Groundwater Services, Inc.

Modeling Intrinsic Remediation With Multiple Electron Acceptors: Results From Seven Sites

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Courtesy of Groundwater Services, Inc.

Recent results from the Air Force Center for Environmental Excellence's (AFCEE) Natural Attenuation Initiative indicate that anaerobic processes dominate the biodegradation of dissolved petroleum hydrocarbons during intrinsic remediation (natural attenuation) of contaminated aquifers. In particular, sulfate reduction and methanogenesis appear to be the major sinks for BTEX compounds at seven AFCEE Natural Attenuation sites. Oxygen and nitrate serve as minor electron acceptors while ferric hydroxide utilization appears to be relatively insignificant.

Data from the seven AFCEE sites are currently being analyzed to determine the best method to simulate natural attenuation with multiple electron acceptors. The seven sites represent a variety of geographic locations (Alaska to Florida) and hydrogeologic settings (both high and low plume length/seepage velocity ratios). Two intrinsic remediation models are being developed, the BIOPLUME III numerical model and BIOSCREEN, a simple spreadsheet-based analytical modeling system. BIOSCREEN has been used to compare two different ways to simulate the kinetics of intrinsic remediation: 1) as a first-order decay expression and 2) as an instantaneous reaction between dissolved hydrocarbons and available electron acceptors.

The BIOSCREEN results indicate that both the first-order decay and instantaneous reaction models can be calibrated to simulate the observed BTEX concentrations in the field. The instantaneous reaction model provides a much closer match to the observed utilization of electron acceptors, however.

These results suggest that 1) the instantaneous reaction model is a better approach to simulating intrinsic remediation than a first-order decay model, and 2) the rate of actual BTEX dissolution from source zones may be much higher than the rate predicted by intrinsic remediation models that do not consider high biodegradation rates in the source zone. Methods for estimating plume lifetimes using the BIOSCREEN model will be presented.

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