Simulated Aquifer Research Articles

Hydrodynamic aquifer or reservoir compartmentalization?

Changes in oil-water contact (OWC) depth across a field are often a signature of reservoir compartmentalization as a result of faulting or stratigraphic barriers, such as shales, but may equally well be caused by an underlying hydrodynamic aquifer. In particular, the pressure-depth data obtained from an aquifer whose flow is changing over time can look very similar to that obtained from a compartmentalized reservoir. Misunderstanding which of these mechanisms causes the observed changes in OWC across the field may result in poor estimates of oil in place and reduced recovery. To address this problem, an analytic expression is presented to estimate the time taken for a steady state tilted OWC to be established once an aquifer starts flowing. A comparison with simulations of hydrodynamic aquifers in homogeneous, compartmentalized, and heterogeneous reservoir models shows that this expression can be used in combination with the one derived by M. K. Hubbert, for the steady state tilt of the OWC, to clarify whether a reservoir contains barriers or baffles to flow or may simply have not yet reached equilibrium.

Fault-controlled deformation and stress from pumping-induced groundwater flow

Four two-dimensional numerical model scenarios are developed to gain a better understanding of how earth fissures form under pumping induced stress. The simulated aquifer–aquitard system contains a low compressibility sub-vertical 100 m wide fault zone that transects the hydrostratigraphic system that includes a 100 m thick vadose zone. Two scenarios represent constant and cyclical pumping over a 5-year period and two scenarios are used to evaluate the difference between using purely elastic aquitards versus elasto-plastic aquitards. The model provides for the analysis of the spatial distribution of deformation, and normal and shear stress that potentially lead to the formation of earth fissures. Results indicate that this fault-zone largely controls the deformation and stress regime of the system during pumping. Patterns of deformation and stress are similar for all scenarios with only the magnitudes being affected. Simulation results show that pumping induces a counterclockwise rotation of the fault plane on the footwall. The distribution of deformation through the fault is damped and the deformation patterns are more subdued on the hanging wall but all deformation is directed toward the pumping well. These deformation patterns result in two areas of stress accumulation that may lead to fissure formation: (a) near the land surface in the fault zone, and (b) near the saturated–unsaturated interface on the hanging wall adjacent to the fault zone. These results reveal the importance of both the fault and vadose zones in fissure genesis.

Construction of high-resolution stochastic geological models and optimal upscaling to a simplified layer-type hydrogeological model

Despite inequalities in spatial resolution between stochastic geological models and flow simulator models, geostatistical algorithms are used for the characterisation of groundwater systems. From available data to grid-block hydraulic parameters, workflows basically utilise the development of a detailed geostatistical model (morphology and properties) followed by upscaling.This work aims to design and test a two-step methodology encompassing the generation of a high-resolution 3D stochastic geological model and simplification into a low-resolution groundwater layer-type model. First, a high-resolution 3D stochastic model of rock types or hydrofacies (sets of rock types with similar hydraulic characteristics) is generated using an enhanced version of the sequential indicator simulation (SIS) with corrections for local probabilities and for two- and three-point template statistics. In a second step, the high-resolution geological model provided by SIS is optimally simplified into a small set of layers according to a supervised simulated annealing (SA) optimisation procedure and at the end equivalent hydraulic properties are upscaled.Two outcomes are provided by this methodology: (1) a regular 2D mesh of the top and bottom limits of each hydrogeological unit or layer from a conceptual model and (2), for each layer, a 2D grid-block of equivalent hydraulic parameters prepared to be inputted into an aquifer simulator. This methodology was tested for the upper aquifer area of SPEL (Sociedade Portuguesa de Explosivos), an explosives deactivation plant in Seixal municipality, Portugal.

Two-dimensional numerical finite volume modeling of processes controlling distribution and natural attenuation of BTX in the saturated zone of a simulated semi-confined aquifer

This paper presents a two-dimensional finite volume model to predict multi-species reactive transport processes in the saturated zone of a simulated semi-confined aquifer. A multipurpose commercial software called PHOENICS was used to solve model equations numerically. Capability of the present model was first confirmed using experimental data and the results obtained by a published one-dimensional finite element reactive transport model by other researchers taking different scenarios into consideration. The model was then expanded to a two-dimensional case to simulate reactive transport of BTX compounds with discontinuous source in the saturated zone of the groundwater flow system. In addition to the physical transport processes, the two-dimensional model also incorporates linear and nonlinear adsorption isotherms, first order and Monod kinetics. The two-dimensional model considers both static and dynamics modes into account. The results show that considering chemical reactions during reactive transport of contaminants could successfully predict the contaminated zone. The results of such studies can be used for monitoring of contaminated areas, designing methods to control pollution transport, and minimize its harmful effects on aquifer systems.

Lumping Analysis for Sorption of Neutral Organic Compounds in Mixtures to Simulated Aquifer Sorbents

The concept of using lumping analysis to reduce the complexity associated with the sorption of 12 neutral organic compounds (NOCs) comprising complex mixtures to simulated aquifer sorbents was evaluated. The sorbates covered a wide range of octanol-water partitioning coefficients, Kow (i.e., � 0:24 ≤ logKow ≤ 4:23), and the sorbents included different types of mineral surfaces and humic acid- mineral complexes with different fractions of organic carbon, f oc (i.e., 0:006% ≤ f oc ≤ 0:221%). Both a priori lumping criteria (i.e., the aqueous activity coefficient at saturation γ sat , and the organic carbon partitioning coefficient, Koc) and experimentally derived lumping criteria (i.e., Freundlich sorption parameters) were evaluated as potential lumping criteria. The results indicated that the sorption behavior of the 12 NOCs contained in mixtures could be approximated reasonably well by lumping them into a fewer number of pseudocompounds (four to six) with similar sorption behaviors based on Freundlich sorption parameters, and that both γ sat and Koc could be used as viable a priori lumping criteria. However, the number and compositions of the pseudocompounds were a complex function of the hydrophobicity (i.e., γ sat and Koc) of the NOCs and varied as a function of the type of mineral surface (e.g., uncoated, α-FeOOH-coated, and Al2O3-coated sands) and the f oc of the sorbent. DOI: 10.1061/(ASCE)EE.1943-7870.0000498. © 2012 American Society of Civil Engineers. CE Database subject headings: Organic carbon; Organic compounds; Organic matter; Pollutants; Soil pollution; Sorption; Subsurface environment. Author keywords: Lumping analysis; Mixtures; Neutral organic compounds; Pseudocompounds; Simulated aquifer sorbent; Sorption.

Screening and evaluation of a saline aquifer for CO 2 storage: Central Bohemian Basin, Czech Republic

This work is a preliminary screening and evaluation of one of the most promising geological structures for CO 2 storage in the Czech Republic, namely the deep saline aquifers of the Central Bohemian Permian–Carboniferous Basin. Archived and new mineralogical, petrophysical, lithological, geochemical, sequence-stratigraphic, thermal history, core analyses, hydrogeological testing, and seismic data for both the reservoir formation and seal rocks were used to build an initial static geological model of the Central Bohemian Basin. Using this static geological model, a sector of the Basin was identified as meeting the basic geological requirements for CO 2 storage; a preliminary reservoir model of this sector was created and used to perform several simulation runs to further evaluate the suitability and potential of the selected formation for storing captured CO 2 from industrial emissions. The initial sensitivity analyses performed using a mechanistic model indicated that while the storage capacity of the structure (based on rock porosity and net-to-gross ratio) may be sufficient, the critical factor was the CO 2 injectivity. The CO 2 injectivity was found to be controlled mainly by the formation heterogeneities, rock permeability, reservoir connectivity, storage formation pore volume, as well as the injection wells type, location and completion scheme. Therefore, based on the current geological model these parameters were the most influential ones defining a potential success of a CO 2 geological storage project. Full-scale sector simulations confirmed the findings of the mechanistic model. The current reservoir model and simulation results have also shown that it would be unlikely to be possible to inject the required target volume of 2 million tonnes of CO 2 per year within a single storage project in the Central Bohemian Basin. The target CO 2 volume could be achieved only for a reservoir scenario that contains an optimistic absolute permeability field, sealing faults in the northern part of the site and the presence of two water production wells. These two production wells could be viewed as actual pressure relief wells or a way of (partially) reflecting pressure communication of the simulated aquifer region to adjacent pore volumes in the geological basin. At present, the available geological data does not allow a detailed formation characterization (there is a lack of reliable permeability and porosity data, sufficient and effective description of reservoir heterogeneities such as existence of faults, fault conductivity, presence of low or high permeability zones, sealing efficiency, etc.). Hence, crucial issues such as CO 2 injectivity, and CO 2 migration and containment in the reservoir rock currently remain uncertain. There is a potential to quantify these parameters more accurately only after additional and reliable seismic, core, and testing data from actual reservoir depths become available. A reduction of the existing uncertainties related to formation characterization is a prerequisite for more accurate predictions of the available reservoir volumes, the required number of wells, the well type, position and completion, CO 2 injection operational issues, and finally the amount of CO 2 that could potentially be stored.

Comparative Evaluation of Genetic Programming and Neural Network as Potential Surrogate Models for Coastal Aquifer Management

Determining the optimal rates of groundwater extraction for the sustainable use of coastal aquifers is a complex water resources management problem. It necessitates the application of a 3D simulation model for coupled flow and transport simulation together with an optimization algorithm in a linked simulation-optimization framework. The use of numerical models for aquifer simulation within optimization models is constrained by the huge computational burden involved. Approximation surrogates are widely used to replace the numerical simulation model, the widely used surrogate model being Artificial Neural Networks (ANN). This study evaluates genetic programming (GP) as a potential surrogate modeling tool and compares the advantages and disadvantages with the neural network based surrogate modeling approach. Two linked simulation optimization models based on ANN and GP surrogate models are developed to determine the optimal groundwater extraction rates for an illustrative coastal aquifer. The surrogate models are linked to a genetic algorithm for optimization. The optimal solutions obtained using the two approaches are compared and the advantages of GP over the ANN surrogates evaluated.

Accuracy of Flowmeters Measuring Horizontal Groundwater Flow in an Unconsolidated Aquifer Simulator

Borehole flowmeters that measure horizontal flow velocity and direction of groundwater flow are being increasingly applied to a wide variety of environmental problems. This study was carried out to evaluate the measurement accuracy of several types of flowmeters in an unconsolidated aquifer simulator. Flowmeter response to hydraulic gradient, aquifer properties, and well‐screen construction was measured during 2003 and 2005 at the U.S. Geological Survey Hydrologic Instrumentation Facility in Bay St. Louis, Mississippi. The flowmeters tested included a commercially available heat‐pulse flowmeter, an acoustic Doppler flowmeter, a scanning colloidal borescope flowmeter, and a fluid‐conductivity logging system. Results of the study indicated that at least one flowmeter was capable of measuring borehole flow velocity and direction in most simulated conditions. The mean error in direction measurements ranged from 15.1° to 23.5° and the directional accuracy of all tested flowmeters improved with increasing hydraulic gradient. The range of Darcy velocities examined in this study ranged 4.3 to 155 ft/d. For many plots comparing the simulated and measured Darcy velocity, the squared correlation coefficient (r2) exceeded 0.92. The accuracy of velocity measurements varied with well construction and velocity magnitude. The use of horizontal flowmeters in environmental studies appears promising but applications may require more than one type of flowmeter to span the range of conditions encountered in the field. Interpreting flowmeter data from field settings may be complicated by geologic heterogeneity, preferential flow, vertical flow, constricted screen openings, and nonoptimal screen orientation.

Aquifer geochemistry at potential aquifer storage and recovery sites in coastal plain aquifers in the New York city area, USA

Abstract The effects of injecting oxic water from the New York city (NYC) drinking-water supply and distribution system into a nearby anoxic coastal plain aquifer for later recovery during periods of water shortage (aquifer storage and recovery, or ASR) were simulated by a 3-dimensional, reactive-solute transport model. The Cretaceous aquifer system in the NYC area of New York and New Jersey, USA contains pyrite, goethite, locally occurring siderite, lignite, and locally varying amounts of dissolved Fe and salinity. Sediment from cores drilled on Staten Island and western Long Island had high extractable concentrations of Fe, Mn, and acid volatile sulfides (AVS) plus chromium-reducible sulfides (CRS) and low concentrations of As, Pb, Cd, Cr, Cu and U. Similarly, water samples from the Lloyd aquifer (Cretaceous) in western Long Island generally contained high concentrations of Fe and Mn and low concentrations of other trace elements such as As, Pb, Cd, Cr, Cu and U, all of which were below US Environmental Protection Agency (USEPA) and NY maximum contaminant levels (MCLs). In such aquifer settings, ASR operations can be complicated by the oxidative dissolution of pyrite, low pH, and high concentrations of dissolved Fe in extracted water. The simulated injection of buffered, oxic city water into a hypothetical ASR well increased the hydraulic head at the well, displaced the ambient groundwater, and formed a spheroid of injected water with lower concentrations of Fe, Mn and major ions in water surrounding the ASR well, than in ambient water. Both the dissolved O 2 concentrations and the pH of water near the well generally increased in magnitude during the simulated 5-a injection phase. The resultant oxidation of Fe 2+ and attendant precipitation of goethite during injection provided a substrate for sorption of dissolved Fe during the 8-a extraction phase. The baseline scenario with a low (0.001 M) concentration of pyrite in aquifer sediments, indicated that nearly 190% more water with acceptably low concentrations of dissolved Fe could be extracted than was injected. Scenarios with larger amounts of pyrite in aquifer sediments generally resulted in less goethite precipitation, increased acidity, and increased concentrations of dissolved Fe in extracted water. In these pyritic scenarios, the lower amounts of goethite precipitated and the lower pH during the extraction phase resulted in decreased sorption of Fe 2+ and a decreased amount of extractable water with acceptably low concentrations of dissolved Fe (5.4 × 10 −6 M). A linear decrease in recovery efficiency with respect to dissolved Fe concentrations is caused by pyrite dissolution and the associated depletion of dissolved O 2 (DO) and increase in acidity. Simulations with more than 0.0037 M of pyrite, which is the maximum amount dissolved in the baseline scenario, had just over a 50% recovery efficiency. The precipitation of ferric hydroxide minerals (goethite) at the well screen, and a possible associated decrease in specific capacity of the ASR well, was not apparent during the extraction phase of ASR simulations, but the model does not incorporate the microbial effects and biofouling associated with ferric hydroxide precipitation. The host groundwater chemistry in calcite-poor Cretaceous aquifers of the NYC area consists of low alkalinity and moderate to low pH. The dissolution of goethite in scenarios with unbuffered injectate indicates that corrosion of the well could occur if the injectate is not buffered. Simulations with buffered injectate resulted in greater precipitation of goethite, and lower concentrations of dissolved Fe, in the extracted water. Dissolved Fe concentrations in extracted water were highest in simulations of aquifers (1) in which pyrite and siderite in the aquifer were in equilibrium, and (2) in coastal areas affected by saltwater intrusion, where high dissolved-cation concentrations provide a greater exchange of Fe 2+ (FeX 2 ). Results indicate that ASR in pyrite-bearing, Cretaceous aquifers of the NYC area can be a cost effective way to both store drinking water supplies and significantly reduce ambient concentrations of dissolved Fe.

Simulation and Multi-Objective Management of Coastal Aquifers in Semi-Arid Regions

Groundwater is the main water resource in many semi-arid coastal regions and water demand, especially in summer months, can be very high. Groundwater withdrawal for meeting this demand often causes seawater intrusion and degradation of water quality of coastal aquifers. In order to satisfy demand, a combined management plan is proposed and is under consideration for the island of Santorini. The plan involves: (1) desalinization (if needed) of pumped water to a potable level using reverse osmosis and (2) injection into the aquifer of biologically-treated waste water. The management plan is formulated in a multi-objective, optimization framework, where simultaneous minimization of economic and environmental costs is desired, subject to a constraint so that cleaned water satisfies demand. The decision variables concern the well locations and the corresponding pumping and recharging rates. The problem is solved using a computationally efficient, multi-objective, genetic algorithm (NSGAII). The constrained multi-objective, optimization problem is transformed to an unconstrained one using a penalty function proportional to constraint violation. This extends the definition of the objective function outside the domain of feasibility. The impact of prolonged droughts on coastal aquifers is investigated by assuming various scenarios of reduced groundwater recharge. Water flow and quality in the coastal aquifer is simulated using a three-dimensional, variable density, finite difference model (SEAWAT). The method is initially applied to a test aquifer and the trade-off curves (Pareto fronts) are determinedl for each drought scenario. The trade-off curves indicate an increase on the economic and environmental cost as groundwater recharge reduces due to climate change.

Simulation of Groundwater Flow in a Sedimentary Aquifer System Subjected to Overexploitation

Groundwater flow models are applied to a large variety of hydrogeological conditions in different aquifer types, in order to simulate the groundwater flow of the investigated system. This paper aims to present the application of a groundwater flow model for the simulation of a sedimentary aquifer, located in Northern Greece. The simulation involves the period between April 2003 and April 2004, and the model is divided into two distinct stress periods, each containing two different time steps. The simulation of the aquifer is found to be satisfactory, conclusion which is based on both graphical as well as arithmetical verifications. The groundwater flow simulation was achieved by the application of the MODFLOW code.

Effects of Primary Substrate Concentration on NDMA Transport during Simulated Aquifer Recharge

N-nitrosodimethylamine (NDMA) is known to be highly carcinogenic and is present in drinking water, wastewater, and a variety of foods. Because of its presence in chloraminated water at nanogram per liter concentrations, NDMA has become an emerging issue for reclaimed water which may be used for aquifer recharge or irrigation. This research investigated the fate of NDMA in two soil column systems used to simulate subsurface transport. One column system was operated under aerobic conditions with increasing primary substrate concentration where the biodegradable organic carbon (BDOC) in reclaimed water was used as the primary substrate. The reclaimed water content in the influent was increased from 0 to 25% in the column to increase the BDOC concentration. Negligible NDMA removal was observed at 0% reclaimed water and increasing the primary substrate in the influent resulted in NDMA removal suggesting that biodegradation of NDMA might be a cometabolic process. The effects of redox conditions on NDMA fate was studied by operating a second column system with 100% reclaimed water under anoxic conditions and then changing the conditions to aerobic. It was observed that NDMA removal was similar under both aerobic and anoxic condition, however, much lower effluent concentrations were observed under aerobic conditions. Under anoxic condition, a normalized mass removal rate of 254 ng NDMA/mg DOC was observed which increased to 273-ng NDMA/mg DOC under aerobic conditions. The majority of NDMA and substrate removal occurred in the first of three columns in series column under both aerobic and anoxic conditions. Normalized mass removal rates of NDMA after the first, second, and third columns were 372, 30, and 20 ng NDMA/mg DOC, respectively. Since the majority of dissolved organic carbon was also removed in the first column, NDMA biodegradation was consistent with cometabolic activity. Batch tests verified the biodegradation removal potential of NDMA. Addition of a methylotrophic substrate, methanol and an aromatic substrate, toluene, did not increase NDMA removal.

Optimal selection of artificial neural network parameters for the prediction of a karstic aquifer's response

AbstractThe simulation of karstic aquifers is difficult using traditional groundwater numerical simulators, as the exact knowledge of the hydraulic characteristics of the physical system in small scale is rarely available and the numerical simulators produce results of limited reliability. In the present work, artificial neural networks (ANNs) are utilized to predict the response of a karstic aquifer, using the hydraulic head change per time step rather than the hydraulic head itself as output parameter of the network. As it will be demonstrated, in the first case a better approximation of the physical system's response is achieved as the change of the hydraulic head is more naturally connected to the input parameters of the network, which model the aquatic equilibrium of the system. The correlation of rainfall and hydraulic head change per time step was initially used to determine the time lag of the rainfall input data, which represents the time needed by the rainfall to percolate and reach the water table. In a second step, a differential evolution (DE) algorithm is utilized for the optimal selection of rainfall time lag as well as ANN's architecture and training parameters. Although a time consuming procedure, the improvement obtained suggests that the empirical determination of the ANN parameters and structure is not always sufficient and an optimization procedure, which minimizes the training and evaluation errors of the ANN, may provide substantially better simulation results. The optimized networks were finally used for midterm predictions (30 to 90 days ahead) of the hydraulic head, showing the ability of the ANN with hydraulic head change as output parameter to provide predictions with high accuracy at the end of the considered time period. Copyright © 2009 John Wiley & Sons, Ltd.

Application of a Discrete‐Continuum Model to Karst Aquifers in North China

A generalized discrete-continuum model is developed to simulate ground water flow in the karst aquifers of North China. The model is a hybrid numerical flow model, which takes into account both quick conduit flow and diffusive fissure flow. The conduit flow is represented by a discrete network model, and the fissure flow is modeled by a continuum approach. The developed model strongly emphasizes the function of the conduits in the flow fields. They control the general drainage pattern, as demonstrated in the simulation of a complex karst aquifer in North China. The model reproduces reasonably well the flow field in response to an unanticipated discharge of ground water from the karst aquifer into an underground mine based on the aquifer parameters that are manually calibrated from a multiple-well pumping test. Sensitivity of the model to the aquifer parameters was evaluated in the context of the case study.

Optimizing the Remedial Process at a Petroleum Hydrocarbon Contaminated Site Using a Three-Tier Approach

Pump and treat (P&T) system is widely used to remove contaminants in groundwater and maintain hydraulic capture of contaminants. A P&T system usually becomes costly in removing contaminant mass after operation for a period of time because the cost of pumping stays high while concentration of contaminants in extracted groundwater decreases significantly over time. However, the P&T system may have to continue operation before contaminants in groundwater reach the cleanup goals. In such cases, the optimization of the existing remedial system and an exit strategy will be necessary and beneficial to all stakeholders. This study examined a P&T system at a remediation site near a coastal wetland. We developed a three-tier approach to optimize the existing remedial system. The remedial process optimization (RPO) was achieved by reducing pump wells and monitoring frequency and developing an exit strategy. In the first tier, we used descriptive trend plot, trend factor analysis, and autocorrelation analysis to capture the temporal trend in the monitoring data and determine optimum sampling frequency; in the second tier, we used multivariate analysis include principal component analysis (PCA) and partial least square regression (PLSR) to identify the spatial and temporal pattern in the monitoring data, and to construct a statistical model to predict future contaminant levels; in the third tier, we used MODFLOW, a USGS developed finite-difference flow module for aquifer simulation, and interpretation of the hydraulic capture of extraction wells and contaminant migration. Based on the tiered analysis, we designed an improved groundwater monitoring program and pumping strategy. The RPO is expected to save 68% groundwater monitoring cost, and 30% P&T operation and maintenance (O&M) cost.

How Processing Digital Elevation Models Can Affect Simulated Water Budgets

For regional models, the shallow water table surface is often used as a source/sink boundary condition, as model grid scale precludes simulation of the water table aquifer. This approach is appropriate when the water table surface is relatively stationary. Since water table surface maps are not readily available, the elevation of the water table used in model cells is estimated via a two-step process. First, a regression equation is developed using existing land and water table elevations from wells in the area. This equation is then used to predict the water table surface for each model cell using land surface elevation available from digital elevation models (DEM). Two methods of processing DEM for estimating the land surface for each cell are commonly used (value nearest the cell centroid or mean value in the cell). This article demonstrates how these two methods of DEM processing can affect the simulated water budget. For the example presented, approximately 20% more total flow through the aquifer system is simulated if the centroid value rather than the mean value is used. This is due to the one-third greater average ground water gradients associated with the centroid value than the mean value. The results will vary depending on the particular model area topography and cell size. The use of the mean DEM value in each model cell will result in a more conservative water budget and is more appropriate because the model cell water table value should be representative of the entire cell area, not the centroid of the model cell.

Temporal response of hydraulic head, temperature, and chloride concentrations to sea-level changes, Floridan aquifer system, USA

Three-dimensional density-dependent flow and transport modeling of the Floridan aquifer system, USA shows that current chloride concentrations are not in equilibrium with current sea level and, second, that the geometric configuration of the aquifer has a significant effect on system responses. The modeling shows that hydraulic head equilibrates first, followed by temperatures, and then by chloride concentrations. The model was constructed using a modified version of SUTRA capable of simulating multi-species heat and solute transport, and was compared to pre-development conditions using hydraulic heads, chloride concentrations, and temperatures from 315 observation wells. Three hypothetical, sinusoidal sea-level changes occurring over 100,000 years were used to evaluate how the simulated aquifer responds to sea-level changes. Model results show that hydraulic head responses lag behind sea-level changes only where the Miocene Hawthorn confining unit is thick and represents a significant restriction to flow. Temperatures equilibrate quickly except where the Hawthorn confining unit is thick and the duration of the sea-level event is long (exceeding 30,000 years). Response times for chloride concentrations to equilibrate are shortest near the coastline and where the aquifer is unconfined; in contrast, chloride concentrations do not change significantly over the 100,000-year simulation period where the Hawthorn confining unit is thick.

Modeling of an MTBE plume at Pascoag, Rhode Island

A numerical groundwater flow and mass transport model was developed to predict the extent of impact from methyl tertiary butyl ether (MTBE) release on a down-gradient drinking water well field. An MTBE incident in Pascoag, Rhode Island, was used as a case study and the plume’s past and future development was simulated using scenario analysis. The numerical code used was GeoSys/Rockflow, which permits a coupled flow and transport simulation as one object, thus alleviating the need for simulating the MTBE fate with separate flow and transport codes. The numerical model was built on available hydrogeological and chemical data as well as on GIS information of the site. By comparing the simulated results with observed field data, it was found that the model could provide reliable results even when the simulated aquifer was simplified to a two-dimensional flow and transport domain. Finally, the calibrated model was used for exploring a location that may be suitable for a new well field. Despite the model limitations associated with uncertainties of data and simplifying assumptions, numerical modeling of this MTBE contaminated site proved a useful tool and provided guidance for future municipal well field operation strategies and aquifer remediation alternatives.

Evaluation of reactive well networks for remediating heterogeneous aquifers

Alternative configurations of non-pumped wells filled with reactive media were evaluated for removing hypothetical contaminant plumes. All wells were screened across the saturated zone of a simulated unconfined aquifer. Three heterogeneous hydraulic conductivity distributions (cases) were considered. A mass transport model accounting for advection and hydrodynamic dispersion produced an initial contaminant plume for each case. Two reactive well configurations were evaluated for each case. In one configuration, evenly spaced wells occupied a linear transect perpendicular to regional groundwater flow, located downgradient of the contaminant plume. A second configuration involved the same number of wells, but along evenly spaced, nonlinear flow lines originating from the downgradient boundary of the contaminant plume. Mass transport modeling simulated contaminant plumes moving through the aquifer and wells. Results suggest that nonlinear configurations, which take into account local flow variations near the downgradient boundary of a contaminant plume, more efficiently reduce contaminant concentrations and better control offsite migration.

Configuring passive wells with reactive media for treating contaminated groundwater

AbstractPassive (nonpumped) wells filled with reactive media may be an effective alternative to permeable reactive barriers for treating contaminated groundwater in deep aquifers. This study's objective was to evaluate alternative passive well configurations for treating contaminated groundwater in a simulated aquifer. Six configurations of 17 wells were tested, each well screened across the saturated zone. Passive wells occupied various locations on monitoring transects. Five configurations involved one or more monitoring transects perpendicular to regional groundwater flow, positioned downgradient of a contaminant plume. In a sixth configuration, monitoring transects were ±45° to groundwater flow, joined downgradient of the contaminant plume. A numerical mass transport model, accounting for advection and hydrodynamic dispersion, tested the ability of each passive well configuration to reduce contaminant concentrations and prevent offsite contaminant migration. For a fixed number of wells, relatively effective strategies include: (a) spanning the lateral width of a contaminant plume; (b) multiple monitoring transects, with relatively close well spacing along upgradient transects; and (c) 45° monitoring transects. Overall, the 45° structure was most effective. © 2008 American Institute of Chemical Engineers Environ Prog, 2008