New Mexico Geological Society Annual Spring Meeting — Abstracts

The Gold King Mine Release of August 5, 2015 mobilized three million gallons of water and sediment into a tributary of the Animas River, which flows south from Colorado into New Mexico. The Animas enters the San Juan River at Farmington, New Mexico, the sixth largest city in the state and one of several communities that rely on the Animas and San Juan rivers for drinking water and (or) crop irrigation. Mines such as the Gold King are sources of metals and acidity to streams of the area, leading to concerns regarding the effects of the release on water quality. Potential problem constituents such as lead and arsenic (among others) were analyzed in surface water and sediment samples collected by the U.S. Geological Survey (USGS) in August 2015. Surface-water samples collected from the San Juan River at Farmington on August 8, 2015 (a time when the orange color indicative of the release was observed) had concentrations in whole water (unfiltered) samples of 552 µg/L lead and 26.3 µg/L arsenic. The concentrations of lead and arsenic were above the lead action level of 15 µg/L and the maximum contaminant level of 10 µg/L for arsenic, as set by the U.S. Environmental Protection Agency for drinking water. In contrast, the dissolved concentrations (0.45 µm filter) of these elements at the same site and time (lead <0.04 µg/L and arsenic 0.23 µg/L) were about two to three orders of magnitude below drinking-water standards and three to 5 orders of magnitude below the whole water sample concentrations. At the San Juan River at Farmington streamgage, located approximately two river miles downstream from the Animas River at Farmington streamgage, concentrations measured in whole water samples collected 4 hours after the samples on the Animas River ranged from 122 µg/L for lead, and 7.89 µg/L for arsenic. These concentrations were likely affected by dilution from the San Juan River which had discharge on August 8, 2015 of about 1,300 cfs, whereas discharge on the Animas River at Farmington was about 800 cfs.

Based on the results for total and dissolved water samples, constituents of concern generally were associated with particulates in the water. Bed sediment samples collected from the Animas River at Farmington on August 12, 2015 had lead concentrations ranging from 33.0 to 179 mg/kg and arsenic concentrations ranging from 3 to 11 mg/kg (n =4). Concentrations of these constituents in bed sediments were generally lower at the San Juan River at Farmington, similar to the trend in water-quality data for the same sites. Concern related to suspended particulates and bed sediment in these rivers continues months after the release. Continuous monitoring of pH, temperature, specific conductance, and turbidity is expected to help to understand the current geochemical interactions in the rivers. Additionally, water-quality sampling during snowmelt and storm events will provide valuable information about sediment and metal mobility during high-discharge events in the Animas and San Juan Rivers.