Concentrations, distribution, and potential sources of aluminum, arsenic, and lead in surface waters of the San Juan River watershed, USA

In remote semi-arid regions like the San Juan River watershed, determining the distribution and sources of Al, As, and Pb can inform water managers and users in mitigating potentially negative effects on human and ecosystem health. Water samples were collected from 33 tributaries and sites along the San Juan River during the monsoon seasons of 2021 and 2022. Synoptic water quality samples collected during baseflow along the San Juan River in February 2021 had Al, As, and Pb concentrations below EPA drinking water thresholds (50-200 µg/L Al, 10 µg/L As, 15 µg/L Pb). Total water concentrations of Al, As, and Pb were generally higher in 2022 than in 2021 with average Al concentrations 9.74%-98.45% higher in 2022 than in 2021. Average As and Pb concentrations were 18.27%-96.29% higher and 11.54%-98.66% higher in 2022 than 2021, respectively. Total water concentrations of Al, As, and Pb were greater than EPA thresholds at most sites and highest at Chinle Creek near Bluff, La Plata River, Comb Wash, and Gallegos Canyon. Most sites also had higher particulate Al, As, and Pb concentrations in total water compared to filtered concentrations. A combination of water chemistry, geology, water use, and geomorphology suggested that the source(s) of Al, As, and Pb in total water is likely geogenic non-point. This study enhances understanding of the distribution and potential sources of Al, As, and Pb in the San Juan River watershed, bringing awareness to the community, and providing tools to evaluate metals concentrations in remote semi-arid regions worldwide.