fate and transport modeling memorandumflow and contaminant transport models. the groundwater model...

Sherwin-Williams Gibbsboro, New Jersey

Fate and Transport Modeling Memorandum Introduction A three-dimensional (3D) numerical groundwater flow and transport model will be developed for the Sherwin-Williams site in Gibbsboro, New Jersey, to evaluate site conditions and potential remedial scenarios. The model will represent subsurface conditions from the ground surface to the low permeable clay layer, which is located at approximately 100 ft below ground surface (bgs). The available geologic, hydrogeologic, and contaminant data will be incorporated into a 3D geospatial model, which will be used to accurately develop the 3D numerical groundwater flow and contaminant transport models. The groundwater model will be developed using industry-accepted modeling protocols, such as those outlined by the American Society for Testing and Materials (ASTM, 1993) the United States Environmental Protection Agency (EPA, 1994), N.J.A.C. 7:26E-4.4(h)3iv and other industry standard publications such as Applied Groundwater Modeling (Anderson and Woessner, 1991). State of the science software will be used for the modeling and pre- and post-processing of the data and modeling results. It is anticipated that the United States Geologic Survey (USGS) model MODFLOW (McDonald and Harbaugh, 1988), which is a widely used and validated modeling package that can simulate complex 3D conditions, will be used for the groundwater flow modeling. The transport modeling will be done using either RT3D (Clement, 1998) or MODFLOW-SURFACT ‘99 (Hydrogeologic, Inc., 1999), both of which are widely used and validated transport models. The groundwater modeling process that will be followed for this project is summarized in the flow chart shown in Figure 1. Following model calibration and sensitivity analysis, the ground water flow model, along with the calibration and sensitivity analysis results, is to be submitted in a report format for review. This modeling process is based on modeling flow charts provided by the EPA, ASTM, and Anderson and Woessner (1991), and was customized for the Sherwin-Williams project. A discussion of each of the modeling process steps is provided below. Conceptual Model Preparation and Data Collection The first and most important step of the modeling process is to clearly define the objectives of the modeling effort. The objectives of this effort currently include the following:

• Develop a 3D geospatial model that numerically represents the geologic conditions and the distribution of selected contaminants at the site.

• Develop a 3D numerical model that adequately represents the groundwater flow pathways and contaminant transport conditions at the site, incorporating the natural physical and hydraulic boundaries into the model.

L:\SHERWIN\REMEDIAL\WP2003\ModelMemo2003.doc 1

• Utilize the 3D numerical model to evaluate remediation scenarios for the site, which may include but not limited to pump and treat, reverse well technology, monitored natural attenuation, containment, permeable and impermeable barrier walls, drains and caps.

• The model will be used to aid in establishing a Classification Exception Area for the site, as required by N.J.A.C. 7:26E-6.2(a)17.

• The model will be used to estimate if, and when, the groundwater contamination will

reach the nearest downgradient receptor, as described in N.J.A.C. 7:26E-6.3(d)5.

• The model will be used to aid in estimating the fate of the groundwater plume, as required by N.J.A.C. 7:26E-6.3(d)6.

• The model will be used to aid in establishing monitoring and performance requirements

for either active or passive groundwater remediation, as set forth in N.J.A.C. 7:26E-6.3 et. seq.

• To the extent possible, the model will be used to evaluate the environmental impact of the

historical lagoons. • Provide written documentation of the modeling effort and electronic copies of the model

input and output files. Every subsequent step of this modeling project is designed to reflect and support these project objectives. An initial conceptual model has already been developed and is presented in Section 4.0 of the Revised Work Plan. There are already more than 30 monitor wells completed in the shallow water-bearing unit and approximately 12 additional wells completed in the deep water-bearing unit. These wells were installed as part of the investigations at The Paint Works Corporate Center (The Paint Works) and the United States Avenue Burn Site, conducted under NJDEP oversight (see Section 3.0 of the Revised Work Plan). While these wells do not cover the entire area proposed for investigations under this Work Plan, they provide a good initial definition of the expected hydrogeologic conditions (Figure 2). The data from the aforementioned wells have shown that there are two sand water-bearing units separated by a semi-confining silt unit. These water-bearing units are believed to be part of the Kirkwood-Cohansey aquifer. The bottom of the deep unit is encountered at approximately 100 ft bgs, which is underlain by a thick clay unit believed to be the ‘400-foot clay’. This thick clay unit separates the Kirkwood-Cohansey aquifer from the underlying Raritan-Magothy aquifer. The purpose of the initial conceptual model is to identify what data currently exist and where and what additional data are needed to complete a working conceptual model. The subsequent field data collection effort is then designed to collect the data needed to complete a modeling effort that fulfills the stated objectives. This data collection effort is already outlined in the project Work Plan for the Gibbsboro Sherwin-Williams facility, and includes the collection of hydraulic


conductivity, water level, geologic, contaminant distribution, and off-site pumping data. The additional data will be used to characterize the site conditions and prepare a working conceptual model. The conceptual model will identify and characterize in adequate detail hydrostratigraphic units, physical boundaries such as lakes, rivers, and low-permeable clay units, hydraulic boundaries such as groundwater divides and flow lines, and a water balance for the area surrounding the site. In addition to data collected from the site, the working conceptual model will also be based on information gained from hydrogeologic and numerical modeling studies previously conducted that include the area of this site. These published reports will include but not be limited to the following: • Martin, Mary. 1990. “Ground-Water Flow in the New Jersey Coastal Plain.” Regional

Aquifer-System Analysis, U.S. Geological Survey, Open-File Report 87-528. West Trenton, New Jersey.

• Navoy, Anthony S. “The Potomac-Raritan-Magothy Aquifer System in the Camden

Metropolitan Area: Cultural Impact on an Outcrop Area.” In the Proceedings from the Second Annual Meeting of the Geologic Association of New Jersey. U.S. Geological Survey, New Jersey District Office, Water Resources Division.

• Sloto, Ronald A. 1988. “Simulation of Ground-Water Flow in the Lower Sand Unit of the

Potomac-Raritan-Magothy Aquifer System, Philadelphia, Pennsylvania.” U.S. Geological Survey, Water-Resources Investigations Report 86-4055.

It is unlikely that these or any other reports will provide information that will be incorporated into the Sherwin-Williams model electronically. However, they do provide very useful data and information that will be used to develop the hydrostratigraphic units, establish reasonable ranges of model input parameters, and to adequately characterize offsite conditions. Information from the literature (such as the modeled behavior of the aquifer units will also be utilized to evaluate the selection of equations that will be implemented in the selected numerical modeling codes. Numerical Model Set Up and Calibration The working conceptual model will be used to set up and calibrate the numerical models. This process includes designing the grid, model layers, internal and perimeter boundary conditions, and preliminary selection of input parameters. The site location, study area, and the estimated numerical model extent are shown on Figure 3. This figure also shows the location of some of the important surface water bodies and offsite supply wells. Important surface water bodies, including Millard Creek, Haney Run, Woodland Lake, and others, will be simulated in the numerical model using either the MODFLOW river package or constant head nodes, depending on their relative depth and influence on the adjacent water bearing unit. At the present time, groundwater monitoring well data exist for The Paint Works and the United States Avenue Burn Site. The installation of additional shallow and deep wells is proposed for


the other sites addressed under this Administrative Order on Consent (AOC). Data and information from all the new and existing wells will be utilized to set up and run the model. The model extent shown on Figure 2 is for illustrative purposes, as the proposed investigations have not yet been implemented, and the extent of the area that will need to be evaluated has not been accurately determined. Conceptually, an area extending approximately 2 miles around The Paint Works (the approximate geometric center of the sites under this AOC) is proposed. Important pumping wells will be incorporated into the model using the well package. The model top boundary will be the water table and the bottom boundary will be the basal clay layer represented by a no-flow boundary condition in MODFLOW. The perimeter model boundaries will be located at natural hydraulic boundaries coincident with groundwater divides and flow lines. It is anticipated that different boundaries will be selected for the shallow and deep zones. Sensitivity analysis runs will be initiated to ensure that the model perimeter boundaries are located sufficiently far enough away from the internal pumping stresses so they don’t inappropriately affect the model predictions. The model grid will be designed to have small nodes (approximately 20 ft by 20 ft) around pumping wells and larger nodes (approximately 200 ft by 200 ft) at the outer edges of the model. The model will consist of at least three layers representing the shallow sand, the silt semi-confining unit, and the deep sand unit. Once the model grid and boundary conditions are established the model will be calibrated to the available data. The calibration process will follow a structured approach such as is outlined by Yeh and Mock (May 1995). In general, this process consists of starting with an effective transmissivity for the model layers and then fine-tuning the boundary condition parameters, such as recharge, so that the residual is minimized. After the ‘correct’ boundary conditions and recharge rates are selected, the transmissivity/hydraulic conductivity distribution is fine-tuned through interpolation and extrapolation of the available data (Yeh and Mock, 1995). The calibrated model will be verified by simulating pumping stresses such as aquifer pump tests that have been conducted in the field, and comparing the model predictions to the actual field test results. The groundwater flow model grid, layering, boundary conditions, and flow simulations will be directly incorporated into the contaminant transport model. The appropriate input parameters will be input into the transport model, including the distribution coefficient, soil bulk density, effective porosity, dispersivity, and, if appropriate, degradation rates. Because insufficient historical contaminant data exist to calibrate the transport model, the currently observed contaminant plumes will be input into the model as initial conditions and the transport model runs will simulate contaminant movement over time beginning with current conditions. A sensitivity analysis will be conducted on the groundwater flow and transport models to evaluate the uncertainty in the model caused by uncertainty in the estimates of model input parameters, stresses, and boundary conditions (ASTM, 1993). Simulation of Model Prediction Scenarios Once the groundwater flow and transport models are developed they will be used to evaluate potential remediation scenarios. It is anticipated that these remediation scenarios may include


pump and treat, reverse well technology, and monitored natural attenuation. The model will be used to evaluate the relative effectiveness of each of the remediation technologies simulated. The evaluation process will include analyzing groundwater flow paths, capture zones, and effective contaminant reduction. A sensitivity analysis will also be done on some of the predictive model scenarios in order to evaluate the effect of parameter uncertainty on the model predictions (ASTM, 1993). Presentation of Modeling Results and Evaluation of Effectiveness Written documentation of the project and electronic copies of the groundwater model input and output files will be provided. The modeling report will include a clear and thorough description of each step of the modeling process with tables and figures describing the input parameters, calibration and sensitivity analysis results, and model simulation predictions. The report will also summarize the results and conclusions of the modeling effort. It is anticipated that the modeling effort will be used to evaluate and design a remediation program for the Sherwin-Williams facility. Once the selected remediation program is implemented, the 3D numerical and geospatial models can be used to evaluate and demonstrate the effectiveness of the remediation program and also to effectively maintain and if necessary make improvements to the remediation system. Once the remediation program begins to meet its objectives the models can be used to gradually reduce and eventually shut down the remediation system. Data collected from the remediation program can be used to conduct a post-audit on the model and evaluate and validate its effectiveness.


References

Anderson, Mary P., and William W. Woessner. 1991. Applied Groundwater Modeling, Simulation of Flow and Advective Transport. Academic Press, Inc., San Diego, CA.

American Society for Testing and Materials (ASTM). Standard Guide for Application of a

Ground-Water Flow Model to a Site-Specific Problem. D 5447-93. ASTM. Standard Guide for Comparing Ground-Water Flow Model Simulations to Site-Specific

Information” D5490-93. ASTM. Standard Guide for Defining Boundary Conditions in Ground-Water Flow Modeling.

D5609-94. ASTM. Standard Guide for Defining Initial Conditions in Ground-Water Flow Modeling.

D5610-94. ASTM. Standard Guide for Conducting a Sensitivity Analysis for a Ground-Water Flow Model

Application. D5611-94. Jim Yeh, T.-C., and Peter A. Mock. 1995. “A Structured Approach for Calibrating Steady-State

Ground-Water Flow Models.” Ground-Water, Vol. 24. No. 2. Martin, Mary. 1990. “Ground-Water Flow in the New Jersey Coastal Plain.” Regional Aquifer-

System Analysis, U.S. Geological Survey, Open-File Report 87-528. West Trenton, New Jersey.

Navoy, Anthony S. “The Potomac-Raritan-Magothy Aquifer System in the Camden

Metropolitan Area: Cultural Impact on an Outcrop Area.” In the Proceedings from the Second Annual Meeting of the Geologic Association of New Jersey. U.S. Geological Survey, New Jersey District Office, Water Resources Division.

Sloto, Ronald A. 1988. “Simulation of Ground-Water Flow in the Lower Sand Unit of the

Potomac-Raritan-Magothy Aquifer System, Philadelphia, Pennsylvania.” U.S. Geological Survey, Water-Resources Investigations Report 86-4055.

U.S. EPA. 1994. Ground-Water Modeling Compendium. Second Edition. Office of Solid Waste

and Emergency Response, Washington, D.C. EPA/500/B-94/004.


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