Abstract
Land subsidence resulting from ground-water level declines has long been recognized as a problem in Antelope Valley, California. At Edwards Air Force Base (EAFB), ground-water extractions have caused more than 150 feet of water-level decline, resulting in about 4.8 feet of subsidence. Differential land subsidence has caused sinklike depressions and earth fissures and has accelerated erosion of the playa lakebed surface of Rogers Lake at EAFB, adversely affecting the runways on the lakebed which are used for landing aircraft such as the space shuttles. Since 1990, about 0.8 foot of aquifer-system compaction has been measured at a deep (840 feet) borehole extensometer (Holly site) at EAFB. More than 16 years of paired ground-water level and aquifer-system compaction measurements made at the Holly site were analyzed for this study. Annually, seasonal water-level fluctuations correspond to step-like variations in aquifer system compaction; summer water-level drawdowns are associated with larger rates of compaction, and winter water-level recoveries are associated with smaller rates of compaction. The absence of aquifer-system expansion during seasonal water-level recovery is consistent with the delayed drainage and resultant delayed, or residual, compaction of thick aquitards.
A numerical one-dimensional MODFLOW model was developed to simulate aquitard drainage and to refine estimates of aquifer-system hydraulic parameters that control compaction and land subsidence at the Holly site. The analyses and simulations of aquifer-system compaction are based on established theories of aquitard drainage. Historical ground-water level and land subsidence data collected near the Holly site were used to constrain simulations of aquifer-system compaction and land subsidence at the site for the period 1908-90, and ground-water level and aquifer-system compaction measurements collected at the Holly site were used to constrain the model for the period 1990-2006.
Simulations indicate that two thick aquitards, with a combined thickness of 129 feet or about one-half the aggregate thickness of the aquitards penetrated by the Holly boreholes, account for most (about 95%) of the compaction measured at the Holly site during the period 1990-2006. Further, simulated pore pressures in these two thick aquitards had not equilibrated with historical drawdowns in the adjacent aquifers by 2006, indicating that the ultimate expected compaction and land subsidence have yet to occur. The simulations show that residual compaction and associated land subsidence attributed to slowly equilibrating aquitards is important to the longterm management of land and water resources at EAFB.