SURFACE-WATER AND GROUNDWATER QUALITY IN NORTHWESTERN NEW MEXICO AFTER THE GOLD KING MINE RELEASE

The Animas River in Colorado and New Mexico (NM) was exposed to three million gallons of water and sediment released from the Gold King Mine on August 5, 2015. During this time, elevated concentrations of iron, manganese, arsenic (As) and lead (Pb) (among other metals) were observed in the surface water. The Animas River enters NM near Cedar Hill and joins the San Juan River at Farmington, NM, one of several communities that rely on the Animas and San Juan rivers for drinking water and irrigation. Between Cedar Hill and Aztec, NM, the Animas River is a gaining reach on a coarse scale; however, observations on a fine scale adjacent to the river suggest it can be a losing reach in places, especially during the winter months when the water table is lowest. Constituents such as major ions, metals, including As, and Pb, as well as field parameters such as pH, temperature, specific conductance, and turbidity, are currently being monitored in surface water by the U.S. Geological Survey and in groundwater by the New Mexico Bureau of Geology and Mineral Resources.

Surface-water samples from the Animas River between Cedar Hill and Aztec and groundwater samples from the alluvial aquifer along the same reach appear to have similar major-ion chemistry, with the water type being calcium-bicarbonate to calcium sulfate. Constituents of concern in surface water and groundwater as a result of the Gold King Mine release include As and Pb. During the snowmelt season from April to June 2016, dissolved and total concentrations of As in surface water between Cedar Hill and Aztec (maximum 0.38 µg/L dissolved and 2.6 µg/L total (n=4)) and nearby groundwater (maximum 3.4 µg/L dissolved and <2.5 µg/L total (n= 4)) were well below the Maximum Contaminant Level of 10 µg/L set by the U.S. Environmental Protection Agency for drinking water. However, total concentrations of Pb in surface water (maximum 34.9 µg/L) exceeded the Action Level of 15 µg/L in samples from the Cedar Hill and Aztec sites (n=4). Dissolved Pb concentrations in surface water (maximum 0.32 µg/L) and dissolved and total Pb concentrations in groundwater (all <2.5 µg/L (n=4)) were well below the Action Level. Collection and interpretation of chemical data may help in understanding the effects of the Gold King Mine release on surface water and groundwater many months after the initial release.