Identification of groundwater nitrate contamination from explosives used in road construction: Isotopic, chemical, and hydrologic evidence

Abstract

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