New Mexico Geological Society Annual Spring Meeting — Abstracts

Following the August 5, 2015 bulkhead breach of the Gold King Mine roughly 3 million gallons of mine waste water and tailings were introduced to a tributary of the Animas River, turning the waters a vivid orange. While the river has since returned to its normal color, there is concern that the toxic metals left on the streambed may begin to seep into the shallow alluvial aquifers and impact the groundwater in the surrounding area. In August 2015, in collaboration with other agencies (USGS, NM OSE, and NMED), groundwater level measurements were collected at over 100 locations along the Animas River. Using the network of wells established in 2015, we developed a groundwater level monitoring program in the Animas River valley utilizing private domestic wells. Future sampling campaigns carried out at different points in the year will help us understand the seasonal fluctuations of the water table and if it will affect the groundwater/ surface water interaction in the area.

Following our preliminary water level measurements in August 2015, a second round of water level measurements were collected in January 2016. The August water level measurements provided us with a snapshot in time of the flow conditions during the monsoon and irrigation season, when water levels are likely at their highest. The January sampling represents base flow conditions in the river. Of the 70 wells with repeat water level measurements the majority of the wells had a decline in water level of 2.1 ft, on average, between August 2015 and January 2016. In general, the wells north of Aztec show more decline than the wells to the south.

A water table map was delineated using the manually collected water level measurements, and paired with high resolution model of the Animas River stage. For the most part, the water table map shows the river is a gaining system, where groundwater from the surrounding alluvial valley flows down gradient and discharges into the river. From a regional perspective, this would suggest that water from the Animas River is not seeping into the alluvium. However, in January we found groundwater levels had declined, and river levels had risen. The January water table map showed that in close proximity to the river, the water table contours were almost perpendicular to the river, and even slightly inverted in places. With a neutral water table, fluctuations in the river stage can quickly turn a slightly gaining reach to a slightly losing reach. The degree to which the river is losing is so slight (less than 1 ft) that it is not detected by coarse resolution, regional water table maps. In areas with dense enough sampling we refined the contouring to 5 ft intervals.