Use of Chemical and Isotopic Signatures to Distinguish Between Uranium Mill‐Related and Naturally Occurring Groundwater Constituents

Abstract

AbstractChemical and isotopic signatures were determined in groundwater samples to aid in distinguishing the source of contamination in three desert arroyos and a buried channel (the swale) near Shiprock, New Mexico. The contamination in the swale and one of the arroyos, Many Devils Wash, was previously attributed to a former uranium mill site because of the similar suite of contaminants (nitrate, selenium, sulfate, and uranium) and the close (0.8 km) proximity. The other two arroyos are far removed from the mill site and could not have received contamination from it. Principal component and cluster analysis indicated similarities in groundwater chemistry among the swale and the three arroyos that contrasted with groundwater chemistry at the disposal cell. Disposal cell groundwater is characterized by high uranium and bicarbonate concentrations, whereas that in remaining study areas is characterized by high sodium and sulfate, but lower uranium concentrations. Mancos Shale forms the bedrock in the region and contains elevated concentrations of the same chemical constituents that appear in the swale and arroyo groundwater. Dissolved sulfate in arroyo groundwater was depleted in sulfur‐34, in contrast to mill‐derived sulfate with more enriched sulfur‐34. Uranium‐234 to uranium‐238 activity ratios (ARs) were near the secular equilibrium value of 1 in mill site groundwater, whereas ARs in all arroyo groundwater samples exceeded 2. Elevated tritium activities present in mill site groundwater (49 to 142 pCi/L) are attributed to the mill being operated during atomic bomb testing in the 1950s and 1960s. The combined chemical and isotopic results indicate that groundwater in Many Devils Wash and the swale was likely derived from the Mancos Shale and not from the milling operation.