Groundwater-Level Altitudes and Changes and Measured Compaction of Fine-Grained Sediments by Borehole Extensometers in the Gulf Coast Aquifer System; 1977–2013
Wednesday, 17 December 2014
Most of the subsidence in the Houston-Galveston region, Texas, has occurred as a direct result of groundwater withdrawals for municipal supply, commercial and industrial use, and irrigation that depressured and dewatered the Chicot and Evangeline aquifers, thereby causing compaction mostly in the clay and silt layers of the aquifer sediments. In 2013, water-level-altitude contours for the Chicot aquifer ranged from 200 feet (ft) below North American Vertical Datum of 1988 (hereinafter, datum) in a small area in southwestern Harris County to 200 ft above datum in central to west-central Montgomery County. Contoured 5-year and long-term changes in water levels in the Chicot aquifer ranged from a 30-ft decline to an 80-ft rise (2008–13), from a 120-ft decline to a 100-ft rise (1990–2013), and from an 80-ft decline to a 200-ft rise (1977–2013). In 2013, water-level-altitude contours for the Evangeline aquifer ranged from 300 ft below datum in south-central Montgomery County to 200 ft above datum in southeastern Grimes and northwestern Montgomery Counties. Contoured 5-year and long-term changes in water levels in the Evangeline aquifer ranged from an 80-ft decline to an 80-ft rise (2008–13), from a 220-ft decline to a 220-ft rise (1990–2013), and from a 360-ft decline to a 260-ft rise (1977–2013). In 2013, water-level-altitude contours for the Jasper aquifer ranged from 200 ft below datum in south-central Montgomery and north-central Harris Counties to 250 ft above datum in northwestern Montgomery County and extending into northeastern Grimes and south-central Walker Counties. Contoured changes in water levels in the Jasper aquifer ranged from a 100-ft decline to 20-ft rise (2008–13) and from a 220-ft decline to no change (2000–13). Compaction of subsurface sediments of the Chicot and Evangeline aquifers was recorded continuously by 13 borehole extensometers. For the period of record beginning in 1973–2012, cumulative measured compaction ranged from 0.100 ft at the Texas City-Moses Lake extensometer to 3.632 ft at the Addicks extensometer. The rate of compaction varies from site to site because of differences in groundwater withdrawals near each site and differences among sites in the clay-to-sand ratio and compressibility in the subsurface sediments. Therefore, compaction rates are unique for each extensometer site.