Postaudit evaluation of conceptual model uncertainty for a glacial aquifer groundwater flow and contaminant transport model

Numerical groundwater flow and contaminant transport modeling incorporating three alternative conceptual models was conducted in 2005 to assess remedial actions and predict contaminant concentrations in an unconfined glacial aquifer located in Milford, Michigan, USA. Three alternative conceptual models were constructed and independently calibrated to evaluate uncertainty in the geometry of an aquitard underlying the aquifer and the extent to which infiltration from two manmade surface water bodies influenced the groundwater flow field. Contaminant transport for benzene, cis-DCE, and MTBE was modeled for a 5-year period that included a 2-year history match from July 2003 to May 2005 and predictions for a 3-year period ending in July 2008. A postaudit of model performance indicates that predictions for pumping wells, which integrated the transport signal across multiple model layers, were reliable but unable to differentiate between alternative conceptual model responses. In contrast, predictions for individual monitoring wells with limited screened intervals were less consistent, but held promise for evaluating alternative hydrogeologic models. Results of this study suggest that model conceptualization can have important practical implications for the delineation of contaminant transport pathways using monitoring wells, but may exert less influence on integrated predictions for pumping wells screened over multiple numerical model layers.     

Variation in time-to-compliance for pump-and-treat remediation of mass transfer-limited aquifers with hydraulic conductivity heterogeneity

In this study, the impact of correlation length (λ) of hydraulic conductivity (K) heterogeneity on pump-and-treat (P&T) remediation period (time-to-compliance) for a mass transfer-limited aquifer is evaluated. Additionally, impacts of variance (σ ²), different distributions of high and low K zones and different initial contaminant masses are explored. Two different P&T policies including different number of wells pumping at different rates are employed for the investigation. Simulation–optimization approach in which a genetic algorithm (GA) is linked with a groundwater flow and contaminant transport model is used. Results show that K heterogeneity, in terms of λ _ln K , s_ln K² \sigma_{\ln \,K}^{2} and respective locations of low and high K zones, significantly impacts the time-to-compliance. Contaminant presence at low K zones can increase the time required to clean up the aquifer. Lower variation is observed in time-to-compliance for the remediation design utilizing higher number of wells pumping at slower rates compared to the design with a single well pumping at a higher rate. Higher number of wells increases the robustness of P&T remediation system when aquifer is heterogeneous in K.     

Aquifer contamination by chlorinated-VOCs: the case of an urban metropolis megasite overlying the Coastal Plain aquifer in Israel

The Tel Aviv metropolitan region (200 km²), located on the sandy and phreatic Mediterranean Coastal Plain aquifer in Israel, is an example of a contaminated megasite that is additionally complicated by being part of a densely populated urban environment. Despite being a major source of fresh potable water, no dedicated aquifer monitoring systems exist. The vast majority of tested water supply wells (82% of 90 wells) were found to be contaminated with one or more chlorinated volatile organic compounds (Cl-VOCs) such as trichloroethene and tetrachloroethene, originating from multiple sources, including former industrial sites. Temporal variations in Cl-VOC concentrations in water supply wells were large and unpredictable. Such aquifer contamination requires immediate action in terms of aquifer management and municipal water distribution. To cope with temporal and spatial variations in contaminant concentrations in water supply wells in an area where monitoring wells are absent, aquifer impact areas were defined based on the concentration of the Cl-VOC contaminant in pumping wells that was greatest in relation to its drinking water standard over a 3-year period. Such a map can be used to define water supply treatment and municipal well monitoring requirements, until an adequate monitoring system is established.     

Groundwater flow and capture zone analysis of the Central Passaic River Basin, New Jersey

Delineating capture zones of pumping wells is an important part of safe drinking water and well protection programs. Capture zones or contributing areas of a groundwater extraction well are the parts of the aquifer recharge areas from which the wells draw their water. Their extent and location depend on the hydrogeologic conditions such as groundwater recharge, pumping scenario and the aquifer properties such as hydraulic conductivity, porosity, heterogeneity of the medium and hydraulic gradient. Different methods of delineation can be used depending on the complexity of the hydrogeologic conditions. In this study, a 3-dimensional transient numerical MODFLOW model was developed for the Central Passaic River Basin (CPRB), and used with a MODPATH particle tracking code to determine 3-dimensional transient capture zones. Analytically calculated capture zones from previous studies at the site were compared with the new numerically simulated capture zones. The study results revealed that the analytical solution was more conservative, estimating larger capture zones than the numerical models. Of all the parameters that can impact the size, shape and location of a capture zone, the hydraulic conductivity is one of the most critical. Capture zones tend to be smaller in lower hydraulic conductivity areas.     

Identifying sources of chlorinated aliphatic hydrocarbons in a residential area in Italy using the integral pumping test method

The results of integral pumping tests (IPTs) performed in the city of Fabriano, Italy, are presented. The IPT methodology was developed by the European Union project INCORE, as a tool for groundwater investigation and source localization in contaminated areas. This methodology consists of a multiple-well pumping test in which the wells are positioned along a control plane downstream of suspected contaminant source zones and perpendicular to the mean groundwater flow direction. During the pumping, concentration time series of target contaminants are measured. In Fabriano, two control planes were realized to identify a chlorinated aliphatic hydrocarbon plume, to estimate the mass fluxes and draw up a ranked list of the main contamination sources. A numerical flow model was implemented to support the IPT design and to interpret the results. This study revealed low-level trichloroethylene contamination (concentration below 8 μg/l), tetrachloroethylene contamination (mean concentration up to 500 μg/l) and a mass flow rate of about 300 g/day. Through the application of the IPT method, the mean contaminant concentrations, the spatial distribution of concentration values along the control planes, and the total contaminant mass flow rates were evaluated, and the investigation area was reduced for further and deeper investigation activities.     

Numerical modeling assessment of three-gate structures for capturing contaminated groundwater

This modeling study evaluated the capability of alternative funnel-and-gate structures with three gates for capturing contaminated groundwater in a hypothetical unconfined aquifer. Simulated interceptor structures were linear and 45 m wide, consisting of three gates and two funnels (walls). One gate occupied the center and two gates occupied the ends of the interceptor structures. The structures, positioned perpendicular to regional groundwater flow, traversed the entire thickness of the aquifer. A total of four structures were evaluated (numbers designate widths of end, center, and end gates, respectively, in meters): 3-3-3, 2-5-2, 1-7-1, and 4-1-4. Particle tracking and zonal water budgets identified shapes of capture zones and discharge patterns for each interceptor structure. A mass transport model, accounting for advection and hydrodynamic dispersion, tested the capability of each structure for capturing a contaminant plume. Results suggest that: time-dependent capture zones underestimate the amount of time to capture a contaminant plume, wide center gates facilitate plume capture, and wide end gates facilitate lateral containment of contaminants. Of the structures simulated, the 2-5-2 configuration was relatively efficient at processing and containing the simulated contaminant plume.     

水力层析法与克立金法估算非均质含水层渗透系数场比较

含水层非均质性空间分布特征的识别,是准确模拟地下水流和污染物运移的前提.基于室内非均质含水层砂箱实验,分别利用水力层析法和克立金插值法刻画了非均质含水层渗透系数场.研究结果表明:(1) 水力层析法与克立金法相比,不仅可以更好地刻画非均质含水层渗透系数场,还可以更高精度地预测地下水流过程;(2) 水力层析抽水实验中,通过增加抽水实验组数可以有效地提高水力层析参数反演的精度,但是抽水实验组数增加到一定程度以后,再增加抽水组数不会显著提升参数反演的效果.后续需要进一步研究水力层析抽水实验合适的组数,进一步对抽水井进行优化布设.      

Dynamics of negative hydraulic barriers to prevent seawater intrusion

Negative hydraulic barriers that intercept inflowing saltwater by pumping near the coast have been proposed as a corrective measure for seawater intrusion in cases where low heads must be maintained. The main disadvantage of these barriers is that they pump a significant proportion of freshwater, leading to contamination with saltwater at the well. To minimize such mixing, a double pumping barrier system with two extraction wells is proposed: an inland well to pump freshwater and a seawards well to pump saltwater. A three-dimensional variable density flow model is used to study the dynamics of the system. The system performs very efficiently as a remediation option in the early stages. Long-term performance requires a well-balanced design. If the pumping rate is high, drawdowns cause saltwater to flow along the aquifer bottom around the seawater well, contaminating the freshwater well. A low pumping rate at the seawards well leads to insufficient desalinization at the freshwater well. A critical pumping rate at the seawater well is defined as that which produces optimal desalinization at the freshwater well. Empirical expressions for the critical pumping rate and salt mass fraction are proposed. Although pumping with partially penetrating wells improves efficiency, the critical pumping rates remain unchanged.     

The dependence of stream depletion by seasonal pumping on various hydraulic characteristics and engineering factors

Compensation pumping is used to alleviate deficiencies in streamflow discharge during dry seasons. Short-term groundwater pumping can use aquifer storage instead of catchment-zone water until the drawdown reaches the edge of the stream. The capacitance is a complex, dimensionless parameter of an aquifer system that defines the delayed effect on streamflow when there is groundwater pumping. This parameter is a function of aquifer hydraulic characteristics, pumping time, and distance between the well and stream edge; the latter can involve stream leakance and vertical leakance of an associated aquitard. Three typical hydraulic cases of combined water systems (major catchment-zone wells close to the stream and compensation pumping wells) were classified depending on their capacitance structure (i.e. the relationship between surface water and groundwater): (1) perfect hydraulic connection between the stream and aquifer; (2) imperfect hydraulic connection between the stream and aquifer; and (3) essentially imperfect hydraulic connection between the stream and the underlying confined aquifer. The impact of various hydraulic characteristics and engineering factors on stream depletion was examined by conceptual and numerical modeling. To predict the suitability and efficiency of a combined water system application, regression tests were undertaken on unit stream depletion and capacitance, and power dependencies were defined.     

Evaluation of unconfined aquifer parameters from flow to partially penetrating wells in Tailan River basin, China

Effective evaluation, management and abstraction of groundwater resources of any aquifer require accurate and reliable estimates of its hydraulic parameters. This study, therefore, looks at the determination of hydraulic parameters of an unconfined aquifer using both analytical and numerical approaches. A long-duration pumping test data obtained from an unconfined aquifer system within the Tailan River basin in Xinjiang Autonomous Region in the northwest of China is used, in this study, to investigate the best method for estimating the parameters of the aquifer. The pumping test was conducted by pumping from a radial collector well and measuring the response in nine observation wells; all the wells used in the test were partially penetrating. Using two well-known tools, namely AquiferTest and MODFLOW, as an aid for the analytical and numerical approaches, respectively, the parameters of the aquifer were determined and their outputs compared. The estimated horizontal hydraulic conductivity, vertical hydraulic conductivity, and specific yield for the analytical approach are 38.1–50.30 m/day, 3.02–9.05 m/day and 0.204–0.339, respectively, while the corresponding numerical estimates are 20.50–35.24 m/day, 0.10–3.40 m/day, and 0.27–0.31, respectively. Comparing the two, the numerical estimates were found to be more representative of the aquifer in the study area since it simulated the groundwater flow conditions of the pumping test in the aquifer system better than the analytical solution.     

某氨氮污染地下水体抽出 -处理系统优化模拟研究

抽出 -处理系统设计多侧重于考虑修复初期的效率,在修复后期通常效率低下,产生拖尾现象,其优化的关键在于布设的井群系统能否高效抽出受污染的地下水体。利用溶质运移数值模拟可为井群布设和抽水方案优化提供依据。本研究旨在优化我国北方某化肥厂高浓度氨氮污染的地下水体的抽出 -处理修复系统,节约时间和成本。在水文地质调查及氨氮浓度监测的基础上,综合考虑井数、抽水天数和总抽水量三个变量,采用中轴线法与三角形法结合的布井方法,利用GMS软件反复试算,筛选出三种较优抽水方案并进一步模拟优化,最终从中选出最优抽水方案。结果,相比最初方案（方案1）,最优方案（方案3）将修复周期缩短了23个月,抽水总量减少了约31.9×104 m3,而抽水井数量仅增加了1口。该模型进行了稳定流水位拟合验证和4期非稳定流实测溶质浓度验证,较符合实际。结果表明,针对抽水井数量不足引起的拖尾问题,关键因素在于合理的井位布设与分阶段的抽水模式。在修复过程中,及时对地下水中污染物进行监测,并随着污染羽变化过程及时调整抽水方案,保证高浓度区一直有抽水井进行较大流量抽水,可有效提高修复效率并缩短修复周期。     

Tracer tests and the structure of permeability in the Corallian limestone aquifer of northern England, UK

The Corallian limestone of northern England (UK) is widely exploited for water supplies and exhibits the karstic phenomena of sinking rivers, conduit development and groundwater velocities of several kilometres per day. To test a number of model-derived source protection zones and elucidate contaminant transport mechanisms in the aquifer, three tracer tests were conducted from a set of swallow-holes draining the River Derwent toward public water supply wells in the eastern part of the aquifer. Tracers used included: Enterobacter cloacae (bacteriophage), Photine C (optical brightener), sodium fluorescein (fluorescent dye) and sulphur hexafluoride (dissolved gas), the varying properties of which make them suitable analogues for different types of potential contaminant. Observed tracer transport times and arrival patterns indicate that tracer transport occurs through karstic channels embedded in a network of primary fissures which exert control over tracer concentrations once initial tracer plumes have passed. A dipole flow system is observed between the swallow-holes and the closest abstraction well, whilst previously modelled source protection zones do not accurately reflect either groundwater velocity or those areas of the aquifer supplying the wells. These findings imply that managing such aquifers for potential contamination should rely upon empirical tracer evidence for source-protection zone modelling.     

Mass transport modelling to assess contamination of a water supply well in Sabarmati river bed aquifer, Ahmedabad City, India

 Drinking water supply wells were constructed in the Sabarmati river bed aquifer of Ahmedabad city using radial pipes and are known as French Collector wells. Contamination of groundwater from one of the French wells near Sabarmati railway bridge was noticed in 1992. The suspected pollution sources are Duff-nala of Shahibaug and two other sources from slum dwellings on either side of Sabarmati river. A combined groundwater flow, pathlines and a mass transport model was constructed covering an area of 9 km² to analyse the capture zone of the French well under two different scenarios. Aquifer parameters of the river bed aquifer were available. Dry river bed condition was simulated under scenario I and controlled flow in the river bed was simulated under scenario II. The groundwater velocity and migration of contaminant particles from sources was analysed in the pathline model. Total dissolved solids (TDS) concentration contours originating from sources in the mass transport model (MT3D) were computed by solving an advection-dispersion equation. The computed pathlines and TDS concentration contours indicate likely migration of contaminant plume from pollutant sources to the French well during 365 days under two scenarios. The model results confirm the tracer injection studies carried out to know the likely migration of contaminants towards the French well. The modelling study emphasised the necessity of controlled release of surface water in Sabarmati river bed from Dharoi reservoir throughout the year. Received: 28 October 1998 · Accepted: 17 June 1999     

Groundwater deterioration in Nankou—a suburban area of Beijing: data assessment and remediation scenarios

Since the 1980s, high-nitrate concentration in one of the groundwater sampling wells at the Nankou site, northwest of the Beijing Plain, has become a major concern for the local water authority. In a previous study (Sun et?al. in Environ Earth Sci 64(5):1323?C1333, 2011), a hydrogeological structural model was developed based on the borehole logs of this area and the steady, as well as transient groundwater-flow models, were calibrated using the measured hydraulic heads. In this paper, the potential pollution sources in this area are investigated. The chemical analysis of the groundwater is also presented. The results demonstrate that the most likely pollution source is the untreated wastewater discharge from a nearby fertilizer factory. Furthermore, a mass transport model is developed to reproduce the nitrate transport process in the aquifer at the Nankou site under different pollution sources, i.e., a fertilizer factory, river with wastewater and an agriculture field. The combined effects of the river and agriculture fields present a better understanding of the nitrate transport in the local aquifer. In addition, a pumping scenario is designed to clean up the current nitrate concentration. The pumping rate and the well location are first estimated with 2-D analytical solutions of the type curves method. Then a 3-D numerical model is used to calculate the nitrate-concentration changes after the pumping activities start. In the downstream direction of the regional groundwater flow, three pumping wells are set up for the clean-up strategy. The calculated pumping rate in each well is about 1,500 m³/day. After 1?year, the nitrate concentration in the observation well recedes to 68?mg/l from the initial value of 72.9?mg/l, and it will be lower than the limitation value (20?mg/l) after 5,400 days of groundwater extraction. The data assessment and clean-up scenarios reported in this paper are fundamental for the contaminated aquifer management in the future.     

Groundwater resources evaluation case study via discrete fracture flow modeling

The Arizona Department of Transportation (ADOT) is preparing to upgrade State Route 260 between Payson and Heber. It is estimated that a total of about one million cubic meters of water will be required for embankment construction during a period of about 84 months to upgrade the first 33.8 km of the highway. ADOT is investigating various sources of construction water for use in the highway improvement project, including groundwater resources along the highway corridor. A region known as the RV site, underlain by fractured granite, is located 12.9 km east of Payson. The site includes three springs, a creek and several wells. Several boreholes and observation wells were made to a maximum depth of 157 m to obtain fracture data and to conduct pumping tests with monitoring. Fracture data recorded by acoustic televiewer logs were used to build a fracture network model for the rock mass. Results of a 24-hour and a 7-day pumping tests were used to calibrate hydraulic parameters of a finite element discrete fracture fluid flow model considering the region as a heterogeneous, anisotropic, fractured medium. A 38-day multi-well pumping test was used to validate the calibrated numerical model. The calibrated model showed the capability to provide reasonably accurate predictions for new pumping tests conducted in the same well field. The validated model was then used to simulate pumping exceeding a 7 year period under different scenarios incorporating different sets of boundary conditions and different pumping rates at multi-wells, with and without recharge, to evaluate the yield of the aquifer and to assess the effect of long-term pumping on the environment. The results indicated that (a) the combined yield of the wells in the RV site is sufficient to meet the water demand for the ADOT highway project and (b) the water levels in the well field would decline between 3.0 and 7.6 m after one year of pumping and by 12.2 to over 30.5 m during the life of the project.     

Physical model test of transparent soil on coupling effect of cut-off wall and pumping wells during foundation pit dewatering

Water level is decreased during foundation pit excavation to avoid water inrush under confined water pressure. Cut-off wall is often used as waterproof curtain to partially cut off the dewatered aquifer. When a foundation pit is located in a built-up area and the underlying confined aquifer is not cut off, the drawdown must be minimized outside the pit to avoid land subsidence in buildings and pipelines. The coupling effect of the cut-off wall and pumping well is used to control the drawdown outside the foundation pit. However, the coupling mechanism is not intuitively well understood because of the limitations of existing experimental methods. In this study, transparent soil was introduced to model the coupling mechanism in the physical model test. High-purity fused silica and mixed paraffin oil were used as skeleton and fluid to simulate the confined aquifer and groundwater. Industrial solid dye and paraffin oil were used as tracers. A camera was used to collect flow information. Tests were performed for the combinations of cut-off wall and partially penetrating pumping wells. The insertion depth ratio of the cut-off wall most effectively influenced the drawdown. The layout of the pumping wells in horizontal direction influenced water level distribution and flow rate. The optimal depth of the pumping wells was 1–5 m above the bottom of the cut-off wall, and the optimal horizontal distance between the cut-off wall and the pumping wells was 25% of the pit width. Non-Darcy flow was observed within the range of 0–10 m around the bottom of the cut-off wall. These results were significant in understanding the cut-off wall and pumping well coupling effect on foundation pit dewatering.

     

Effects of the planned ephesus recreational canal on freshwater–seawater interface in the Selçuk sub-basin, İzmir-Turkey

An artificial water canal opening is planned between the Agean Sea and the historical Ephesus site for the sake of tourism in the Selçuk sub-basin. In order to predict the effects of the planned canal on freshwater–seawater interface and related contamination in the aquifer, 3-D numerical density dependent flow and solute transport simulations were carried out. The simulations included the pre-pumping and pumping periods without a canal and the prediction period in the presence of the canal. Chloride concentration comparisons of the results obtained from the pre-pumping period and the pumping period indicate that the freshwater-seawater interface in the aquifer has progressed inland due to artificial discharge in the sub-basin. Drawdown during the pumping period is about 15 cm. The planned canal opening could further lower the groundwater levels in the area and would change the groundwater flow directions in the first 4 years. Then the levels and flow directions will nearly recover. However, the canal opening could cause further seawater intrusion into the aquifer to the extent that groundwater would be unfit to use for irrigation after the seventh year of the canal opening in the irrigation cooperative II wells area and would be unfit to use for drinking purposes after the tenth year in the municipality wells area located at the south of the cooperative II wells. On the other hand, the cooperative I wells would not be effected by the opening of the canal.     

Evaluation of the water resources potential at Dahaban region, western Saudi Arabia

Shallow renewable groundwater sources have been used to satisfy the domestic needs and the irrigation in many parts of Saudi Arabia. Increased demand for water resulting from accelerated development activities has placed excess stress on the renewable sources especially in coastal aquifers of the western region of Saudi Arabia. It is expected that the current and future development activities will increase the rate of groundwater mining of the coastal aquifer near the major city Jeddah and surrounding communities unless management measures are implemented. The current groundwater development of Dahaban coastal aquifer located at alluvial fan at the confluence of three major Wadis is depleting the shallow renewable groundwater sources and causes deterioration of its quality. Numerical models are known tools to evaluate groundwater management scenarios under a variety of development options under different hydrogeological regimes. In this study, two models are applied—the MODFLOW for evaluating the hydrodynamic behaviors of the aquifer and MT3D salinity distribution to the costal aquifer near Dahaban town. The models’ simulation evaluates two development scenarios—the impact of excessive abstraction and the water salinity variation keeping abstraction at its current or increases in levels with or without groundwater recharge taking place. The simulation evaluated two scenarios covering a 25-year period—keeping the current abstraction at its current and the other scenario is increasing the well abstraction by 50% for dry condition (no recharge) and wet condition (with recharge). The analysis reveals that, under the first scenario, the continuation of the current pumping rates will result in depletion of the aquifer resulting in drying of many wells and quality deterioration at the level of 2,500 ppm. The results are associated with the corresponding salinity distribution in the region. Simulation of salinity in the region is a density-independent problem as salt concentration does not exceed 2,000 ppm, which is little value compared with sea salinity that amounts to 40,000 ppm. It is not recommended to increase the pumping rate than the current values. However, for the purpose of increasing water resources in the region, it is recommended to install new wells in virgin zones west of Dahaban main road. Maps of high/low potential groundwater and maps of salinity zones (more or less than 1,000 ppm) are provided and could be used to identify zones of high groundwater potential for the four studied scenarios. The implemented numerical simulation of Dahaban aquifer was undertaken to assess the water resources potential in order to reduce the depletion of sources in the future.     

Site characterization and monitoring of natural attenuation indicator parameters in a fuel contaminated coastal aquifer: Karaduvar (Mersin,SE Turkey)

This paper presents results from a site characterization and monitoring study at Karaduvar area (Mersin, SE Turkey), where high concentrations of refined petroleum products have been detected in domestic and irrigation water wells. The saturated and unsaturated zones in the deltaic aquifer are contaminated by large quantities of gasoline and diesel range fuel hydrocarbons (GRHs and DRHs) released from diverse sources that include accidental spills, storage tank fires, pipeline breaks, deliberate discharge of waste petroleum products from slop tanks and illegal tanker truck washing facilities. At the site, due to the complex nature of the pollution sources, overlapping contaminant plumes exist and cover an area of about 0.5 km². In both polluted and unpolluted parts of the aquifer, monitoring of groundwater physicochemical parameters in a total of 55 sampling points was carried out between 2006 and 2007. The results show that the terminal electron acceptors (e.g. dissolved oxygen, nitrate, Mn(IV), Fe(III), sulfate) were reduced near the source area(s) indicating presence of actively operating biodegradation processes at the site. Close to the contaminant source area(s), conditions in the plume are highly anoxic and reducing; where high amounts of transformation products (e.g. bicarbonate, dissolved iron, and manganese) are present in solution. Additionally, at the site, excessive pumping, careless land use, and deliberate wastewater discharges significantly deteriorated the quality and quantity of groundwater. Excessive groundwater pumping for industrial and agricultural uses has resulted in substantial water level declines (2–3 m) near the coastal part where seawater intrusion threatens the groundwater resources.     

Contamination,risk, and heterogeneity: on the effectiveness of aquifer remediation

The effectiveness of aquifer remediation is typically expressed in terms of a reduction in contaminant concentrations relative to a regulated maximum contaminant level (MCL), and is usually confirmed by sparse monitoring data and/or simple model calculations. Here, the effectiveness of remediation is re-examined from a more thorough risk-based perspective that goes beyond the traditional MCL concept. A methodology is employed to evaluate the health risk to individuals exposed to contaminated household water that is produced from groundwater. This approach explicitly accounts for differences in risk arising from variability in individual physiology and water use, the uncertainty in estimating chemical carcinogenesis for different individuals, and the uncertainties and variability in contaminant concentrations within groundwater as affected by transport through heterogeneous geologic media. A hypothetical contamination scenario is developed as a case study in a saturated, alluvial aquifer underlying an actual Superfund site. A baseline (unremediated) human exposure and health risk scenario, as induced by contaminated groundwater pumped from this site, is predicted and compared with a similar estimate based upon pump-and-treat exposure intervention. The predicted reduction in risk in the remediation scenario is not an equitable one—that is, it is not uniform to all individuals within a population and varies according to the level of uncertainty in prediction. The importance of understanding the detailed hydrogeologic connections that are established in the heterogeneous geologic regime between the contaminated source, municipal receptors, and remediation wells, and its relationship to this uncertainty is demonstrated. Using two alternative pumping rates, we develop cost-benefit curves based upon reduced exposure and risk to different individuals within the population, under the presence of uncertainty.