Surface water chemistry from the San Juan River watershed, 2021-2022, Four Corners USA

This study was done because elevated concentrations of metals including aluminum, arsenic, and lead were identified by Navajo Nation EPA staff in the San Juan River from below the Navajo Dam, through the Navajo Nation to Mexican Hat, Utah in the Four Corners Region of the United States (Colorado, Arizona, New Mexico, Utah). An interdisciplinary team applied approaches and principles of geology, geochemistry, geomorphology, hydrology, and statistics to gain a better understanding of the tributaries supplying the source(s) of metals to the San Juan River. Water samples were collected in single-siphon samples and by grab sample at 59 ephemeral and perennial tributaries to the San Juan River in 2021 and 2022. Eighteen surface water sites along the San Juan River were sampled as grab samples in 2021 and 2022 and fifteen sites along the San Juan River were sampled by equal width increment and composited in a churn during a baseflow synoptic sampling campaign in February 2021. A total of 765 unfiltered and 282 filtered samples were analyzed for major and trace cations. All samples were analyzed through the USGS Analytical Chemistry Laboratory in Denver, Colorado (Taggart, 2022; USGS, 2023 Method 37).