GEOCHEMICAL FINGERPRINTING TO DETERMINE SOURCES OF METALS TO THE SAN JUAN RIVER, FOUR CORNERS REGION, USA

The Four Corners Region in the United States encompasses areas of New Mexico, Colorado, Utah, and Arizona. It is a semi-arid and scarcely populated region with one main source of surface water: the San Juan River. The Navajo Nation EPA (NNEPA) has conducted water quality monitoring along the San Juan River for many years. During these investigations, NNEPA has found water quality exceedances in aluminum (Al), arsenic (As), and lead (Pb) concentrations. To determine the source(s) of these metals, the NNEPA tasked staff at the U.S. Geological Survey New Mexico Water Science Center with creating a sampling system to collect water quality samples in rural and rugged terrain where most precipitation occurs only as unpredictable monsoonal events. Surface water samples collected over two years were analyzed for major and trace elements, rare earth elements, and sulfur, strontium, and lead isotopic composition. Many sampling locations had elevated concentrations of Al, As, and Pb in unfiltered water samples, likely from geogenic sources. Concentrations of these elements were higher in samples collected in 2022 compared to 2021, likely due to increased precipitation in 2022. Combining the results of these water quality samples with investigations of the local geology, morphometric parameters, and sediment chemistry allowed investigators to develop a fingerprint of metals sources. These fingerprints will ultimately be used to identify, and potentially quantify, different sources that are actively contributing metals to the San Juan River.