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1.
Soil and groundwater contamination are often managed by establishing on‐site cleanup targets within the context of risk assessment or risk management measures. Decision‐makers rely on modeling tools to provide insight; however, it is recognized that all models are subject to uncertainty. This case study compares suggested remediation requirements using a site‐specific numerical model and a standardized analytical tool to evaluate risk to a downgradient wetland receptor posed by on‐site chloride impacts. The base case model, calibrated to observed non‐pumping and pumping conditions, predicts a peak concentration well above regulatory criteria. Remediation scenarios are iteratively evaluated to determine a remediation design that adheres to practical site constraints, while minimizing the potential for risk to the downgradient receptor. A nonlinear uncertainty analysis is applied to each remediation scenario to stochastically evaluate the risk and find a solution that meets the site‐owner risk tolerance, which in this case required a risk‐averse solution. This approach, which couples nonlinear uncertainty analysis with a site‐specific numerical model provides an enhanced level of knowledge to foster informed decision‐making (i.e., risk‐of‐success) and also increases stakeholder confidence in the remediation design.  相似文献   

2.
A systematic hydrogeologic site characterization has been completed in a fractured rock flow system, with the objective of identifying contaminant migration and fate pathways from a historical release of 1,1,1-trichloroethane (TCA). The study integrated hydrogeologic analysis techniques such as borehole geophysical logging, pumping test analysis, and hydrochemical facies analysis to study the impact of a dense nonaqueous phase liquid (DNAPL) in a sparsely fractured crystalline bedrock. The assessment methodology can be divided into two parts: (1) characterization of the source area, where DNAPL is acting as a residual source of TCA, and (2) characterization of the downgradient plume. Reduction in DNAPL mass in the source area has resulted in significant and sustained reductions in downgradient concentrations, suggesting that remediation of fractured crystalline bedrock contaminated with DNAPL is possible and not "technically infeasible."  相似文献   

3.
The geochemical computer model PHREEQC can simulate solute transport in fractured bedrock aquifers that can be conceptualized as dual-porosity flow systems subject to one-dimensional advective-dispersive transport in the bedrock fractures and diffusive transport in the bedrock matrix. This article demonstrates how the physical characteristics of such flow systems can be parameterized for use in PHREEQC, it provides a method for minimizing numerical dispersion in PHREEQC simulations, and it compares PHREEQC simulations with results of an analytical solution. The simulations assumed a dual-porosity conceptual model involving advective-reactive-dispersive transport in the mobile zone (bedrock fracture) and diffusive-reactive transport in the immobile zone (bedrock matrix). The results from the PHREEQC dual-porosity transport model that uses a finite-difference approach showed excellent agreement compared with an analytical solution.  相似文献   

4.
A two-dimensional, transient-flow, and transport numerical model was developed to simulate in situ chemical oxidation (ISCO) of trichloroethylene and tetrachloroethylene by potassium permanganate in fractured clay. This computer model incorporates dense, nonaqueous phase liquid dissolution, reactive aquifer material, multispecies matrix diffusion, and kinetic formulations for the oxidation reactions. A sensitivity analysis for two types of parameters, hydrogeological and engineering, including matrix porosity, matrix organic carbon, fracture aperture, potassium permanganate dosage, and hydraulic gradient, was conducted. Remediation metrics investigated were the relative rebound concentrations arising from back diffusion and percent mass destroyed. No well-defined correlation was found between the magnitude of rebound concentrations during postremedy monitoring and the amount of contaminant mass destroyed during the application. Results indicate that all investigated parameters affect ISCO remediation in some form. Results indicate that when advective transport through the fracture is dominant relative to diffusive transport into the clay matrix (large System Peclet Number), permanganate is more likely to be flushed out of the system and treatment is not optimal. If the System Peclet Number is too small, indicating that diffusion into the matrix is dominant relative to advection through the fracture, permanganate does not traverse the entire fracture, leading to postremediation concentration rebound. Optimal application of ISCO requires balancing advective transport through the fracture with diffusive transport into the clay matrix.  相似文献   

5.
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6.
Refractive flow and treatment (RFT) systems are designed for passive or low-maintenance in situ ground water remediation for rock or soil of low to moderate permeability. An RFT system captures and refracts contaminated ground water and conveys it to an in situ permeable treatment zone without the need for pumping. Flow to the treatment zone is through one or more high-permeability collection cells, and flow from the treatment zone back into the adjacent native media is through one or more high-permeability dispersal cells.
Conceptual, analytical, and numerical modeling demonstrates the potential for RFT systems to be successful. Analytical modeling shows that the most important factor for this success is that RFT system components be engineered to have comparatively high hydraulic conductivities. A numerical model, capable of representing site-specific conditions, is required for actual RFT system design.  相似文献   

7.
Field characterization of a trichloroethene (TCE) source area in fractured mudstones produced a detailed understanding of the geology, contaminant distribution in fractures and the rock matrix, and hydraulic and transport properties. Groundwater flow and chemical transport modeling that synthesized the field characterization information proved critical for designing bioremediation of the source area. The planned bioremediation involved injecting emulsified vegetable oil and bacteria to enhance the naturally occurring biodegradation of TCE. The flow and transport modeling showed that injection will spread amendments widely over a zone of lower‐permeability fractures, with long residence times expected because of small velocities after injection and sorption of emulsified vegetable oil onto solids. Amendments transported out of this zone will be diluted by groundwater flux from other areas, limiting bioremediation effectiveness downgradient. At nearby pumping wells, further dilution is expected to make bioremediation effects undetectable in the pumped water. The results emphasize that in fracture‐dominated flow regimes, the extent of injected amendments cannot be conceptualized using simple homogeneous models of groundwater flow commonly adopted to design injections in unconsolidated porous media (e.g., radial diverging or dipole flow regimes). Instead, it is important to synthesize site characterization information using a groundwater flow model that includes discrete features representing high‐ and low‐permeability fractures. This type of model accounts for the highly heterogeneous hydraulic conductivity and groundwater fluxes in fractured‐rock aquifers, and facilitates designing injection strategies that target specific volumes of the aquifer and maximize the distribution of amendments over these volumes.  相似文献   

8.
Soil vapor extraction (SVE) is a prevalent remediation remedy for volatile organic compound (VOC) contaminants in the vadose zone. To support selection of an appropriate condition at which SVE may be terminated for site closure or for transition to another remedy, an evaluation is needed to determine whether vadose zone VOC contamination has been diminished sufficiently to keep groundwater concentrations below threshold values. A conceptual model for this evaluation was developed for VOC fate and transport from a vadose zone source to groundwater when vapor‐phase diffusive transport is the dominant transport process. A numerical analysis showed that, for these conditions, the groundwater concentration is controlled by a limited set of parameters, including site‐specific dimensions, vadose zone properties, and source characteristics. On the basis of these findings, a procedure was then developed for estimating groundwater concentrations using results from the three‐dimensional multiphase transport simulations for a matrix of parameter value combinations and covering a range of potential site conditions. Interpolation and scaling processes are applied to estimate groundwater concentrations at compliance (monitoring) wells for specific site conditions of interest using the data from the simulation results. The interpolation and scaling methodology using these simulation results provides a far less computationally intensive alternative to site‐specific three‐dimensional multiphase site modeling, while still allowing for parameter sensitivity and uncertainty analyses. With iterative application, the approach can be used to consider the effect of a diminishing vadose zone source over time on future groundwater concentrations. This novel approach and related simulation results have been incorporated into a user‐friendly Microsoft® Excel®‐based spreadsheet tool entitled SVEET (Soil Vapor Extraction Endstate Tool), which has been made available to the public.  相似文献   

9.
Boreholes drilled through contaminated zones in fractured rock create the potential for vertical movement of contaminated ground water between fractures. The usual assumption is that purging eliminates cross contamination; however, the results of a field study conducted in a trichloroethylene (TCE) plume in fractured sandstone with a mean matrix porosity of 13% demonstrates that matrix-diffusion effects can be strong and persistent. A deep borehole was drilled to 110 m below ground surface (mbgs) near a shallow bedrock well containing high TCE concentrations. The borehole was cored continuously to collect closely spaced samples of rock for analysis of TCE concentrations. Geophysical logging and flowmetering were conducted in the open borehole, and a removable multilevel monitoring system was installed to provide hydraulic-head and ground water samples from discrete fracture zones. The borehole was later reamed to complete a well screened from 89 to 100 mbgs; persistent TCE concentrations at this depth ranged from 2100 to 33,000 microg/L. Rock-core analyses, combined with the other types of borehole information, show that nearly all of this deep contamination was due to the lingering effects of the downward flow of dissolved TCE from shallower depths during the few days of open-hole conditions that existed prior to installation of the multilevel system. This study demonstrates that transfer of contaminant mass to the matrix by diffusion can cause severe cross contamination effects in sedimentary rocks, but these effects generally are not identified from information normally obtained in fractured-rock investigations, resulting in potential misinterpretation of site conditions.  相似文献   

10.
Diffusion of dissolved contaminants in multilayer porous media is an important phenomenon affecting both contamination and remediation in natural aqueous environments, including diffusion in groundwater aquitards and contaminated bed sediments. This study presents a new analytical solution for solute diffusion in a semi-infinite two-layer porous medium for arbitrary boundary and initial conditions. The solution was obtained by using the Green's function approach in the Laplace domain with the application of the binomial theorem to facilitate inversion back to the real time domain. Results based on this solution were found to be simple both in form and ease of calculation and to be in good agreement with those obtained using numerical calculations based on the Crank-Nicolson finite difference method. Applications of the solution are presented in the context of a contaminated groundwater aquitard to demonstrate how different boundary and initial conditions can greatly affect the contamination and decontamination of porous media, and to illustrate how diffusion modeling might be used in a forensic sense.  相似文献   

11.
The hydrogeologic framework of fractured sedimentary bedrock at the former Naval Air Warfare Center (NAWC), 1 1 For more information on the USGS Toxics Substances Hydrology Program at the Naval Air Warfare Center visit the NAWC website at http://nj.usgs.gov/nawc/
Trenton, New Jersey, a trichloroethylene (TCE)-contaminated site in the Newark Basin, is developed using an understanding of the geologic history of the strata, gamma-ray logs, and rock cores. NAWC is the newest field research site established as part of the U.S. Geological Survey Toxic Substances Hydrology Program, Department of Defense (DoD) Strategic Environmental Research and Development Program, and DoD Environmental Security Technology Certification Program to investigate contaminant remediation in fractured rock. Sedimentary bedrock at the NAWC research site comprises the Skunk Hollow, Byram, and Ewing Creek Members of the Lockatong Formation and Raven Rock Member of the Stockton Formation. Muds of the Lockatong Formation that were deposited in Van Houten cycles during the Triassic have lithified to form the bedrock that is typical of much of the Newark Basin. Four lithotypes formed from the sediments include black, carbon-rich laminated mudstone, dark-gray laminated mudstone, light-gray massive mudstone, and red massive mudstone. Diagenesis, tectonic compression, off-loading, and weathering have altered the rocks to give some strata greater hydraulic conductivity than other strata. Each stratum in the Lockatong Formation is 0.3 to 8 m thick, strikes N65°E, and dips 25° to 70°NW. The black, carbon-rich laminated mudstone tends to fracture easily, has a relatively high hydraulic conductivity and is associated with high natural gamma-ray count rates. The dark-gray laminated mudstone is less fractured and has a lower hydraulic conductivity than the black carbon-rich laminated mudstone. The light-gray and the red massive mudstones are highly indurated and tend to have the least fractures and a low hydraulic conductivity. The differences in gamma-ray count rates for different mudstones allow gamma-ray logs to be used to correlate and delineate the lithostratigraphy from multiple wells. Gamma-ray logs and rock cores were correlated to develop a 13-layer gamma-ray stratigraphy and 41-layer lithostratigraphy throughout the fractured sedimentary rock research site. Detailed hydrogeologic framework shows that black carbon-rich laminated mudstones are the most hydraulically conductive. Water-quality and aquifer-test data indicate that groundwater flow is greatest and TCE contamination is highest in the black, carbon- and clay-rich laminated mudstones. Large-scale groundwater flow at the NAWC research site can be modeled as highly anisotropic with the highest component of permeability occurring along bedding planes.  相似文献   

12.
Bair ES  Metheny MA 《Ground water》2002,40(6):657-668
Remediation of ground water and soil contamination at the Wells G & H Superfund Site, Woburn, Massachusetts, uses technologies that reflect differences in hydrogeologic settings, concentrations of volatile organic compounds (VOCs), and costs of treatment. The poorly permeable glacial materials that overlie fractured bedrock at the W.R. Grace property necessitate use of closely spaced recovery wells. Contaminated ground water is treated with hydrogen peroxide and ultraviolet (UV) oxidation. At UniFirst, a deep well completed in fractured bedrock removes contaminated ground water, which is treated by hydrogen peroxide, UV oxidation, and granular activated carbon (GAC). The remediation system at Wildwood integrates air sparging, soil-vapor extraction, and ground water pumping. Air stripping and GAC are used to treat contaminated water; GAC is used to treat contaminated air. New England Plastics (NEP) uses air sparging and soil-vapor extraction to remove VOCs from the unsaturated zone and shallow ground water. Contaminated air and water are treated using separate GAC systems. After nine years of operation at W.R. Grace and UniFirst, 30 and 786 kg, respectively, of VOCs have been removed. In three years of operation, 866 kg of VOCs have been removed at Wildwood. In 15 months of operation, 36 kg of VOCs were removed at NEP. Characterization work continues at the Olympia Nominee Trust, Whitney Barrel, Murphy Waste Oil, and Aberjona Auto Parts properties. Risk assessments are being finalized that address heavy metals in the floodplain sediments along the Aberjona River that are mobilized from the Industri-Plex Superfund Site located a few miles upstream.  相似文献   

13.
This article studies the effect of drought and pumping discharge on groundwater supplies and marine intrusion in the Korba aquifer (Cap‐Bon peninsula, Tunisia). The Groundwater Modelling System has been used to model the groundwater flow and to simulate the seawater intrusion. The calibration is based on the groundwater levels in the steady state from 1963, and in the transient state from the groundwater levels from 1963 to 2005. The main objective is to quantify the components of the groundwater mass balance and to estimate the hydraulic conductivity distribution. The impact of pumping discharge on the groundwater level evolution has been examined by two pumping scenarios P1 (no. 8420) and P2 (no. 8862) wells. The hydrodynamic modelling shows the increasing drawdowns after 14 years of pumping: 4 m in P1 well and about 5 m in P2 well below sea level. The drawdowns are accompanied by the inverse hydraulic gradient. The numerical model was used to discuss the management of the groundwater resources of Cap‐Bon. As the population continues to grow and the demand for groundwater pumping intensifies beyond the 1963 level, it can be expected that the actual extent of seawater intrusion in the future would be more severe than the model prediction. Better strategies for groundwater development and management will be necessary to protect the freshwater aquifers to the marine intrusion. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

14.
A new probabilistic remediation simulation package, PREMChlor, was used to simulate the effect of contaminant source and plume remediation at a site contaminated by trichloroethylene (TCE). First, the PREMChlor model was calibrated to the plume using a deterministic approach to represent the site conditions prior to remediation activities, which occurred in 1999. The calibrated model was then used in a probabilistic mode to conduct a simulation of the effects of field source and plume remediation activities during the period after 1999. This probabilistic simulation considers uncertainties in seven key parameters: the initial source mass and concentration, the relationship between source mass removal and source concentration, the effectiveness of the source remediation, the groundwater velocity, the background plume degradation rate, and the plume treatment effectiveness. The simulation results compare favorably with the observed data collected after 1999, and show the influence of the remediation efforts on the plume.  相似文献   

15.
The Motooka region in the Fukuoka prefecture in western Japan is a coastal area, where groundwater is utilized as the main water resource for greenhouse agriculture and domestic use. Over-exploitation of groundwater has resulted in seawater intrusion and thus in the contamination of the freshwater aquifer. Fluctuations in electric conductivities caused by such intrusion are a crucial problem, since even slight changes in electric conductivities of the water used for agricultural purposes significantly affect the crops’ growth and yield. However, no study has thus far been conducted on the electric conductivity fluctuations caused by groundwater pumping and seasonal recharge of groundwater in the Motooka region. To this end, an attempt is made in the present study to develop a numerical variable-density solute transport model and then apply it to simulate the electric conductivity fluctuations with groundwater pumping and rainwater recharge. This model is developed under a finite difference scheme, and the method of characteristics is used as the numerical technique for solving the advection term of the advection–dispersion solute transport equation. The results from this numerical model are compared with the field measurements.  相似文献   

16.
We present an analytical expression for the shear dispersion during solute transport in a coupled fracture–matrix system. The dispersion coefficient is obtained in a fracture with porous walls by taking into account an accurate boundary condition at the interface between the matrix and fracture, and the results were compared with those in a non-coupled system. The analysis presented identifies three regimes: diffusion-dominated, transition, and advection-dominated. The results showed that it is important to consider the exchange of solute between the fracture and matrix in development of the shear dispersion coefficient for the transition and advection-dominated regimes. The new dispersion coefficient is obtained by imposing the continuity of concentrations and mass fluxes along the porous walls. The resulting equivalent transport equation revealed that the effective velocity in a fracture increases while the dispersion coefficient decreases due to mass transfer between the matrix and fracture. A larger effective advection term leads to greater storage of mass in the matrix as compared with the classical double-porosity model with a non-coupled dispersion coefficient. The findings of this study can be used for modeling of tracer tests as well as fate, transport, and remediation of groundwater contaminants in fractured rocks.  相似文献   

17.
Heat as a tracer in fractured porous aquifers is more sensitive to fracture-matrix processes than a solute tracer. Temperature evolution as a function of time can be used to differentiate fracture and matrix characteristics. Experimental hot (50 °C) and cold (10 °C) water injections were performed in a weathered and fractured granite aquifer where the natural background temperature is 30 °C. The tailing of the hot and cold breakthrough curves, observed under different hydraulic conditions, was characterized in a log–log plot of time vs. normalized temperature difference, also converted to a residence time distribution (normalized). Dimensionless tail slopes close to 1.5 were observed for hot and cold breakthrough curves, compared to solute tracer tests showing slopes between 2 and 3. This stronger thermal diffusive behavior is explained by heat conduction. Using a process-based numerical model, the impact of heat conduction toward and from the porous rock matrix on groundwater heat transport was explored. Fracture aperture was adjusted depending on the actual hydraulic conditions. Water density and viscosity were considered temperature dependent. The model simulated the increase or reduction of the energy level in the fracture-matrix system and satisfactorily reproduced breakthrough curves tail slopes. This study shows the feasibility and utility of cold water tracer tests in hot fractured aquifers to boost and characterize the thermal matrix diffusion from the matrix toward the flowing groundwater in the fractures. This can be used as complementary information to solute tracer tests that are largely influenced by strong advection in the fractures.  相似文献   

18.
Risk assessment of contaminated sites is crucial for quantifying adverse impacts on human health and the environment. It also provides effective decision support for remediation and management of such sites. This study presents an integrated approach for environmental and health risk assessment of subsurface contamination through the incorporation of a multiphase multicomponent modeling system within a general risk assessment framework. The method is applied to a petroleum-contaminated site in western Canada. Three remediation scenarios with different efficiencies (0, 60, and 90%) and planning periods (10, 20, 40, 60, and 80 years later) are examined for each of the five potential land-use plans of the study site. Then three risky zones with different temporal and spatial distributions are identified based on the local environmental guidelines and the excess lifetime cancer risk criteria. The obtained results are useful for assessing potential human health effects when the groundwater is used for drinking water supply. They are also critical for evaluating environmental impacts when the groundwater is used for irrigation, stockbreeding, fish culture, or when the site remains the status quo. Moreover, the results indicate that the proposed method can effectively identify risky zones with different risk levels under various remediation actions, planning periods, and land-use patterns.  相似文献   

19.
This study presents an approach for delineating groundwater basins and estimating rates of recharge to fractured crystalline bedrock. It entailed the use of completion report data (boring logs) from 2500 domestic wells in bedrock from the Coventry Quadrangle, which is located in northeastern Connecticut and characterized by metamorphic gneiss and schist. Completion report data were digitized and imported into ArcGIS® for data analysis. The data were processed to delineate groundwater drainage basins for the fractured rock based on flow conditions and to estimate groundwater recharge to the bedrock. Results indicate that drainage basins derived from surface topography, in general, may not correspond with bedrock drainage basins due to scale. Estimates of recharge to the bedrock for the study area indicate that only a small fraction of the precipitation or the amount of water that enters the overburden recharges the rock. The approach presented here can be a useful method for water resource‐related assessments that involve fractured rock aquifers.  相似文献   

20.
Groundwater dependency is increasing globally, while millions of potentially contaminated sites are yet to be characterized for contamination levels. In particular, groundwater contamination due to light nonaqueous phase liquids (LNAPLs) continues to be a global challenge. Mathematical approaches (i.e., analytical, semi-analytical, empirical, numerical) are preferred for an initial site assessment to circumvent the high characterization costs and limited site data availability. However, the site-specific nature of contamination restricts the generalization of any single approach. Hence, the requirement is for an easy-to-use computing interface that provides site-specific data management, the selection and use of multiple-model interfaces for computing, and site characterization, with extension for the latest models as they become available. This work provides one such interface called CAST or Contamination Assessment and Site-management Tool. CAST is an open-source browser-based (online/offline) tool that provides an interface for six different analytical models (e.g., BIOSCREEN-AT), a MODFLOW based numerical model, and two empirical models (including a hybrid numerical-analytical model). Additionally, CAST includes interfaces for site data management, their evaluation, and scenario-based modeling. CAST's development is in a modular format, which simplifies the addition of new computing or data interfaces. Furthermore, the entire code-base of CAST is based on open-source (dominantly Python programming) libraries and standards. This further simplifies the modification or extension of this tool. This paper introduces CAST, its different computing, and data management interfaces and provides examples of the tool's functionality primarily for the initial evaluation of contaminated sites.  相似文献   

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