Examination of dissolved uranium concentrations in regional shallow groundwater relative to Operable Unit 8 of the Denver Radium Superfund Site

A radium industry existed between about 1914 and 1920 in Denver, Colorado, with operations located along the South Platte River. Sites associated with that industry were contaminated with radium and uranium processing residues and were incorporated into clean-up efforts as Operating Units (OUs) of the Denver Radium Superfund Site. Concentrations of uranium exceeding the U.S. Environmental Protection Agency maximum contaminant level of 0.03 milligrams per liter for drinking water are present in shallow groundwater at OU8. However, previous studies have shown concentrations of dissolved uranium can be naturally high in shallow groundwater of the South Platte River valley compared to other rivers of the world. This report compares dissolved uranium concentrations measured by the U.S. Geological Survey across the South Platte River valley to data collected at the OU8 of the Denver Radium Superfund Site. The U.S. Geological Survey data represent 5 distinct urban or agricultural geographic areas and included 230 sampling events at 114 wells during 1993 to 2013. The OU8 data represent 13 wells and groundwater discharge locations sampled during the years 2017 and 2018. Dissolved uranium concentrations were statistically significantly greater for both years of the OU8 data compared to three datasets from shallow groundwater beneath urban areas in the Denver metropolitan area. However, compared to OU8, concentrations were significantly greater in shallow groundwater from an agricultural area of the South Platte River valley distant from Denver. Additionally, each of the urban area datasets contained some individual dissolved uranium concentrations greater than the greatest concentrations from the two OU8 datasets. Thus, naturally occurring concentrations of dissolved uranium in shallow groundwater that are greater than those observed at OU8 are not uncommon in the South Platte River valley.