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Geologists have known for decades that the sandstones near Santa Fe and Española contain uranium. The yellow and brown mineral coatings on sand grains will kick a Geiger counter but are not rich enough to be mined at the present time. Most water wells in the area contain detectable uranium. More than 200 private and public wells contain uranium in excess of the drinking water health standard of 30 parts-per-billion and some wells contain hundreds of ppb. The primary health concern of uranium in drinking water is kidney toxicity.
Los Alamos National Laboratory sits west of the Santa Fe-Española area, west of the Rio Grande. A New Mexico Environment Department investigation in 1995 concluded that the uranium in rocks and groundwater east of the Rio Grande, in the Santa Fe-Española area, was naturally occurring and did not come from LANL. These findings have been confirmed by a second study by NMED, LANL and three water-quality businesses in Santa Fe.
One way to distinguish between natural and manmade uranium is by testing for its different isotopes. U-234, U-235 and U-238 are naturally occurring isotopes. U-236 is a manmade isotope that does not occur in nature. The ratios of natural isotopes, and the presence or absence of U-236, can be used to fingerprint sources of uranium.