HR: 14:10h
AN: GP23A-03    [Abstracts]
TI: Use of Absolute Gravity Measurements to Monitor Groundwater in the Española Basin, New Mexico
AU: * Cogbill, A H
EM: ahc@lanl.gov
AF: Earth & Environmental Sciences Div. Los Alamos National Laboratory, MS F665 P. O. Box 1663, Los Alamos, NM 87545 United States
AU: Ferguson, J F
EM: ferguson@utdallas.edu
AF: Center for Lithospheric Studies University of Texas at Dallas, P. O. Box 830688, Richardson, TX 75083-0688 United States
AU: Keating, E H
EM: ekeating@lanl.gov
AF: Earth & Environmental Sciences Div. Los Alamos National Laboratory, MS F665 P. O. Box 1663, Los Alamos, NM 87545 United States
AB: We present early results of three-year project using absolute gravity instrumentation 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 Solutions, 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 relative gravimeters. We have used geodetic-quality GPS surveys to directly measure any possible elevation changes at the gravity sites; thus far, no significant changes in elevation have been observed. For the A-10 gravity sites, we have learned that sites must be constructed rather carefully to minimize noise levels due to certain characteristics of the A-10 measurement system. At good sites, away from regions where we expect changes due to groundwater removal, reproducibility of the A-10 measurements is ±4~μGal. To date, we have data from repeat campaigns over a period of 22 months. We have observed systematic changes in gravity of as much as 14~μGal at certain sites. We have directly incorporated gravity modeling into a detailed 3D groundwater model of the basin. On the basis of groundwater modeling, we believe that such gravity changes are due to increased recharge at some sites, as precipitation began to return to normal amounts after a long, pronounced drought about a year into the study. Somewhat surprisingly, no significant gravity changes have been observed at the Buckman Well Field, a spatially small well field that is heavily pumped as a municipal supply field for Santa Fe, New Mexico. One interpretation of this observation is that pumping at the Buckman Field is accessing nearby surface sources rather than groundwater, despite the fact that pumping is occurring from more than 300~m depth.
DE: 1244 Standards and absolute measurements
DE: 1299 General or miscellaneous
DE: 1829 Groundwater hydrology
DE: 1836 Hydrologic budget (1655)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2005 Joint Assembly