USE OF RADIUM ISOTOPES TO EVALUATE NATURALLY-OCCURRING URANIUM AND RADIUM SOURCES AND BEHAVIOR IN NEW MEXICO GROUNDWATERS: EXAMPLES FROM THREE SITES

Elevated Ra activities and U concentrations occur in some New Mexico groundwaters, and represent health hazards as Ra can be a significant contributor to the total radioactivity, and U is a nephrotoxin. Data are presented here from three contrasting sites in bedrock aquifers: (1) Wells sampled in Permian sandstones near Glorieta have elevated U (8.5 to 24.2 µg/L) and in one case, elevated Ra (up to >14 pCi/L); (2) a deep well near the Zuni Mountains has highly elevated Ra (>20 pCi/L) and low U; and (3) wells sampled near Laguna Pueblo, in the Jackpile and Brushy Basin Members of the Morrison Formation (units hosting roll-front U deposits nearby), are reducing and have low U and Ra.

Trends in Ra isotope ratios (²²⁴Ra/²²⁸Ra and ²²⁸Ra/²²⁶Ra) provide insights into the sources and mobility of U and Ra in these wells, and can influence decisions about water treatment vs. obtaining alternative water sources to meet drinking water standards. The Glorieta waters are oxic, with variable ²²⁸Ra/²²⁶Ra values, indicating a variable degree of U enrichment in the aquifer sandstones, while ²²⁴Ra/²²⁸Ra ratios close to one suggest that Ra is labile, sorbing and desorbing rapidly relative to the half-life of ²²⁴Ra (3.6 days). In reducing groundwaters near the Zuni Mountains, U concentrations are low. Ra is low in one well but highly elevated in a nearby well, and ²²⁸Ra/²²⁶Ra values are very low (<0.25), implying enrichment of U in the aquifer sediments. Although U concentrations were low in reducing groundwaters from the Laguna area, low ²²⁸Ra/²²⁶Ra values (<0.4) suggest enrichment of U relative to Th in the clastic sedimentary aquifer units. ²²⁴Ra/²²⁸Ra values are variable but overall close to 1. This study used an innovative low-cost method for ²²⁶Ra analyses. Radium was concentrated onto Mn oxide coated fibers and counted via decay products after a 20 day incubation period using an inexpensive radon-in-air detector. This technique could be used for low-cost long-term monitoring of water sources or pilot tests for radium treatment.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.