PRELIMINARY INTERPRETATIONS OF GROUND WATER SAMPLE GEOCHEMICAL RESULTS, SAN MATEO CREEK AREA, GRANTS MINING DISTRICT, NEW MEXICO

In New Mexico, the Grants Mining District (GMD) produced more uranium (U) than any other district in the world during the period of 1951-1980. In Ambrosia Lake there are 96 documented former producing mines and four mills, some of which have documented contaminant releases. The San Mateo Creek (SMC) area is located in the GMD in northwestern New Mexico. In 2008-2009 the New Mexico Environment Department (NMED) performed a Site Investigation (SI), which included the collection, analysis, and evaluation of ground water samples to characterize-evaluate the impacts of legacy U mining-milling activities on the SMC regional ground water system. This SI was conducted under authority of the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), and it focused on ground water in the vicinity of U mill and mine sites in the SMC area. Sample data was acquired and used to evaluate sites to determine if threats to human health and the environment exist such that further action under CERCLA is warranted. Variations in water sample geochemistry were used to determine if anthropogenic impacts to ground water exist from the Bluewater Disposal site and upgradient sites in the SMC area. Deep-shallow well samples from the Bluewater (BW)-SMC areas were collected from existing private, municipal, industrial, and monitoring wells. Samples were analyzed for dissolved major ions, metals, and radiochemistry. Isotopes of U, O, H, C, and S were used as an environmental forensic tool to identify potential anthropogenic components in some water samples. Evaluation of the water analyte concentrations compared to the federal EPA drinking water and state ground water quality standards indicated 38 samples had 107 concentrations that exceeded one or more standards. The average total dissolved solids concentration in the BW-SMC samples was 1,051 and 1,432 mg/l, respectively. The average dissolved U concentration in the BW-SMC samples was 12.4 and 67.3 ug/l, respectively. The alpha activity ratio of ²³⁴U/²³⁸U is generally close to 1.0 in samples with elevated U concentrations compared to samples of natural U concentration. Data and interpretations from this investigation are under further review to determine the validity and potential for these geochemical tools to be used in future investigations in the SMC area.