Differentiating anthropogenic and natural sources of uranium by geochemical fingerprinting of groundwater at the Homestake uranium mill, Milan, New Mexico, USA

A multiparameter geochemical-isotopic fingerprinting approach was used to differentiate anthropogenic and natural signatures of uranium contamination near the Homestake uranium mill site (Site), near Milan, New Mexico, USA. The Site consists of two tailings piles from milling operations and groundwater contamination from these tailings has been noted. The Site lies within the lower San Mateo Creek Basin and has multiple regional sources of uranium contamination from mining and mill operations. The Site is underlain by a heterogeneous alluvial aquifer, which is in turn underlain by basement rock of the Chinle Group aquifer and the underlying San Andres-Glorieta Formation aquifer. To help decipher signatures, several statistical approaches were used including principal component analysis, non-metric multidimensional scaling, and cluster analysis. Piper diagrams indicate two end-member water types at the Site, sulfate-Na-K generally in the Chinle Group aquifer and sulfate-Ca generally in the alluvial aquifer. There are wells from both aquifers that plot between the two end members. Uranium concentrations from the Site fall into three broad categories: less than the drinking water standard of 30 mu g/L (n = 3), from 30 to 100 mu g/L (n = 9), and greater than 100 mu g/L (n = 8). Component loadings in a principal component analysis are highest for uranium isotopes, uranium, molybdenum, chloride, sodium, (228)radium, and gross alpha-beta, which affect the similarities or differences among wells sampled. Results suggest that several alluvial wells north of the Site have groundwater with anthropogenic fingerprints from regional sources related to upgradient mining. Well water with higher uranium concentrations has uranium activity ratios close to 1, which is indicative of mining or milling signatures. These same wells have elevated radon activities. This information can be used to inform Site managers regarding the source of water related to uranium at the Site and provide an approach for geochemical fingerprinting.