δ15N of nitrate derived from explosive sources in a karst aquifer beneath the Ammunition Burning Ground, Crane Naval Surface Warfare Center, Indiana, USA

Military institutions involved in the production and demolition of explosives, propellants, and pyrotechnics have the potential to degrade groundwater aquifers through the addition of numerous contaminants including nitrate. A nitrate plume has been identified in a karst aquifer beneath the Ammunition Burning Ground (ABG) at the Crane Naval Surface Warfare Center, Indiana, USA. Wells located in the vicinity of surface impoundments and burn pans used for treatment of explosive materials show the highest concentrations of nitrate ranging from 11.2 to 19.6 mg l(-1) as NO3-. Little is known about the isotopic composition of nitrates originating from these processes. Eight wells within the ABG were sampled and analyzed for nitrogen isotopic composition of nitrate. An enrichment in the delta(15)N (delta(15)N = +8.9, +12.0, +13.1. and +13.5 parts per thousand) occurred at four wells located near the primary areas of disposal activities within the ABG. Four wells located near the outer limits of the ABG had delta(15)N values significantly lower than those observed in the central area of the ABG (delta(15)N = +4.0, +4.1, +4.6, and +2.0 parts per thousand). Soil samples and burn-pan ash samples were collected and analyzed for the nitrogen isotopic composition of nitrate. Three soil nitrate samples had low delta(15)N values of -1.7, -1.8. and +2.2 parts per thousand. The burn-pan ash sample produced nitrate with a delta(15)N value of +2.9 parts per thousand. The observed enrichment in delta(15)N from samples taken from wells located near the ABG has been postulated to be a result of photodegradation or biochemical modification of RDX and TNT contaminated sludges and volatilization of NH3 in storage lagoons within the ABG. (C) 1998 Elsevier Science B.V. All rights reserved.