10.1021/acs.est.5b03671.s005 James R. Degnan James R. Degnan J. K. Böhlke J. K. Böhlke Krystle Pelham Krystle Pelham David M. Langlais David M. Langlais Gregory J. Walsh Gregory J. Walsh Identification of Groundwater Nitrate Contamination from Explosives Used in Road Construction: Isotopic, Chemical, and Hydrologic Evidence American Chemical Society 2016 source highway construction site breakthrough δ15 N identification groundwater contamination mass balance calculations unexploded NH 4NO denitrification biogenic concentration Groundwater Nitrate Contamination data δ18 2016-01-19 00:00:00 Dataset https://acs.figshare.com/articles/dataset/Identification_of_Groundwater_Nitrate_Contamination_from_Explosives_Used_in_Road_Construction_Isotopic_Chemical_and_Hydrologic_Evidence/2086213 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.