An Evaluation of Hydrogeologic Parameters on Natural Attenuation of Explosives

By

HARRELSON, D. W., and ZAKIKHANI, M.

U.S. Army Engineering Research and Development Center, Vicksburg, MS

The Louisiana Army Ammunition Plant (LAAP) is a 14,974-acre government-owned was placed on the National Priorities List in 1989 due to contamination caused by past disposal of explosives-laden wastewater in 16 unlined surface impoundment's located in an area designated as Area P.

The near surface geology at LAAP consists of Pleistocene, terraced fluvial deposits (basal gravels fining upward to clays) uncomfortably overlying Eocene, nonmarine, massive sands, silty sands, silty clays, and occasional lignitic beds. These sediments occur as fining upwards sequences of materials that were deposited as fluvial Terraces associated with the ancestral Red River of the South. Maximum thickness of these deposits is about 60 feet, but local variations are present. Immediately underlying the Terrace deposits is an effectively impermeable boundary, the Cane River Formation. This unit consists of over consolidated claystones and is not an aquifer in this area.

Data from the late 1980s indicated that the explosives contaminants from the Area P lagoons had entered the two terrace aquifers below the site. Groundwater plumes containing RDX, TNT, and 1,3,5-trinitrobenzene (TNB, a photodecomposition product of TNT) had been detected. Concentrations in the upper terrace aquifer were lower in 1994 than in 1990, suggesting an improvement in the groundwater quality since the removal of the lagoons. The groundwater contaminant plumes had not advanced very far laterally, suggesting very slow transport and the potential for attenuation.

In this presentation, environmental monitoring and assessment tools that were applied to the site will be discussed and illustrated.