Effects of Uranium Mining and Milling on Ground Water in the Grants Mineral Belt, New Mexico

ABSTRACT Ground‐water contamination from uranium mining and milling results from the infiltration of mine, mill, and ion‐exchange plant effluents containing elevated concentrations of radium, selenium, and nitrate. Available data indicate that radium concentrations in the discharge waters of a producing mine tend to increase substantially as the ore body is developed. Whereas natural background radium concentrations are generally about several picocuries/liter (pCi/l), 100 to 150 pCi/l appear in the effluents of operating mines. The discharge of such highly contaminated mine effluents to streams and seepage from tailings ponds, creates a long‐lived source of ground‐water contamination. Seepage of mill tailings at two active mills ranges from 126,000 to 491,000 m3/yr and, to date, has contributed an estimated 2400 Curies of uranium, radium, and thorium to the ground‐water reservoir. The shallow aquifer in use and downgradient from another mill has been grossly contaminated with selenium, attributable to excessive seepage from a nearby tailings pond. Radium, selenium, nitrate and, to a lesser extent, uranium, are of most value as indicators of ground‐water contamination. Gross alpha results are not consistent indicators of radium or uranium in water, although uranium does appear to be the principal contributor of alpha activity. Accurate radium‐s226 analyses yield the most information for radiological evaluation of drinking water. To date, no adverse impacts on municipal groundwater supplies have been observed. However, industry‐sponsored environmental monitoring programs are inadequately designed and implemented, and may not define the full, long‐term impact of mining and milling operations on the ground‐water quality of the study area.