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Models created for remediating groundwater contamination.

Where do you start when you have to remediate a 40-year-old groundwater contamination site, and you have little or no knowledge of the site's geology or extent of contamination, and have limited financial resources?

In the past, clean-up crews performed some cursory surface remediation and roped the area off, or walked away from it for lack of any reliable informational tools.

I and other researchers at Battelle Memorial Institute, Columbus, Ohio, have addressed the decision-making part of this problem by developing a software package, 3D Subsystem, that characterizes the subsurface contamination of soil and groundwater based on collected data, creates 3-D groundwater flow and solute transport models, and provides input to a management decision process.

The 3D Subsystem is being applied at U.S. Dept. of Defense and U.S. Dept. of Energy facilities to provide solutions for specific subsurface contamination investigations. It currently runs on a Silicon Graphics Iris workstation and can also be implemented on Sun or Digital Equipment workstations.

Site characterization

During this phase of the investigation, a large amount of data is collected, including water level data from one or more aquifer zones; the thickness and properties of various geologic layers; and chemical sampling results from both soil and groundwater.

At one site, for example, we drilled and collected samples in each water-bearing zone from nearly 250 wells. The wells were drilled to varying depths from 40 to 250 ft.

Included in the 3D Subsystem is a version of Earth Vision software by Dynamics Graphics Inc., Alameda, Calif. This segment is used to integrate site characterization data by translating all data types into a uniform coordinate system. The data are then used to produce a 3-D image of the site features.

These images give engineers and project managers a full 3-D interactive visualization and analysis of the data from any vantage point. The database used to generate the images contains surface topography, unsaturated zone water table elevations, the location of groundwater samples taken for chemical analysis, and the results of the lab analyses of contaminant concentrations.

Contaminant concentrations are represented as shells of average concentration. The analytical results for groundwater samples are passed to an algorithm that creates a 3-D spatial representation of the contaminant plume. Each shell shows the location, extent, and thickness at a particular concentration level. High levels of contamination occurring in several distinct areas often indicate multiple sources of contamination.

Models

The unifying tool in subsurface investigations is the groundwater flow and solute transport model. This model visualizes, simulates, and performs predictive modeling of subsurface characteristics, including hydrologic features and contaminant distributions.

The flow modeling segment of our 3D Subsystem includes database programs, 3-D visualization techniques, established groundwater flow models, and fate and transport models. It uses raw site characterization data and provides direct input to risk-management and -reduction systems.

The flow model uses a modified version of the U.S. Geological Survey's groundwater code, MODFLOW, and an in-house developed 3-D solute transport code, RANDOM Walk 3D (RWLK3D).

MODFLOW is publicly available software and has been offered since 1982. It is a widely used and respected groundwater flow modeling system based on finite difference modeling techniques.

RWLK3D uses groundwater flow system output from MODFLOW to simulate contaminant transport from single or multiple simultaneous sources. It simulates the effects of molecular diffusion and mechanical dispersion, radioactive decay, biodegradation, and nonlinear sorption on the transport of selected contaminants within the groundwater flow system.

Contaminants can be non-aqueous phase liquids (NAPLs), such as trichlorethylene and gasoline; metals; or any dissolved material.

We modeled two NAPL contaminant sources at a DOD contamination site, for example, using MODFLOW and RWLK3D. The two sources were located on opposite sides of a groundwater flow divide, or mound. The site was surrounded by upper and lower aquifer zones separated by a low permeability shale layer. The 3D Subsystem's predictive tools were used to analyze the migration of the contaminant plume in time and space for a period of 15 years.

The plume on one side of the mound migrated with the local groundwater flow in the uppermost aquifer zone. However, the plume on the opposite side descended vertically to the lower aquifer zone in areas where a low permeability shale layer became thin or nonexistent.

The integration of EarthVision, MODFLOW, and RWLK-3D into the 3D Subsystem is useful for making informed environmental restoration (risk management) decisions.

The 3-D format enables the viewer--including site owners, regulatory agencies, and community action groups--to visualize the shape, extent, and concentration levels of a contaminant plume from any vantage point and to predict the change in plume configuration through movement or remediation over time.

These models are required by many regulatory agencies and can demonstrate that effective aquifer remediation or risk reduction is being performed.

Our 3D Subsystem has already been applied successfully at several contamination sites and has shown to be a highly cost-effective means of identifying and prioritizing the risks and costs of remediation solutions.

Future work in this area is directed at combining these subsystems into an environmental control system. Sub-surface modeling subsystems can be combined, for example, with surface subsystems to provide a more complete integrated solution to environmental problems.

The total system could in turn feed into an environmental decision support system where informed decisions on the best available cleanup method are made, based on the available data and analyses performed within each subsystem.

N. Joseph Gantos is a staff scientist in the Environmental Systems & Technology Div. at Battelle Memorial Institute, Columbus, Ohio.
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Author:Gantos, N. Joseph
Publication:R & D
Date:Oct 1, 1994
Words:913
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