­­ISOTOPIC FINGERPRINTING OF NITRATE SOURCES IN GROUNDWATER NEAR HIGHWAY BLASTING SITES

Explosives used in blasting operations can potentially contaminate groundwater with nitrate; however, other natural and anthropogenic sources of nitrate make identification of blasting impacts difficult. In 2013, the U.S. Geological Survey and the New Hampshire Department of Transportation (NHDOT) began a study to determine the source and fate of nitrogen compounds near blasting sites using a combination of isotopic, geochemical, hydrologic, and geologic data. Approximately 1.75 million cubic yards of rock will be removed by blasting in several locations for roadway construction at a major highway construction site in southern NH. Sources of nitrate and other nitrogen compounds can potentially be determined through analysis of isotopic ratios of light stable elements. Common sources of groundwater nitrogen concentrations that may impart different isotopic signatures include septic systems, animal wastes, agricultural fertilizers, decomposing vegetation, landfill leachate, and explosives.

Isotope ratios of nitrogen and oxygen in nitrate were used to identify sources of nitrate concentrations in groundwater from wells near the blasting sites. Wells near a rock excavation site where blasting was used shortly after the start of this study and wells with existing persistent nitrate contamination suspected to be the result of septic and past blasting were targeted for temporal sampling and analysis in an attempt to characterize nitrate sources.

In general results show a low d15N signature from synthetic nitrate sources (including explosives) and a high d15N signature from waste sources. Results also indicate that nitrate pulses in wells following blasting events can be distinguished isotopically from other local sources, and that reducing conditions in this geologic setting locally cause denitrification, resulting in lower nitrate concentrations. Transport and persistence of nitrate due to blasting operations and other nitrogen sources in fractured rock environments will be presented.