Characterization Of A VOC Plume Migrating From Fractured Shale Into A Karst Limestone Aquifer

Delineation of the non-aqueous phase liquid (NAPL) source zone in the vicinity of an oil burn pit (OBP) at Letterkenny Army Depot (LEAD) revealed the presence of source material on both sides of a nearby groundwater divide resulting in a bilobate plume. While the northern lobe of the plume migrating from the OBP had been well defined, the southern lobe of the plume had been previously undetected. An investigation was initiated to determine whether the southern lobe of the plume had reached the Pinola Fault (900 ft downgradient), the formation contact between the Martinsburg Shale and the St. Paul Limestone, and if so, what impacts the plume was having on groundwater/surface water quality in the limestone aquifer. Dye tracing and aquifer tests had shown significant differences in groundwater velocities between the two rock types (i.e., ft/year in the shale versus \u3e2,000 ft/day in the limestone). The migration pathway and extent of the plume moving south was mapped using geoprobe/soil gas sampling, and verified by well installations. The soil gas results correlated extremely well with the travel path and extent of the contaminated groundwater within the shale. The strong soil gas/groundwater quality correlation was attributed to both shallow groundwater and slow velocities within the shale, which allowed for the accumulation of soil gas vapors. In this manner, the plume was effectively mapped down to the fault contact and the plume entry location into the limestone was identified. Volatile organic compound (VOC) sampling results to date indicate minimal impact from the plume on limestone groundwater quality. Rapid groundwater velocities and extensive karst development result in significant declines in contaminant levels a short distance from the fault. Dye tracing results from wells at the plume entry location and at numerous downgradient wells throughout the limestone basin appear to substantiate these findings