APPROACHES TO FINGERPRINTING URANIUM IN WATER AND SOLIDS WITH APPLICATIONS ON THE SAN JUAN RIVER, SOUTHWEST USA

Identifying sources of uranium (U) to surface waters poses a challenge to drinking water managers, regulatory agencies, and remediation staff especially in locations where there are point and non-point sources. In large-scale hydrologic settings there are often multiple potential sources of U. There are numerous approaches to fingerprinting, which allow scientists to identify the source of natural and anthropogenic U. Spatial analysis, multivariate statistics, mixing models, and ternary diagrams of major and trace element concentrations in both water and solids can help to fingerprint the U in each media and locate sources of U. In addition, metal isotopes in water or sediments and the mineralogy of the sediments can be useful. As part of a larger project on the San Juan River in the southwestern USA, unfiltered surface water samples were analyzed for U as well as major and trace element concentrations. Sediment samples were collected from sediment traps from major ephemeral tributaries and total bulk chemistry and scanning electron microscopy (SEM) was performed to identify mineral assemblages. Unfiltered surface water uranium concentrations in the San Juan River during baseflow in February 2021 ranged from 0.40 to 2.60 µg/L while U concentrations in the San Juan River during monsoonal storms in summer 2021 ranged from 0.41 to 45.6 µg/L. The USEPA MCL for U is 30 µg/L. Concentrations of U from tributary surface waters vary as well. Total bulk chemistry of sediments from the tributaries revealed a range of U concentrations from 0.50 to 6.45 mg/kg. Preliminary principal component analysis reveals similar and different U fingerprints in the San Juan River water samples, suggesting a range of sources. SEM sediment results indicate locations of potential U sources including Fe-oxides, Fe-Ti oxides, and clays. Using multiple lines of evidence to identify the source(s) of U to surface waters can be useful when evaluating risk to a watershed and potential remediation strategies.