Groundwater Withdrawal and Aquifer Compaction: A Case Study in Addicks, Texas

Abstract

This study examines the long history of aquifer compaction observed at the Addicks Facility using the continuous measurements of one USGS borehole extensometer (1975-2012) and two Subsidence District GPS stations (1996-2012). The Addicks Facility is located in west Houston, near an area of recent and rapid subsidence. GPS positions are generated within the IGS08 Reference Frame using the GIPSY-OASIS (version 6.1.2) software package, which employs the Precise Point Positioning with Single Receiver Phase Ambiguity (PPP-SRPA) method. Ellipsoid height is used to assess vertical displacements. The GPS and extensometer measurements verify that the vertical displacement is caused by aquifer compaction within the shallowest 549 meters of the subsurface. The antenna of the ADKS-CORS GPS station is mounted to the inner pipe of the extensometer, and the absence of observed displacement demonstrates that the base of the pipe is anchored to stable strata. The PAM05 GPS station is anchored at the surface, only fifty meters from the extensometer site. PAM05 recorded nearly 30 centimeters of vertical displacement from 1996-2010, which corresponds well with the compaction observed by the extensometer. Long-term (1972-2012) groundwater level data from the USGS were also investigated in this study. A strong correlation is identified between groundwater level fluctuations and aquifer compaction. In particular, the most rapid declines in groundwater level always correspond to severe and rapid compaction. This study carefully examines rapid vertical displacement of approximately 7 cm/year observed in 2005. An additional relationship is defined between the potentiometric surface and the rate of groundwater withdrawal. No groundwater was extracted by local pumping wells from January-May, 2005. Following this period, withdrawals rose to an average of approximately 350 million gallons per month. This increase in the rate of withdrawal caused the potentiometric surface to decline by more than 90 meters, and the land surface to subside nearly 5 centimeters in eight months. This study indicates that the rate of groundwater withdrawal, more than the volume of withdrawal, exerts the strongest control over aquifer compaction. This correlation is important as local regulatory bodies continue to limit only the volume of groundwater extracted at the Addicks Facility.