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Identification of Groundwater Nitrate Contamination from Explosives Used in Road Construction: Isotopic, Chemical, and Hydrologic Evidence

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posted on 19.01.2016 by James R. Degnan, J. K. Böhlke, Krystle Pelham, David M. Langlais, Gregory J. Walsh
Explosives used in construction have been implicated as sources of NO3 contamination in groundwater, but direct forensic evidence is limited. Identification of blasting-related NO3 can be complicated by other NO3 sources, including agriculture and wastewater disposal, and by hydrogeologic factors affecting NO3 transport and stability. Here we describe a study that used hydrogeology, chemistry, stable isotopes, and mass balance calculations to evaluate groundwater NO3 sources and transport in areas surrounding a highway construction site with documented blasting in New Hampshire. Results indicate various groundwater responses to contamination: (1) rapid breakthrough and flushing of synthetic NO3 (low δ15N, high δ18O) from dissolution of unexploded NH4NO3 blasting agents in oxic groundwater; (2) delayed and reduced breakthrough of synthetic NO3 subjected to partial denitrification (high δ15N, high δ18O); (3) relatively persistent concentrations of blasting-related biogenic NO3 derived from nitrification of NH4+ (low δ15N, low δ18O); and (4) stable but spatially variable biogenic NO3 concentrations, consistent with recharge from septic systems (high δ15N, low δ18O), variably affected by denitrification. Source characteristics of denitrified samples were reconstructed from dissolved-gas data (Ar, N2) and isotopic fractionation trends associated with denitrification (Δδ15N/Δδ18O ≈ 1.31). Methods and data from this study are expected to be applicable in studies of other aquifers affected by explosives used in construction.