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

• Published in: Environmental science & technology 2016-01, Vol.50 (2), p.593-603
• Main Authors: Degnan, James R, Böhlke, J. K, Pelham, Krystle, Langlais, David M, Walsh, Gregory J
• Format: Article
• Language: English
• Subjects: Construction Industry; Contamination; Environmental Monitoring; Explosive Agents - analysis; Explosives; Groundwater; Groundwater - analysis; Highway construction; Hydrology; New Hampshire; Nitrates; Nitrates - analysis; Nitrogen Isotopes - analysis; Oxygen Isotopes - analysis; Transportation; Water Pollutants, Chemical - analysis
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/acs.est.5b03671

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.