New Mexico Geological Society Annual Spring Meeting — Abstracts

The Gold King Mine spill that occurred in August 2015 drained water that was backed up in the mine adit rapidly, quickly eroding the waste rock pile that was outside of the mine. As result, roughly 490,000 kg of sediment was released with the water, turning the water an orange color. While the river has since returned to its normal color, there is still concern that the metals left on the streambed may affect the shallow alluvial aquifers and impact the communities in the surrounding area.

To determine if water from the river was infiltrating in to the alluvial aquifers of New Mexico during or after the spill, a water-level monitoring network was developed. The monitoring network consisted of roughly 60 existing wells and was used to construct groundwater elevation contour maps to determine the direction of groundwater flow in the Animas Valley between Farmington and the NM/CO boarder. The monitoring network of wells was measured four times per year, over two years, during hydraulically significant periods to understand the seasonal fluctuations of the water table and how it affects the groundwater/ surface water interactions; during baseflow conditions (late-January), the initial snowmelt/onset of irrigation season (mid-March), peak snowmelt/extended irrigation season (early-June), and at the end of irrigation season (mid-October).

A subset of wells were instrumented with pressure transducers to continuously collect water-level data. The groundwater hydrographs recorded at these wells showed distinct patterns that were used to categorize most of the measured wells based on their different hydrograph characteristics. Wells close to the river typically had a direct correlation to river stage with a distinct increase that correlates to peak snowmelt in the river followed by a rapid drop in groundwater levels through August. Most of the hydrographs begin increasing in late March at the beginning of irrigation season and continue to increase through July and do not begin to decrease until the end of irrigation in late October.

In most areas, the Animas River is gaining water from the groundwater, as groundwater from the surrounding valley flows down gradient, discharging to the river. However, in some areas, water-levels in close proximity to the river have a nearly flat hydraulic gradient between groundwater and the river, where small seasonal fluctuations in groundwater levels and river stage can turn a slightly gaining reach to a slightly losing reach. Groundwater levels in the valley are generally lowest in March, before the irrigation season begins, and highest in October, near the end of the irrigation season. High seasonal water-level fluctuations were observed near the Cedar Hill and Inca communities, where we observed an apparent reversal in gradient that changes the river in those areas from a gaining stream in the summer during irrigation season to a losing reach in the winter. The results of this study indicate river water does infiltrate into the alluvial aquifer primarily from irrigation return or infiltration through the ditch network, as demonstrated by the hydrograph response to the irrigation season.