Animas River Valley - Long Term Groundwater Monitoring

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Our agency collaborated with the New Mexico Environment Department (NMED) to conduct a hydrogeology study along the Animas River in New Mexico in response to the Gold King Mine (GKM) spill of August 2015. The water released from the spill was loaded with dissolved metals and contaminated sediments, which posed a possible risk to groundwater quality in the Animas Valley. This study aimed to assess potential effects of the GKM spill on groundwater quality in New Mexico. For the first phase of this study, which began in January 2016, we measured groundwater levels and collected up to 26 water samples for geochemical analysis from existing wells in the shallow alluvial aquifer along the Animas Valley north of Farmington, NM. Return sampling trips were conducted at different seasons, under different Animas River flow conditions in 2016 and 2017. The second phase of this study focused on groundwater chemistry for water samples collected from 54 and 74 wells in the fall of 2018 and spring of 2019 respectively. We increased in the number of wells sampled in an effort to fill data gaps from phase 1. Results from this study have helped to construct a working conceptual model of the hydrogeologic system that includes the identification of the primary recharge sources and mechanisms, groundwater flow paths and some groundwater contaminants of concern.

General findings include:

1) There is no evidence of any adverse impacts to groundwater quality related to the GKM spill within the study area.

2) The Animas River in New Mexico is largely a gaining stream, meaning groundwater discharges to the river.

3) All water samples exhibited inorganic chemical concentrations below the maximum contaminant levels (MCLs) as defined by the EPA National Primary Drinking Water Regulations that were established to protect against consumption of drinking water contaminants that present a risk to human health.

4) Some wells produce water that exceeds US EPA secondary MCLs for sulfate, total dissolved solids, chloride, iron, manganese, and aluminum. US EPA secondary drinking water regulations are non-enforceable guidelines regarding cosmetic or aesthetic effects.

5) Geologic controlled upwelling of regional groundwater results in observed increases in sulfate and total dissolved solids concentrations in the southern portion of the study area (south of Aztec).

6) High iron and manganese concentrations observed in localized areas of the shallow aquifer are likely due to manganese oxides and iron (hydr)oxides that were deposited as sediments that make up the shallow aquifer.

7) While hydrogeologic and geochemical conditions (pH and Eh) in the shallow aquifer make the system resilient to potential contamination related to legacy acid-mine drainage and mine water spills in the Animas headwaters in Colorado, the system is very dynamic. Therefore we recommend the continued monitoring of groundwater quality in the Animas Valley in New Mexico.

Funding for this project is from the U.S. Environmental Protection Agency (U.S. EPA), distributed through the NMED.

For more information, please contact:
Dr. Talon Newton, Hydrogeologist