Monday, February 12, 2007
6:00 - 7:30 pm
Use of Trend Analysis and Linear Regression Modeling
to Derive Proportional Rate Change of Nitro Aromatic Explosives
in Groundwater: Application to Remedial Alternatives Evaluations
Moshiur Rahman (presenter),
Brian Caldwell, and Frank Bogle
Tetra Tech, Inc.
Oak Ridge, Tennessee
Since the end of World War II, the existence of explosive contaminants such as TNT, RDX, and their metabolites in soil and groundwater at a number of operational and non-operational sites has become a growing concern for protection of human health and the environment. The United States Environmental Protection Agency (EPA) is working with various organizations to identify and remediate such sites to acceptable concentrations. As with other organic contaminants in dissolved phase, these types of explosive contaminants decay (i.e., transformation to daughter products and/or direct reduction of mass) through the process of natural bacterial degradation (biodegradation) over time. In some cases, in direct response to accelerated remedial goals or site geology, the degradation process is facilitated through in-situ introduction of bacterial amendment. Remedial alternatives evaluations should incorporate the often extensive existing data to determine the optimal approach. At an example site in the Valley and Ridge of the southeastern US, both a residuum mantle and fractured bedrock are impacted by nitro-aromatic explosives; because of the co-occurrence of species, DNT was used as the target COC. Concentrations in the saturated residuum indicate the presence of product phase, and this has been released to underlying bedrock groundwater at rates consistent with calculated infiltration rates and partition coefficients. Specific criteria were used to screen DNT results for statistical trend analysis, and this provided focus and defensibility to calculate concentration rate changes for DNT. Time series plots of log-transformed concentrations from wells that demonstrated significant trends were then prepared, and a linear regression model using a least-squares fit line to the data was used to calculate the proportional rate change in terms of micrograms/liter-yr per unit concentration. Spatial analysis of the data indicates that the source mass is in large part attenuating naturally and does narrow remedial considerations. The resulting data are being used as a tool in the evaluation of the remedial alternatives for both the residuum and bedrock groundwater.
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