EBTAG Annual Workshop and Field Trip
May 13, 2014

¹N.M. Environment Department, PO Box 5469, 1190 St. Francis Drive, Santa Fe, NM, 87502, dennis.mcquillan@state.nm.us

²Good Water Company, 933 Baca.St., Santa Fe, NM, 87505

³Los Alamos National Laboratory, PO Box 1663, MS C333, Los Alamos, NM, 87544

⁴Indepth Water Testing, 2019 Galisteo St., Suite G-2, Santa Fe, NM, 87505

⁵N.M. Bureau of Geology and Mineral Resources, 801 Leroy Pl., Socorro, NM, 87801

Low-grade uranium mineralization in the Tesuque Formation east of the Rio Grande between Española and Santa Fe was discovered in 1954, and a small amount of uranium was produced from the San Jose mining district in 1957. Carnotite, schroekingerite, and autunite occur in sandstones and conglomerates, often associated with clay galls, carbonaceous material, chert, opal and fossil bone fragments. Uranium mineralization also occurs in some pegmatites that have intruded Proterozoic rocks in the Sangre de Cristo Mountains. Within this mineralized area, elevated levels of uranium (up to 74 pCi/L = 110 µg/L) were discovered in several public water supply systems during a statewide testing initiative conducted in 1975-79. Extensive testing of private domestic water wells began in 1995 and approximately half of all wells tested in this area, to date, produced water with uranium in excess of the 30 µg/L human health standard. A significant number of wells had uranium concentrations greater than 100 µg/L, with one well measuring 1,820 µg/L. High levels of uranium in drinking water pose a risk of kidney toxicity. Some area residents have installed treatment devices to reduce uranium concentrations produced by public and private water supply wells.

Since industrial sources of uranium also exist in the Española Basin, studies were performed to determine the isotopic signatures of the uranium in groundwater. U-234 to U-238 ratios were determined for samples collected from 52 water supply wells located in the mineralized area east of the Rio Grande in 1995, and were determined to be consistent with natural uranium. An additional 13 wells in the mineralized zone were sampled in 2012 for isotopic analysis by three different laboratories. Ratios of U-234 to U-238 were similar to the naturally occurring ratios detected in the 1995 study. Ratios of U-235 to U-238 also were consistent with natural geologic sources. Manmade U-236 was not detected in any well water sample.

Uranium mineral zones have not been found in the Española Basin west of the Rio Grande, and groundwater uranium concentrations are much lower in this area. Uranium originating from historical waste discharges at Los Alamos National Laboratory (LANL) has been detected in monitoring wells drilled into shallow groundwater in canyons near the lab. This industrial uranium sometimes has an enriched ratio of U-235 to U-238, and detectable U-236, but total uranium concentrations are less than 10 µg/L. No public or private water supply wells have been impacted by uranium originating from LANL effluent.

In summary, naturally occurring uranium, sometimes far in excess of the 30 µg/L health standard, has impacted numerous water supply wells east of the Rio Grande between Española and Santa Fe. This uranium originates from natural mineralization that occurs widely in basin fill and Proterozoic rocks. Anthropogenic uranium has been detected in shallow observation wells near LANL, but this uranium does not exceed the health standard, has not contaminated any water supply wells, and there is no indication it migrated east of the Rio Grande.