Source Apportionment of Lead in the San Juan Watershed (USA): Influences from Weathering of Mineralized and Sedimentary Deposits

Natural and anthropogenic sources of lead (Pb) can adversely impact water and sediment quality within large watersheds and the ecosystems they support. This study examined the sources and distribution of Pb within the San Juan watershed located in southwestern Colorado and the Four Corners region of Colorado, New Mexico, Arizona, and Utah (western United States). Samples for this project were collected from 2018 to 2021 and included seeps and springs located within the mineralized headwaters region, surface water, and sediments along an approximately 570 km stretch of riverbed. Concentrations and isotopic compositions of Pb showed that (1) source attribution using all stable Pb isotope ratios, ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb, allowed for an analysis of metal dilution and changing sources of Pb; (2) in upstream reaches, Pb from landscape disturbance related to mining operations and weathering of mineralized geologic units represented the most significant Pb source, accounting for as much as 90% of the Pb within the upper Animas River sediments; and (3) Pb attributed to the mining-impacted headwaters decreased downstream through the Animas River and San Juan River and represented up to about 50% of the Pb in downstream sediments. The proportion and mass of Pb derived from the mining district were reduced in downstream areas due to increased sediment delivery to the central river channels from tributaries and weathering of Paleozoic- to Tertiary-aged sedimentary deposits. Our analysis demonstrates that Pb isotope ratios can be used to effectively trace Pb transport through watershed systems where multiple Pb sources exist and where Pb concentrations may be similar to geogenic values. The study results indicate that the spatial and temporal variation of Pb isotopic signatures is associated with multiple contributions from natural sources, which are influenced by seasonality and hydrological factors.