The contamination of groundwater in the United States by perchlorate (ClO4-) has become an issue of national concern, as it is estimated that the drinking water of millions of people may be impacted. New evidence suggests that, in addition to man-made sources, the past application of Chilean nitrate fertilizer as well as natural atmospheric and mineral sources of perchlorate in the United States may result in groundwater and drinking water contamination (SERDP project ER-1435). The isotopic compositions of both chlorine and oxygen in the perchlorate molecule have been observed to differ significantly and consistently among natural perchlorate in Chilean caliche fertilizer, natural indigenous perchlorate, and man-made perchlorate from numerous sources (ESTCP project ER-200509). The objective of this study was to evaluate the use of isotopic data to distinguish sources of ClO4- in groundwater in a specific region of the Rialto-Colton and Chino, California, groundwater subbasins (Study Area).
This project was a collaborative effort among scientists at CB&I Federal Services, LLC in Lawrenceville, New Jersey, the University of Delaware in Newark, Delaware, and the U.S. Geological Survey (USGS) in San Diego, California and Reston, Virginia. During this study, perchlorate was collected from groundwater wells in the Study Area, which includes two groundwater ClO4- plumes emanating from known military/industrial source areas, and a larger area outside of these plumes having measurable ClO4-. Perchlorate extracted from these wells was analyzed for chlorine and oxygen stable isotope ratios (δ37Cl, δ18O, δ17O) and radioactive chlorine-36 (36Cl) isotopic abundance, along with other geochemical, isotopic, and hydrogeologic parameters.
Isotope data indicate synthetic, Atacama, and indigenous natural ClO4- are present in the Study Area. Stable isotope data from nearly all sampled wells within the contours of the two characterized plumes, including those located in a perched zone and within the regional groundwater aquifer, were consistent with a dominant synthetic ClO4- source. In wells downgradient from the synthetic plumes and in the Chino subbasin to the southwest, isotopic data indicate the dominant source of ClO4- largely was Atacama, presumably from historical application of nitrate fertilizer in this region. Past agricultural land use and historical records are consistent with this source being present in groundwater. The 36Cl and δ18O data indicate that wells having largely synthetic or Atacama ClO4- also commonly contained small fractions of indigenous natural ClO4-. The indigenous ClO4- was most evident in wells having the lowest overall ClO4- concentrations (< 1 µg/L), consistent with its occurrence as a low-level background constituent in the region. The stable isotope and 36Cl data provided relatively unambiguous discrimination of synthetic and Atacama ClO4- sources in most wells having relatively high concentrations, providing regional perspective on anthropogenic ClO4- contamination in the Rialto-Colton and Chino subbasins. Where indigenous natural ClO4- was indicated as a substantial component, total ClO4- concentrations were low and concentrations of anthropogenic components were near background levels.
Isotopic analysis can be used as a forensic technique to evaluate whether perchlorate in groundwater is from natural or anthropogenic sources. This technique is useful for site assessment and source attribution.