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In 2016, the U.S. Geological Survey, in cooperation with the U.S. Environmental Protection Agency, collected data on isotopes, age dating, and geochemistry including aqueous uranium concentrations of samples from 20 locations in the vicinity of the Homestake Mining Company Superfund site near Milan, New Mexico. The 20 sampled locations include 19 groundwater wells and 1 treatment plant for water used for injection into aquifers. At 6 of the 19 wells, multiple samples were collected by several different sampling methods, including passive, micropurge, and volumetric methods. Aqueous uranium concentrations were adjusted on the basis of comparisons between three sampling methods (called sample adjustments). These adjustments were specific to passive sample results because they underestimated uranium concentrations compared with results from purge samples (micropurge and volumetric). Sample adjustments were also made on aqueous selenium concentrations from previously published data for passive sampler results following a similar procedure. Aqueous uranium concentrations in dissolved and total form were adjusted from the original analytical values (called laboratory analytical adjustments) on the basis of a rigorous comparison to several external tests, including reruns and analysis by a different laboratory after accuracy issues were identified in data from the original laboratory. The original laboratory analytical results were found to be two to five times greater than historical concentrations at the same locations, which prompted further evaluation, as described in this report. Introduction Reliable sampling procedures and analytical results are critical for understanding water chemistry. Unsatisfactory results, either from sampling or laboratory analysis, of a category of chemicals or selective constituent can prevent interpretation of findings and completion of projects. In these situations, results may need to be carefully assessed, and adjusted on the basis of a series of external testing procedures and comparisons. In 2016, the U.S. Geological Survey (USGS), in cooperation with the U.S. Environmental Protection Agency (EPA), collected data on isotopes, age dating, and geochemistry of samples from 20 locations at the Homestake Mining Company Superfund site near Milan, New Mexico. Data were collected to characterize the groundwater chemistry in the vicinity of the Homestake Superfund site (fig. 1). Version 1.0 of the USGS data release for this work (Becher and others, 2017) contained information on isotopes, age dating, and geochemistry, including aqueous concentrations of metals—excluding dissolved and total uranium. All metals previously reported in data release version 1.0 (Becher and others, 2017) as well as uranium samples documented in the present report, were analyzed by inductively coupled plasma mass spectrometry1 (ICPMS; EPA method 6020A; U.S. Environmental Protection Agency, 1998). These analyses were conducted by RTI Laboratories, Inc. (RTI). However, uranium concentrations from RTI were found to be two to five times greater than historical concentrations of uranium at the site (U.S. Environmental Protection Agency, 2016). In some wells, the historical sampling record spanned 40 years and the uranium concentrations reported by RTI exceeded all previously reported concentrations. This discrepancy prompted followup laboratory analysis, multiple comparisons between lab results, and postanalytical adjustments. Version 1.1 of the USGS data release (Blake and others, 2017) reported aqueous uranium concentrations from a subset of the original samples (17 samples from 16 wells), which were analyzed by EPA method 6020A at the EPA Region 6 laboratory (EPA lab). Uranium concentrations measured by the EPA lab were comparable to historical concentrations of uranium. All analytical results for uranium samples, both previously unpublished (RTI) and published (EPA lab), are documented in this report and in Harte and others (2018). 1Terms listed in the glossary at the back of this report are in bold typeface at first use in the text. 2 Uranium and Selenium Concentrations From Groundwater, Homestake Mining Company Superfund Site, Milan, N. Mex. Ap pr ox im at e su bs ur fa ce fa ul t l oc at io n Evaporation ponds !(

Determination of representative uranium and selenium concentrations from groundwater, 2016, Homestake Mining Company Superfund site, Milan, New Mexico