New Mexico Geological Society Annual Spring Meeting — Abstracts

Following the August 2015 mine-water release into Cement Creek and the Animas River, the New Mexico Bureau of Geology and Mineral Resources (NMBGMR) undertook a hydrologic assessment of the Animas River and its alluvial aquifer in New Mexico, from Colorado-state line to Farmington, NM. The purpose of the project is to evaluate seasonal changes to the hydraulic or groundwater quality conditions along the NM-reach of the Animas River.

In August 2015, in collaboration with other agencies including U.S. Geological Survey, NM Office of the State Engineer, and NM Environment Department, we measured groundwater level at over 100 locations along the Animas River. Building from this network of wells, with funding from the NM Environment Department, the NMBGMR developed a repeatable monitoring network in the Animas River valley utilizing private domestic wells. Twenty of these wells were selected for water chemistry sampling as part of our monitoring efforts. Samples were analyzed for cations, anions, trace metals and the stable isotopes of oxygen and hydrogen. We also compiled and evaluated water quality data from groundwater samples collected in August 2015 by the U.S. EPA.

Water chemistry sampling revealed groundwater with elevated levels of Fe, Mn, SO₄, and Al above recommended drinking water standards. From our January 2016 results, the highest observed total iron from a groundwater sample was 4.58 mg/L, which is above the secondary recommendation (0.3 mg/L), and total manganese at 6.4 mg/L, which is also above the health advisory level (1.6 mg/L). We compared results from the January 2016 water chemistry sampling with previous results from samples collected by US EPA in August 2015. Similar to our results, the US EPA samples had elevated levels of Fe, Mn, SO₄, and Al. Most samples collected in January 2016 exhibited higher total manganese and iron levels than those sampled in August 2015.

Maps of the water chemistry results show an increase in ion content and total dissolved solids in groundwater in a down river direction from the Colorado-state line to Farmington. Whereas, the elevated levels of iron and manganese appear to be more common in the Cedar Hill and Inca regions. Results from stable isotopes of hydrogen range from -95.9 to -104.1‰, and oxygen from -13.4 to -14.4‰. The very “light” ranges of these values suggest that groundwater in the Animas River valley is predominantly recharged by winter precipitation.

With long-term environmental, mine-related, and anthropogenic impacts to the water quality along the Animas River, it is not surprising to find groundwater with elevated constituents observed in these limited samples. Continued monitoring of groundwater quality, through consistent and repeated measurements, is the only way to accurately examine any affects to groundwater related to the Gold King Mine or other contaminant concerns over time.