HYDROGEOCHEMICAL INVESTIGATION OF SANTA FE COUNTY, NEW MEXICO, USA

Groundwater within Santa Fe County, NM is relatively oxidizing with variable concentrations of total dissolved solids (TDS) and trace elements including uranium. From June 23 through July 2, 2009, we collected groundwater samples from 428 domestic wells throughout the Santa Fe region. Groundwater samples were analyzed at Los Alamos National Laboratory for major cations and anions, trace elements, and stable isotopes of hydrogen and oxygen. Initial results from this sampling effort and analysis of previous work are presented.  Oxygen isotope ratios from hydrothermal springs east of Nambe, NM are heavier than the local meteoric water line suggesting enhanced water-rock interaction at these sites.  Investigation of the source and distribution of uranium were evaluated using GIS, geochemical modeling, and data analysis. Concentrations of natural uranium range from less than 1 mg/L to a maximum of 1820 mg/L. Wells with elevated concentrations of uranium are typically less than 61 meters in depth. Calculated correlation coefficients show that natural uranium is variably correlated to sulfate, sodium, lithium, arsenic, and TDS, and is negatively correlated with calcium, magnesium, and barium. This suggests that cation exchange is an important process for mobilizing uranium (VI) within aquifer systems. Isotope signatures of uranium in groundwater are consistent with natural sources including small, laterally discontinuous, roll-front uranium deposits in the Tesuque Formation, devitrification of local volcanic ash, weathering of Precambrian rocks in the Sangre de Cristo Mountains, and hydrothermal waters. Between Santa Fe and Española, approximately 50% of the wells have uranium concentrations exceeding EPA's drinking water standard of 30 µg/L. High concentrations of uranium in drinking water can cause kidney toxicity, especially to persons with diabetes.