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

Explosives used in construction have been implicated as sources of NO<sub>3</sub><sup>–</sup> contamination in groundwater, but direct forensic evidence is limited. Identification of blasting-related NO<sub>3</sub><sup>–</sup> can be complicated by other NO<sub>3</sub><sup>–</sup> sources, including agriculture and wastewater disposal, and by hydrogeologic factors affecting NO<sub>3</sub><sup>–</sup> transport and stability. Here we describe a study that used hydrogeology, chemistry, stable isotopes, and mass balance calculations to evaluate groundwater NO<sub>3</sub><sup>–</sup> 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 NO<sub>3</sub><sup>–</sup> (low δ<sup>15</sup>N, high δ<sup>18</sup>O) from dissolution of unexploded NH<sub>4</sub>NO<sub>3</sub> blasting agents in oxic groundwater; (2) delayed and reduced breakthrough of synthetic NO<sub>3</sub><sup>–</sup> subjected to partial denitrification (high δ<sup>15</sup>N, high δ<sup>18</sup>O); (3) relatively persistent concentrations of blasting-related biogenic NO<sub>3</sub><sup>–</sup> derived from nitrification of NH<sub>4</sub><sup>+</sup> (low δ<sup>15</sup>N, low δ<sup>18</sup>O); and (4) stable but spatially variable biogenic NO<sub>3</sub><sup>–</sup> concentrations, consistent with recharge from septic systems (high δ<sup>15</sup>N, low δ<sup>18</sup>O), variably affected by denitrification. Source characteristics of denitrified samples were reconstructed from dissolved-gas data (Ar, N<sub>2</sub>) and isotopic fractionation trends associated with denitrification (Δδ<sup>15</sup>N/Δδ<sup>18</sup>O ≈ 1.31). Methods and data from this study are expected to be applicable in studies of other aquifers affected by explosives used in construction.