New Mexico Geological Society Annual Spring Meeting & Ft. Stanton Cave Conference — Abstracts

The San Juan River is a vital source of water for drinking, irrigation, and recreational use for individuals in the arid Four Corners Region, much of which encompasses the Navajo Nation. The Navajo Nation Environmental Protection Agency (NNEPA) discovered elevated concentrations of aluminum, arsenic, and lead in surface water samples from routine monitoring of the San Juan River. Tributaries, most of which are ephemeral, that flow into the San Juan River may be contributing elevated concentrations of aluminum, arsenic, and lead. These metals could come from local oil drilling operations, abandoned uranium mines and mills, agricultural land, natural gas power plants, geology and erosion, illegal dumping of waste, and other sources. In cooperation with the NNEPA, the U.S. Geological Survey (USGS) is working to determine which tributaries are contributing aluminum, arsenic, and lead to the San Juan River, as well as the relative contributions from anthropogenic and natural sources. Thirty-four tributaries and numerous sites on the San Juan River, from Navajo Dam, NM to Mexican Hat, UT are being sampled routinely for surface water and sediments by scientists at the NNEPA and the USGS. Surface water samples are being analyzed for major ions and trace metals, and sediment mineralogy is being analyzed using a scanning electron microscope (SEM) at the University of New Mexico. Water chemistry and sediment mineralogy data analysis is ongoing. Results from a February 2021 baseflow synoptic survey of the San Juan River, and from tributary samples collected in July 2021, show concentrations of aluminum as high as 3.77x10⁶ parts per billion (ppb), above the Environmental Protection Agency (EPA) suggested goal for aluminum of 200 ppb. Arsenic results were as high as 27 ppb, above the EPA maximum contaminant level (MCL) for arsenic of 10 ppb. Lead results were as high as 218 ppb, above the EPA MCL for lead of 15 ppb. SEM analysis of sediments collected in August 2021 show a large amount of aluminum in tributary sediments. Arsenic and lead have not been found in SEM analysis, which could be due to their low concentrations, or desorption from sediments. To better determine the effect of tributary chemistry on the San Juan River, USGS scientists are beginning to use aerial imaging to model streamflow so that metal loads can be calculated. Determining the relative contributions of metals to the San Juan River is vital to the public for recreational use, for water managers in the region, and for future scientific inquiry.