14 Geologic Hazards of the Southeastern U.S. Region

Thursday, 29 March 2007
1:20 PM - 5:20 PM

NITROAROMATIC EXPLOSIVES DECAY TRENDS IN GROUNDWATER

Paper number: 14-1
Program: THEME SESSIONS
Time: 1:20 PM – 1:40 PM

Authors

Abstract

Since the end of World War II, the existence of selective explosive contaminants like RDX, HMX, Freon 113, primarily found in soil and groundwater of various operational and non-operational sites has become a growing concern for human health. The explosive contaminants by parts, decay (reduction and breakdown of mass) through the process of natural bacterial degradation (biodegradation) over time and in some cases, are forced to biodegrade by method of enhanced remediation technique. To support remedy evaluation and selection for residuum and bedrock groundwater, statistically significant trends (increasing and decreasing) were identified using non-parametric statistical tests. 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 the wells for statistical analysis, and this provided focus and defensibility to calculate concentration rate changes for 2,4,6 TNT, 2 NT, 4 NT, and DNT.

Time series plots of concentrations for these nitro aromatic explosives from wells that demonstrated significant trends were then prepared, and a simple linear regression model using a least-squares fit line to the data was used to calculate the constant rate change in terms of micrograms/Liter-yr per unit concentration. The resulting data is being used in evaluation of the remedial alternatives for both the residuum and bedrock groundwater.

Based on the current and historical data of a former Army Ammunition Plant, Time Series plots coupled with an appropriate statistical methods were applied to quantify the trend in an individual sample set. Statistics was judged against alpha value of significance. Variance between time and concentration was computed assuming the linear correlation model, based on the least squares fits to the data. For each contaminant, selective regions showing either a clear decreasing trend or undetermined but stable trend with at least 80 % confidence level were further considered to obtain an average decay rate, a key identifier to further compute the time to achieve the cleanup goal, using principal of Fundamental Age equation.