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 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.

在建筑工程中使用的炸药已被证实为地下水中NO3-污染的来源，然而直接的法医证据却相当有限。由于农业、废水处理等其他NO3-来源以及影响NO3-迁移和稳定性的水文地质因素，识别与爆破相关的NO3-可能变得复杂。本研究采用水文地质学、化学分析、稳定同位素和物质平衡计算等方法，评估了位于新罕布什尔州一处有爆破记录的高速公路建设场地周边的地下水NO3-来源和迁移情况。研究结果揭示了地下水对污染的不同响应：（1）氧化性地下水溶解未爆炸的NH4NO3爆破剂后，合成NO3-（低δ15N，高δ18O）的快速突破和冲洗；（2）部分反硝化作用下的合成NO3-（高δ15N，高δ18O）的延迟和降低突破；（3）由NH4+硝化产生的与爆破相关的生物来源NO3-（低δ15N，低δ18O）的相对持久浓度；（4）与化粪池补给一致（高δ15N，低δ18O），受反硝化作用影响的生物来源NO3-浓度在空间上稳定但存在变异性。通过溶解气体数据（氩气、氮气）和与反硝化作用相关的同位素分馏趋势（Δδ15N/Δδ18O ≈ 1.31），重建了反硝化样品的来源特征。本研究的方法和数据预计可应用于研究其他受建筑工程中使用的炸药影响的含水层。