Use of absolute gravity measurements to monitor groundwater in the Española Basin, New Mexico

We present results of 32-month project using absolute gravity measurements to monitor groundwater in an arid to semi-arid region in northern New Mexico. Over 100 permanent gravity stations have been established in the groundwater basin. A-10 absolute gravity meters, manufactured by Micro-g LaCoste, Inc., have been used to monitor long-term gravity changes in the groundwater basin. Over fifty A-10 sites have been established; other gravity sites have been established by reference to the primary A-10 sites using Scintrex CG-3M and CG-5 relative gravimeters. Geodetic-quality GPS surveys were used to directly measure any possible elevation changes at the gravity sites. At good sites, away from regions where we expect changes due to groundwater removal, reproducibility of the A-10 measurements is ±4 μGal. Gravity stations were observed approximately every six months over the course of the project. Systematic changes in both gravity and station elevation have been observed at a number of sites. On the basis of our observations, we can delineate three areas which appear to have systematic changes in storage. We conclude that the use of high-precision absolute gravity measurements, in conjunction with precise observations of station elevation, can provide useful information for evaluating groundwater storage changes over time.