New Mexico Geological Society Annual Spring Meeting — Abstracts

On August 5, 2015 the accidental breech of the Gold King Mine (GKM), located in Colorado, resulted in the movement of millions of gallons of bright orange water through the Animas River in northwestern New Mexico. This water, which was loaded with dissolved metals and contaminated sediments, posed a potential risk to groundwater quality in the Animas Valley. The Animas River from the Colorado-New Mexico border flows through Quaternary alluvial deposits, which are largely made up of sediment eroded from Paleogene rocks into which the Animas River has incised. While most water for domestic use and irrigation in the area is largely sourced from the Animas River, there are many private domestic and irrigation wells in the valley completed in the alluvial aquifer with depths of about 30 to 60 feet.

We collected water samples from up to 26 wells within and near the Animas Valley between the NM-CO border and Farmington, NM several times between January 2016 and June 2017. The objectives of this study were to characterize the hydrogeologic system, investigate groundwater/surface water interactions, and assess the possible impacts of the GKM spill to shallow groundwater.

General water chemistry, stable isotope and environmental tracer data, and modeling of two-endmember mixing indicate that shallow groundwater is primarily comprised of young river water and older regional groundwater from the underlying Nacimiento Formation. The river water end-member is characterized by total dissolved solids (TDS) concentrations less than 500 mg/L, a calcium bicarbonate water type, and tritium values above 5 tritium units. The regional groundwater end-member is characterized by much higher TDS concentrations close to 10,000 mg/L, a sodium sulfate water type, undetectable tritium content, and an apparent carbon-14 age of approximately 20,000 years before present. The upwelling of regional groundwater due to the gradual thinning of the Nacimiento Formation to the south, significantly affects water quality by increasing the TDS content to above 1,000 mg/L in some areas south of Aztec.

The main process that may potentially introduce contaminants from upstream mines into the shallow groundwater is the seepage of irrigation water (diverted river water) through the bottoms of ditches and agricultural fields to recharge the aquifer. Potential groundwater contaminants associated with the GKM spill, which include iron, aluminum, manganese, lead, copper, arsenic, zinc, cadmium, and mercury, were found to be below U.S. Environmental Protection Agency (USEPA) maximum contaminant levels (MCLs). Several wells in the shallow aquifer produced water that exceeds USEPA secondary MCLs for dissolved iron and manganese. It is difficult to determine the source of these trace metals, which were observed to be present in the shallow groundwater before the GKM spill. While the GKM spill exhibited high iron and manganese concentrations, these metals are also known to be ubiquitous in fluvial sediments, such as those that make up the alluvial aquifer. Therefore, results from this study do not suggest that the groundwater quality has necessarily been impacted by the GKM spill. However, continued monitoring of groundwater quality is recommended.