Wednesday, April 22, 2009: 2:00 p.m.
Joshua Tree (Hilton Tucson El Conquistador Golf & Tennis Resort )
Michelle Sneed
,
Western Remote Sensing and Visualization Center, USGS, Sacramento, CA
Devin L. Galloway
,
USGS, Sacramento, CA
David G. Evans
,
Geology, California State University, Sacramento, Sacramento, CA
Land subsidence caused by ground-water extraction is a long-standing problem in Antelope Valley, California, where ground-water withdrawals since the early 1900s have lowered water levels by more than 45 m, and have resulted in nearly 1.5 m of subsidence at Edwards Air Force Base (EAFB). Erosion and fissuring of runway surfaces related to differential subsidence at EAFB are persistent problems that confront the current and future management of the ground-water resource. From 1990 to 2006, 230 mm of compaction was measured over a 256-m-depth interval of the aquifer system using a borehole extensometer located on EAFB. Seasonal water-level fluctuations were accompanied by step-like variations in compaction rates. The absence of aquifer-system expansion during periods of water-level recovery is characteristic of the slow equilibration of fluid pressure and the accompanying residual compaction of thick aquitards following water-level declines in the aquifers.
The residual compaction processes were evaluated at EAFB by developing two one-dimensional, vertical numerical models using MODFLOW-2000 with different subsidence packages (IBS1 and SUB). The models incorporate the available hydrogeologic information, use identical parameter values, and were constrained by aquifer-system compaction measured at the extensometer site, and by historical subsidence measured at nearby benchmarks. The IBS1 approach uses a model domain composed of 1 layer, 1 row, and 272 columns turned on its side to implicitly simulate delayed drainage and residual compaction of the thick aquitards represented by numerous adjacent model cells. The SUB approach uses a model domain composed of 14 layers, 1 row, and 1 column to explicitly simulate delayed drainage and residual compaction of the thick aquitards. Both approaches explicitly simulate instantaneous drainage and compaction of the thin aquitards. The SUB approach more closely approximates the measured compaction and subsidence at short (monthly) and long (multi-annual) time scales.
