Estimates of groundwater age from till and carbonate bedrock hydrogeologic units at Jefferson Proving Ground, Southeastern Indiana, 2007-08

During 2007-08, the U.S. Geological Survey, in cooperation with the U.S. Department of the Army, conducted a study to evaluate the relative age of groundwater in Pre-Wisconsinan till and underlying shallow and deep carbonate bedrock units in and near an area at Jefferson Proving Ground (JPG), southeastern Indiana, which was used during 1984-94 to test fire depleted uranium (DU) penetrators. The shallow carbonate unit includes about the upper 40 feet of bedrock below the bedrock-till surface; the deeper carbonate unit includes wells completed at greater depth. Samples collected during April 2008 from 15 wells were analyzed for field water-quality parameters, dissolved gases, tritium, and chlorofluorocarbon (CFC) compounds; samples from 14 additional wells were analyzed for tritium only. Water-level gradients in the Pre-Wisconsinan till and the shallow carbonate unit were from topographically higher areas toward Big Creek and Middle Fork Creek, and their tributaries. Vertical gradients were strongly downward from the shallow carbonate unit toward the deep carbonate unit at 3 of 4 paired wells where water levels recovered after development; indicating the general lack of flow between the two units. The lack of post development recovery of water levels at 4 other wells in the deep carbonate unit indicate that parts of that unit have no appreciable permeability. CFC and tritium-based age dates of Pre-Wisconsinan till groundwater are consistent with infiltration of younger (typically post-1960 age) recharge that 'mixes' with older recharge from less permeable or less interconnected strata. Part of the recharge to three till wells dated from the early to mid-1980s (JPG-DU-03O, JPG-DU-09O, and JPG-DU-10O). Age dates of young recharge in water from two till wells predated 1980 (JPG-DU-04O and JPG-DU-06O). Tritium-based age dates of water from seven other till wells indicated post-1972 age recharge. Most wells in the Pre-Wisconsinan till have the potential to produce groundwater that partially was recharged during or after DU penetrator testing; their water quality can indicate the presence of DU-related contaminants. The shallow carbonate unit near Big Creek is a karst flow system that may be recharged in part from areas with smaller thicknesses of overlying till or through more permeable parts of the till. This is indicated by CFC- and tritium-based piston-flow (non-mixing) model age dates of early-1980s for water from JPG-DU-02I, similar tritium-based ages of water produced from nearby wells MW-5 and MW-11, and cave development along the creek. The CFC and tritium-based age dates indicate that water samples from JPG-DU-01I and JPG-DU-03I were best described as mixtures of post-1984 modern recharge and submodern (1953 or older) recharge. These five wells produced groundwater that was recharged, at least partially, during or after DU-penetrator testing and are within or downgradient from the DU Impact Area with respect to groundwater flow directions inferred from water-level contours. Wells with groundwater age dates that are near to or after the onset (1984) of DU penetrator testing and that have a plausible connection to a contaminant source can be used to indicate the presence or absence of contaminants from DU penetrator or DU-related corrosion products in groundwater. Groundwater-age dates indicate that the ages of recharge sampled from shallow carbonate unit wells JPG-DU-04I, JPG-DU-05I, JPG-DU-06I, JPG-DU-09I, and JPG-DU-10D in easternmost (upgradient) and southernmost wells in the shallow carbonate unit are submodern (1953 or older) and predate the DU testing by at least 30 or more years. Water-quality data from these five wells are not likely to represent effects from DU-projectile testing or corrosion for years. Well JPG-DU-09D in the deep carbonate unit produced groundwater samples with a submodern (1953 or older) age date. The slow recovery of water levels in most wells in the deep carbonate unit is consis