Theme: 10. Characterization, Fate and Transport

Topic Area: 10e. Groundwater Modeling: Advancements and Applications

Formatted Title:

Groundwater Modeling Used in Determining Likelihood of Future Endpoint Concentration Exceedance in Declining Groundwater Contaminant Levels

Background/Objectives:

Many hazardous waste sites with groundwater contamination exhibit declining contaminant concentrations at compliance monitoring wells, under both active remediation and natural attenuation treatment approaches. Typically, remedy completion is achieved once a specific number of contaminant of concern (COC) detections fall below the designated cleanup goal. However, the attainment of specified goals can be disrupted by detected concentration variations that may sporadically rise above the cleanup goal for some monitoring events. The USEPA document “An Approach for Evaluating the Progress of Natural Attenuation in Groundwater” (EPA 600/R-11/204) addresses this specific situation using actual site data, and establishes techniques for determining statistically whether remedy complete is attained. The groundwater and contaminant parameters responsible for fluctuations in groundwater concentration include variable infiltration rates, variation in soil-to-water phase distribution due to a fluctuating water table, especially at or near a contaminant smear zone, and changes in subsurface contaminant degradation rates due to natural or anthropogenic changes in reactive agents or groundwater conditions. If contaminant concentrations are close to the cleanup goal but fluctuations are substantial, then the progress towards achieving remedy complete may be disrupted. In some cases, a regulatory agency may require a responsible party to restart the attainment period over again. The ability to estimate the chances of future sporadic exceedance, based on historic site fluctuations and matching to contaminant fate and transport modeling results, would be a useful tool for hazardous waste site program managers to have at their disposal.

Approach/Activities:

Constant values for parameter infiltration rate and degradation rate are most commonly used in groundwater fate and transport modeling. The approach used here in evaluation of modeling versus summary site data is to perform hydraulic and contaminant modeling using time variation for these parameters, to match in magnitude the concentration fluctuations that result as compared to site-specific data. Comparisons are made with site data and COCs found in the literature, such as in the EPA “Evaluating the Progress ……” document cited above and in “Multiyear Temporal Changes in Chlorinated Solvent Concentrations at 23 Monitored Natural Attenuation Sites (J. Environ. Eng., 2006, 132(6): 653-663). The chlorinated aliphatic contaminants are the focus in this evaluation, since these contaminants are frequently dealt with using monitored natural attenuation at compliance well locations. Modeling is performed using MODFLOW and MT3D. The modeling parameters that were input in a time-varying manner are water infiltration magnitude and periodicity, and contaminant degradation rate associated with MNA of the COC of interest.

Results/Lessons Learned:

The extent to which fate and transport modeling can replicate the level of groundwater concentration fluctuations exhibited in site contaminant data sets will be presented as a means to validate this analysis approach. For any one particular site, if the modeling is able to match site-specific fluctuations, then the site-specific model is used for further evaluation. This evaluation entails estimating the probability that future monitoring results may sporadically exceed cleanup goals when the contaminant levels at a compliance well approach the concentration goal and a cleanup monitoring program is initiated or is in force.

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Matthew Alexander