, and water quality analyses with nitrogen loading and transport models. The approach utilizes a numerical groundwater flow model, a nitrogen-loading model, and a multi-species contaminant transport model. Nitrogen from each source is modeled as an independent component in order to trace the impact from individual land-use activities. Model results are calibrated and validated with dissolved groundwater TN concentrations and inorganic δ15N values, respectively. Results indicate that OSDS contribute significantly more TN to Tutuila’s aquifers than other sources, and thus should be prioritized in future water-quality management efforts.

Numerical groundwater-flow modeling to evaluate potential effects of pumping and recharge: implications for sustainable groundwater management in the Mahanadi delta region, India

Process-based groundwater models are useful to understand complex aquifer systems and make predictions about their response to hydrological changes. A conceptual model for evaluating responses to environmental changes is presented, considering the hydrogeologic framework, flow processes, aquifer hydraulic properties, boundary conditions, and sources and sinks of the groundwater system. Based on this conceptual model, a quasi-three-dimensional transient groundwater flow model was designed using MODFLOW to simulate the groundwater system of Mahanadi River delta, eastern India. The model was constructed in the context of an upper unconfined aquifer and lower confined aquifer, separated by an aquitard. Hydraulic heads of 13 shallow wells and 11 deep wells were used to calibrate transient groundwater conditions during 1997–2006, followed by validation (2007–2011). The aquifer and aquitard hydraulic properties were obtained by pumping tests and were calibrated along with the rainfall recharge. The statistical and graphical performance indicators suggested a reasonably good simulation of groundwater flow over the study area. Sensitivity analysis revealed that groundwater level is most sensitive to the hydraulic conductivities of both the aquifers, followed by vertical hydraulic conductivity of the confining layer. The calibrated model was then employed to explore groundwater-flow dynamics in response to changes in pumping and recharge conditions. The simulation results indicate that pumping has a substantial effect on the confined aquifer flow regime as compared to the unconfined aquifer. The results and insights from this study have important implications for other regional groundwater modeling studies, especially in multi-layered aquifer systems.

Tools to simulate changes in hydraulic flow systems in complex geologic settings affected by tunnel excavation

Geotechnical problems during and after tunnel construction are often related to groundwater circulation. In tunnelling projects, however, groundwater flow systems are often only partly known. This uncertainty is manifested by the typically scarce hydraulic data that limits the understanding of subsurface hydrogeological processes. In particular, there is a general lack of data documenting groundwater flow changes caused by tunnelling. The present paper presents a concept involving an iterative understanding of subsurface hydrogeological systems influenced by tunnelling. A major challenge of our approach consists of integrating complex geological geometries from a 3D geological model (GOCAD) into a numerical groundwater flow model (COMSOL Multiphysics). The starting point is a 3D geological model representing a regional tectonic system located in the Jura Mountains in Switzerland. This geological model is transferred into regional and local-scale groundwater flow models. Due to the lack of hydrogeological data, a 3D view of geological–hydrogeological systems is often required to respond to groundwater-induced geotechnical problems in tunnelling. Numerical groundwater flow models make it possible to perform sensitivity analysis and to test how boundary conditions and hydraulic property distributions influence calculated groundwater flow regimes. In addition, our approach enables testing the effects of changes of hydraulic regimes due to tunnel excavation at different scales.

Effects of different boundary conditions on the simulation of groundwater flow in a multi-layered coastal aquifer system (Taranto Gulf, southern Italy)

The evaluation of the accuracy or reasonableness of numerical models of groundwater flow is a complex task, due to the uncertainties in hydrodynamic properties and boundary conditions and the scarcity of good-quality field data. To assess model reliability, different calibration techniques are joined to evaluate the effects of different kinds of boundary conditions on the groundwater flow in a coastal multi-layered aquifer in southern Italy. In particular, both direct and indirect approaches for inverse modeling were joined through the calibration of one of the most uncertain parameters, namely the hydraulic conductivity of the karst deep hydrostratigraphic unit. The methodology proposed here, and applied to a real case study, confirmed that the selection of boundary conditions is among the most critical and difficult aspects of the characterization of a groundwater system for conceptual analysis or numerical simulation. The practical tests conducted in this study show that incorrect specification of boundary conditions prevents an acceptable match between the model response to the hydraulic stresses and the behavior of the natural system. Such effects have a negative impact on the applicability of numerical modeling to simulate groundwater dynamics in complex hydrogeological situations. This is particularly important for management of the aquifer system investigated in this work, which represents the only available freshwater resource of the study area, and is threatened by overexploitation and saltwater intrusion.

Inferring hydraulic properties of alpine aquifers from the propagation of diurnal snowmelt signals

AbstractAlpine watersheds source major rivers throughout the world and supply essential water for irrigation, human consumption, and hydroelectricity. Coarse depositional units in alpine watersheds can store and transmit significant volumes of groundwater and thus augment stream discharge during the dry season. These environments are typically data scarce, which has limited the application of physically based models to investigate hydrologic sensitivity to environmental change. This study focuses on a coarse alpine talus unit within the Lake O'Hara watershed in the Canadian Rockies. We investigate processes controlling the hydrologic functioning of the talus unit using field observations and a numerical groundwater flow model driven with a distributed snowmelt model. The model hydraulic parameters are adjusted to investigate how these properties influence the propagation of snowmelt‐induced diurnal signals. The model results expectedly demonstrate that diurnal signals at the talus outlet are progressively damped and lagged with lower hydraulic conductivity and higher specific yield. The simulations further indicate that the lag can be primarily controlled by a higher hydraulic conductivity upper layer, whereas the damping can be strongly influenced by a lower hydraulic conductivity layer along the base of the talus. The simulations specifically suggest that the talus slope can be represented as a two layer system with a high conductivity zone (0.02 m s−1) overlying a 10 cm thick lower conductivity zone (0.002 m s−1). This study demonstrates that diurnal signals can be used to elucidate the hydrologic functioning and hydraulic properties of shallow aquifers and thus aid in the parameterization of hydrological models.

Numerical modelling of groundwater flow to understand the impacts of pumping on arsenic migration in the aquifer of North Bengal Plain

In this paper, numerical simulations of regional-scale groundwater flow of North Bengal Plain have been carried out with special emphasis on the arsenic (As)-rich alluvium filled gap between the Rajmahal hills on the west and the Garo hills on the east. The proposed concern of this modelling arose from development that has led to large water table declines in the urban area of English Bazar block, Malda district, West Bengal and possible transport of As in the near future from the adjacent As-polluted aquifer. Groundwater occurs under unconfined condition in a thick zone of saturation within the Quaternary alluvial sediments. Modelling indicates that current pumping has significantly changed the groundwater flowpaths from pre-development condition. At the present pumping rate, the pumping wells of the urban area may remain uncontaminated till the next 25 yrs, considering only pure advection of water but some water from the As-polluted zone may enter wells by 50 yrs. But geochemical and other processes such as adsorption, precipitation, redox reaction and microbial activity may significantly retard the predicted rate by advective transport. In the rural areas, majority of the water pumped from the aquifer is for irrigation, which is continuously re-applied on the surface. The near-vertical nature of the flowpaths indicates that, where As is present or released at shallow depths, it will continue to occur in pumping wells. Modelling also indicates that placing all the pumping wells at depths below 100 m may not provide As-free water permanently.

Development of a model to simulate groundwater inundation induced by sea-level rise and high tides in Honolulu, Hawaii

Many of the world's largest cities face risk of sea-level rise (SLR) induced flooding owing to their limited elevations and proximities to the coastline. Within this century, global mean sea level is expected to reach magnitudes that will exceed the ground elevation of some built infrastructure. The concurrent rise of coastal groundwater will produce additional sources of inundation resulting from narrowing and loss of the vertical unsaturated subsurface space. This has implications for the dense network of buried and low-lying infrastructure that exists across urban coastal zones.Here, we describe a modeling approach that simulates narrowing of the unsaturated space and groundwater inundation (GWI) generated by SLR-induced lifting of coastal groundwater. The methodology combines terrain modeling, groundwater monitoring, estimation of tidal influence, and numerical groundwater-flow modeling to simulate future flood scenarios considering user-specified tide stages and magnitudes of SLR.We illustrate the value of the methodology by applying it to the heavily urbanized and low-lying Waikiki area of Honolulu, Hawaii. Results indicate that SLR of nearly 1 m generates GWI across 23% of the 13 km2 study area, threatening $5 billion of taxable real estate and 48 km of roadway. Analysis of current conditions reveals that 86% of 259 active cesspool sites in the study area are likely inundated. This suggests that cesspool effluent is currently entering coastal groundwater, which not only leads to degradation of coastal environments, but also presents a future threat to public health as GWI would introduce effluent at the ground surface.

GROUNDWATER FLOW MODELLING OF THE ALLUVIAL AQUIFER IN THE MOURIA AREA, SW GREECE

The objective of this paper is to study the impact of a reflooding of the former Mouria Lake on the hydraulic state of the Pyrgos area. The hydrogeological data acquired through field work were combined with the volumetric budget and the hydrological data from the entire Alfios River catchment in order to build the conceptual and numerical model of the groundwater flow system, which confirmed the hydraulic state before the drainage of the Mouria Lake. The model was also used to predict the future hydraulic state in case stresses change. For this purpose, Flowpath II, a numerical groundwater flow model, was used to evaluate the impacts of groundwater exploitation in the alluvium unconfined aquifer that is developing in the Holocene deposits. Due to the connection of the aquifer with the surface drainage canals near the coastal zone, the conceptual model was built upon irrigation data, rainfall data, and pumping rate data from the pumping stations that drain the area of the former Mouria Lake. These data were inserted in the model which was calibrated using a 24- month set of piezometric measurements. The simulation results show that the groundwater level before the drainage of the lake was 2m higher than the present situation and the same scenario will happen in case of reflooding the drained lake. Today, the pumping stations keep the groundwater level near the sea level throughout the hydrological year.

Challenges of Groundwater Flow Model Calibration Using MODFLOW in Ethiopia: With Particular Emphasis to the Upper Awash River Basin

In this work, most important problems related to model calibration have been assessed using MODFLOW. Particular emphasis is given to the Upper Awash river basin where many boreholes have been drilled for municipal and industrial uses compared with other regions in Ethiopia. Static Water Level (SWL) records from water supply wells drilled for about 32 years in the Upper Awash basin is considered to illustrate the commonly used groundwater flow model calibration procedures and associated problems. The assumptions made in the modeling procedures to use SWL data collected over many years from water supply boreholes to calibrate steady state models is too much of an assumption. Alternatives on steady and pseudo transient model calibration approaches in data scarce areas based on logical assumptions and reasonable representation of groundwater systems has been suggested. Hence, numerical groundwater flow models may play the expected key role for the sustainable groundwater resource management of the country, which is solving practical ground-water related problems.

Groundwater flow in the Venice lagoon and remediation of the Porto Marghera industrial area (Italy)

This study aims to determine the groundwater flow in a large area of the Venice (northeast Italy) lagoon that is under great anthropogenic pressure, which is influencing the regional flow in the surficial aquifer (about 30 m depth). The area presents several elements that condition the groundwater flow: extraction by means of drainage pumps and wells; tidal fluctuation; impermeable barriers that define part of the coastline, rivers and artificial channels; precipitation; recharge, etc. All the elements were studied separately, and then they were brought together in a numerical groundwater flow model to estimate the impact of each one. Identification of the impact of each element will help to optimise the characteristics of the Porto Marghera remediation systems. Longstanding industrial activity has had a strong impact on the soil and groundwater quality, and expensive and complex emergency remediation measures in problematic locations have been undertaken to ensure the continuity of industrial and maritime activities. The land reclamation and remediation works withdraw 56–74% of the water budget, while recharge from the river accounts for about 21–48% of the input. Only 21–42% of groundwater in the modelled area is derived from natural recharge sources, untouched by human activity. The drop of the piezometric level due to the realization of the upgradient impermeable barrier can be counteracted with the reduction of the pumping rate of the remediation systems.

The effects of soil organic matter on soil water retention and plant water use in a meadow of the Sierra Nevada, CA

Tuolumne Meadows is a groundwater dependent ecosystem in the Sierra Nevada of California, USA, that is threatened by hydrologic impacts that may lead to a substantial loss of organic matter in the soil. In order to provide a scientific basis for management of this type of ecosystem, this paper quantifies the effect of soil organic content on soil water retention and water use by plants. First, we show a substantial dependence of soil water retention on soil organic content by correlating Van Genuchten soil water retention parameters with soil organic content, independent of soil texture. Then, we demonstrate the impact of organic content on plants by simulating the degree to which Root Water Uptake (RWU) is affected by soil water retention with the use of a physically-based numerical model of variably-saturated groundwater flow. Our results indicate that the increased water retention by soil organic matter contributes as much as 8.8 cm to transpiration, or 35 additional water-stress free days, during the dry summer when plants experience increased water stress.

Neotectonic intra-plate fault zone mapping and hydrogeology in floodplain sediments: an inter-disciplinary approach

Over the past decade, a relatively rich record of neotectonics has been revealed in continental Australia, however very few investigations into the hydrogeological implications have been undertaken. While the most active intra-plate deformation zones are readily identified by seismicity monitoring and satellite and airborne terrain mapping, advances in airborne electromagnetic (AEM) technology and data optimization have made it possible to map numerous, more subtle ‘blind’ intra-plate fault systems concealed in near-surface floodplain landscapes. To date, fault geometries, displacements, and fault zone properties remain ambiguous due to the combination of AEM footprint resolution, the non-uniqueness of the conductivity models and derived hydrostratigraphy and fault geometry solutions produced by AEM equivalent inversion models, and the inherent uncertainty of stitched 1D AEM inversion models. The resultant uncertainty in fault zone characterisation inhibits investigations into the permeability heterogeneity and anisotropy introduced by these faults, making it difficult to resolve the significance of these structures for groundwater processes. In this study, a novel, inter-disciplinary approach has been developed that helps characterise the hydrogeology of one such intra-plate fault zone in unconsolidated, near-surface floodplain sediments. The approach integrates the mapping of tectonic geomorphology, with mapping of sub-surface hydrostratigraphy and ‘blind’ intra-plate fault systems using AEM. Validation of fault zone geometry and displacement at local scales is provided by ground geophysics (e.g. seismic reflection, resistivity and surface nuclear magnetic resonance (SNMR)) and drilling. Fault zone hydrogeology, including permeability variability, has been assessed through the integration of geophysics with hydrochemistry, hydrodynamics, and studies of vegetation response to water availability (using Landsat time-series analysis). A combination of deterministic and stochastic approaches is then used to unravel complex fault zone conduit-barrier system behaviour that determines lateral and vertical groundwater flow, inter-aquifer leakage and recharge. This inter-disciplinary methodology has been used to parameterise numerical groundwater flow models and target potential groundwater resources.

Gaussian process modelling for uncertainty quantification in convectively-enhanced dissolution processes in porous media

Numerical groundwater flow and dissolution models of physico-chemical processes in deep aquifers are usually subject to uncertainty in one or more of the model input parameters. This uncertainty is propagated through the equations and needs to be quantified and characterised in order to rely on the model outputs. In this paper we present a Gaussian process emulation method as a tool for performing uncertainty quantification in mathematical models for convection and dissolution processes in porous media. One of the advantages of this method is its ability to significantly reduce the computational cost of an uncertainty analysis, while yielding accurate results, compared to classical Monte Carlo methods. We apply the methodology to a model of convectively-enhanced dissolution processes occurring during carbon capture and storage. In this model, the Gaussian process methodology fails due to the presence of multiple branches of solutions emanating from a bifurcation point, i.e., two equilibrium states exist rather than one. To overcome this issue we use a classifier as a precursor to the Gaussian process emulation, after which we are able to successfully perform a full uncertainty analysis in the vicinity of the bifurcation point.

Investigating groundwater flow components in an Alpine relict rock glacier (Austria) using a numerical model

Relict rock glaciers are complex hydrogeological systems that might act as relevant groundwater storages; therefore, the discharge behavior of these alpine landforms needs to be better understood. Hydrogeological and geophysical investigations at a relict rock glacier in the Niedere Tauern Range (Austria) reveal a slow and fast flow component that appear to be related to the heterogeneous structure of the aquifer. A numerical groundwater flow model was used to indicate the influence of important internal structures such as layering, preferential flow paths and aquifer-base topography. Discharge dynamics can be reproduced reasonably by both introducing layers of strongly different hydraulic conductivities or by a network of highly conductive channels within a low-conductivity zone. Moreover, the topography of the aquifer base influences the discharge dynamics, which can be observed particularly in simply structured aquifers. Hydraulic conductivity differences of three orders of magnitude are required to account for the observed discharge behavior: a highly conductive layer and/or channel network controlling the fast and flashy spring responses to recharge events, as opposed to less conductive sediment accumulations sustaining the long-term base flow. The results show that the hydraulic behavior of this relict rock glacier and likely that of others can be adequately represented by two aquifer components. However, the attempt to characterize the two components by inverse modeling results in ambiguity of internal structures when solely discharge data are available.

Rising groundwater levels and impacts in urban and semirural are around Naples (southern Italy)

The rising of groundwater levels in urban and rural areas is a relevant topic of the Urban Hydrogeology because it affects different areas of the world requiring specific analyses and mitigation measures by the institutions involved in the protection and security of the territory.In the last century, in many developed countries processes of deindustrialization and urban transformation have caused the rising of groundwater levels and severe economic and social impacts.The aim of this paper is to analyse the rising of groundwater levels in a multi-layered pyroclastic-alluvial aquifer, located in the coastal plain eastward of Naples (southern Italy) and its interactions with buildings and agricultural lands.The research has been carried out by means of: i) analysis of stratigraphic settings by 86 boreholes and reconstruction of hydraulic conductivity logs; ii) reconstruction of a 2D hydrostratigraphic model of the multi-layered pyroclastic-alluvial aquifer; iii) slug test and multi-temporal hydrogeological monitoring of 83 wells, 9 piezometers and 12 river water levels; iv) analysis of daily rainfall time series; v) hydrogeological monitoring of impacted buildings and agricultural lands.For the monitored period (November 2013-March 2015), the groundwater table showed a generalized rise, although with a spatial variability. The rise of groundwater levels ranged from a minimum of 0.10 meters, in the central and eastern sector of the study area, to a maximum of 2.05 meters, in the south-western sector, with an average growth rate of about 0.16 m.The distribution of the impacted areas is not spatially homogeneous, and it was observed to vary over time. The hydrogeological interactions between groundwater and urban and rural areas are affected by: i) foundation types of buildings and the depth of the underground structures; ii) presence of shallow aquicludes (marshy clay-sands, clay-silts and peat levels) that determine local conditions of confinement for groundwater; iii) hydraulic efficiency and maintenance state of the dense network of drainage channels and micro-channels, which were historically built to drain surface water and groundwater in rural areas.The obtained results provide the basic data to set up a numerical groundwater flow model, which is an indispensable tool to simulate and predict the hydrogeological effects of possible safeguard actions on urban and agricultural areas.

Numerical modeling of regional groundwater flow in the Adda-Ticino Basin: advances and new results

The Adda Ticino basin is a densely-populated area, dependent massively on groundwater resources not only for industrial uses but also for agricultural and domestic ones. A large scale hydrogeological assessment study over 8000 Km2 area northwest of Lombardy Region was done in reason to implement a groundwater flow model. The main objective was to give a quantitative estimate of groundwater budget into the Adda-Ticino basin and to deliver a tool to support Public Authorities in regional groundwater management required by the European Directive 2000/60/CE. The approach consists of improving the hydrogeological conceptual model and better define hydraulic properties of the aquifer units and all components in the basin influencing the groundwater flow (rivers, wells withdrawal). Then the collected GIS information were added into the numerical model Modflow. Regional steady-state flow was calibrated under current stress conditions with an inverse calibration code (Doherty, 2005) mainly by changing hydraulic conductivity and vertical recharge inflow. The water mass balance computed shows a total flow of 3279 Mm3/y. Vertical recharge provides 65% of the total groundwater inflows, whereas the 30% is coming from pre-Alps up-gradient boundary. Discharge from basin aquifers occurs through flow to rivers (60%), groundwater supply wells (30%) and underground outflow at the S-SE border (10%).

Analysis of transient groundwater flow through a stochastic modelling approach

This study demonstrates the application of a stochastic procedure to enhance confidence in the predictions of numerical groundwater flow models. The objective was to assess the hydrogeological conditions and groundwater potential of a semi-confined aquifer system in Legon, Accra. The study applied the parameter randomization procedure in the Groundwater Modelling System (GMS) to evaluate model uncertainties arising from uncertainties in the data of model input parameters. A total of 2500 equally likely solutions of the numerical model were simulated through the Latin hypercube stochastic option. Regarding the size of the terrain and the number of parameters used for calibrating the model, the 2500 model realizations were adjudged to be largely sufficient to represent the possible solutions that could arise from the various permutations of the parameter data. High variances would suggest significant differences in the realizations, indicating high levels of non-uniqueness in the solutions. The 2500 solutions were compared using standard deviations computed from the differences in the estimated horizontal hydraulic conductivity, specific yield and specific storage estimates. The transient model calibrated for this domain suggests very close similarities in the model-computed aquifer parameters for all the 2500 solutions. This suggests that any of the solutions could be used to represent the conditions prevailing in the domain. Risk analyses were then performed on abstractions through all the 2500 solutions. The study shows that the domain is sustained largely by lateral subsurface inflows from adjoining areas amounting to 399,278 m3/day, with direct vertical infiltration and percolation of precipitation accounting for an average of 2450 m3/day. The analysis suggests that at this rate of input, the domain can sustain groundwater abstraction to meet all water needs in the area. In addition, the domain has demonstrated resilience against annual increment in abstraction rates by 5 % over a 20-year period at the current recharge conditions. However, increasing abstraction rates by up to 200 % of the current abstraction rates over the 20-year period (2015–2035) but with a gradual decrease in groundwater recharge by up to 10 % over the period will result in drastic lowering of the hydraulic head especially in the south-central portions of the domain. This situation might induce reversal of the current flow and potentially lead to contamination and/or aquifer dewatering in the domain.

Impacts of physical and chemical aquifer heterogeneity on basin-scale solute transport: Vulnerability of deep groundwater to arsenic contamination in Bangladesh

Aquifer heterogeneity presents a primary challenge in predicting the movement of solutes in groundwater systems. The problem is particularly difficult on very large scales, across which permeability, chemical properties, and pumping rates may vary by many orders of magnitude and data are often sparse. An example is the fluvio-deltaic aquifer system of Bangladesh, where naturally-occurring arsenic (As) exists over tens of thousands of square kilometers in shallow groundwater. Millions of people in As-affected regions rely on deep (≥150m) groundwater as a safe source of drinking water. The sustainability of this resource has been evaluated with models using effective properties appropriate for a basin-scale contamination problem, but the extent to which preferential flow affects the timescale of downward migration of As-contaminated shallow groundwater is unknown. Here we embed detailed, heterogeneous representations of hydraulic conductivity (K), pumping rates, and sorptive properties (Kd) within a basin-scale numerical groundwater flow and solute transport model to evaluate their effects on vulnerability and deviations from simulations with homogeneous representations in two areas with different flow systems. Advective particle tracking shows that heterogeneity in K does not affect average travel times from shallow zones to 150m depth, but the travel times of the fastest 10% of particles decreases by a factor of ∼2. Pumping distributions do not strongly affect travel times if irrigation remains shallow, but increases in the deep pumping rate substantially reduce travel times. Simulation of advective-dispersive transport with sorption shows that deep groundwater is protected from contamination over a sustainable timeframe (>1000y) if the spatial distribution of Kd is uniform. However, if only low-K sediments sorb As, 30% of the aquifer is not protected. Results indicate that sustainable management strategies in the Bengal Basin should consider impacts of both physical and chemical heterogeneity, as well as their correlation. These insights from Bangladesh show that preferential flow strongly influences breakthrough of both conservative and reactive solutes even at large spatial scales, with implications for predicting water supply vulnerability in contaminated heterogeneous aquifers worldwide.

Groundwater Assessment of the Bleone Catchment Karst Aquifer in Southern France

Karst aquifer is an important water resource in southern France. It is the main source for agriculture and for domestic water supply. Hence, it is necessary to assess the quantity and quality of water in the Bleone Catchment. In order to achieve this aim, a groundwater chemistry analysis and regional numerical groundwater flow modelling using MODFLOW were conducted. Groundwater samples from springs and wells analyzed for water quality in the Bléone Catchment demonstrate different water types dominated mostly by fresh water, which is of moderate alkalinity and contains calcium and magnesium as major cations and bicarbonate as a common anion. The saturation indices for calcite and dolomite reveal that dissolution of calcite and dolomite can still take place. In addition, there is a very complex interaction between surface water and groundwater in the catchment.