Land Elevation Changes Due to Groundwater Withdrawals Indicate Regional Shifts in Houston-Galveston Area
Extensive groundwater withdrawals have caused the loss of land-surface elevation, or subsidence, in parts of the Houston-Galveston region in Texas, according to a recent U.S. Geological Survey annual report.
The Houston-Galveston region represents one of the largest areas of land surface subsidence in the United States. Most of the land-surface subsidence in this region has occurred as a direct result of groundwater withdrawals for municipal supply, commercial and industrial use and irrigation. Groundwater withdrawn from the Chicot, Evangeline and Jasper aquifers, within the Gulf Coast aquifer system, has been the primary source of water for these uses since the early 1900s.
“The water-level and compaction data provided by the USGS is absolutely critical for water managers and planners to make informed resource management decisions throughout the region,” said Mike Turco, general manager of the Harris Galveston and Fort Bend Subsidence Districts. “This information is used by the District to provide a better understanding of the impact of our regulatory plan on aquifer water levels and subsidence.”
The latest annual report from the USGS marks 40 years of long-term monitoring of groundwater levels and land-surface subsidence in the greater Houston area. Water-level changes for 2015–2016 ranged from a 65-foot decline in some areas to a 61-foot rise in other areas. The monitoring equipment in Addicks, Texas, adjacent to Interstate 10, has measured as much as 3.7 feet of subsidence since 1974. The new report includes various maps depicting water-level altitudes, short and long-term water-level changes and aquifer compaction.
“The demand for water to meet the rapidly developing Houston-Galveston area is derived from groundwater withdrawals and surface water pumpage,” said USGS scientist and lead author Mark Kasmarek. “Because the availability of surface-water sources fluctuates seasonally and in times of drought, groundwater from the Gulf Coast Aquifer System continues to be an important natural resource for the region.”
This new report illustrates water-level altitude and water-level changes in the Chicot, Evangeline and Jasper aquifers, and compaction in the Chicot and Evangeline aquifers. Since compaction monitoring began in 1973 or later, the current amount of compaction of fine-grained sediments of the aquifers ranges from 0.095 feet at the Texas City-Moses Lake extensometer site in Galveston County to 3.66 feet at the Addicks extensometer site in Harris County. Water levels in the southeastern parts of the study area in the Chicot and Evangeline aquifers have generally continued to rise since 1977, while water levels in the northern and western parts of Harris County and Southern Montgomery County have continued to decline. For the Jasper aquifer, primarily used in Montgomery County, water levels have continued to decline with development in the area.
The USGS has been monitoring water levels in the Houston-Galveston region since the 1930s, and has published the data annually since 1977. Presently, scientists monitor more than 690 water wells throughout an 11-county area on an annual basis to collect data on groundwater levels of the aquifers. Water levels are measured annually from December to March because the water levels are usually higher during these months and groundwater withdrawals are at a minimum.
In this study, compaction was measured using equipment called borehole extensometers, which is a device that basically uses boreholes and a long continuous string of pipes to measure the compaction of the aquifer that corresponds to land-surface-elevation changes. Thirteen extensometers at 11 sites have continuously recorded data since they were activated or installed during 1973–1980.
The Houston-Galveston region consists of Harris, Galveston, Fort Bend, Montgomery, Brazoria, Chambers, Liberty, San Jacinto, Walker, Grimes and Waller Counties. By 1979, as much as 10 feet of subsidence had occurred in the region, and approximately 3,200 square miles of the 11,000 square mile geographic area had subsided more than one foot.
This annual report summarized results from long-term monitoring and was created in cooperation with the Harris-Galveston Subsidence District, City of Houston, Fort Bend Subsidence District, Lone Star Groundwater Conservation District and the Brazoria County Groundwater Conservation District.