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.