A Microgravity Pilot Study: Insights into Storage Change, Specific Yield, and Groundwater/Surface Water Interaction in the Mesilla Groundwater Basin, New Mexico.

The transboundary Mesilla groundwater basin (Basin) is one of a series of alluvial basins formed by the Rio Grande Rift and is located along the Rio Grande in south-central New Mexico, west Texas, and northern Chihuahua Mexico. Water resources in the Basin are under growing pressure due to increased demand by industrial, agricultural, and municipal users in both the United States and Mexico. Hydrologic monitoring has been ongoing in the Basin since 1987. Differences in repeat microgravity measurements are sensitive enough to capture temporal changes in groundwater storage and enable direct calculation of specific yield values when collocated with a monitoring well. Specific yield is typically poorly constrained using traditional methods such as pumping tests but is important for water production and groundwater modeling purposes. A network of 22 regionally-distributed microgravity stations was established in the United States' portion of the Basin to 1) monitor magnitude and location of groundwater storage changes and 2) gain information on aquifer properties in the Basin. Microgravity data were collected two to three times annually between 2016 and 2018, discrete groundwater levels were collected concurrently, and continuous surface water flow data were recorded through gages on the Rio Grande. Microgravity surveys were scheduled to capture seasonal differences in streamflow and water use, including surface-water releases from the upstream reservoir, groundwater withdrawals, and agricultural irrigation. Microgravity stations were collocated with active monitoring wells at 13 locations to allow for calculation of specific yield values. Preliminary results suggest that groundwater storage increased over the course of the study, and seasonal differences at some locations were observed in conjunction with irrigation and surface-water flows. As expected, water levels in wells close to the Rio Grande appear to be positively correlated with surface flows. Specific yields were calculated for each of the microgravity stations collocated with a monitoring well with varying results.