The cumulative effects of 40 years of groundwater withdrawals in the Houston-Galveston region are documented in a report from the United States Geological Survey. Decades of extensive withdrawals have caused the land to sink, or subside.
The Houston-Galveston region is one of the largest areas of land surface subsidence in the U.S., according to the agency. Water withdrawals started as early as 1836, but routine, precise data gathering in the region began in the 1970s. The information has been used to inform water managers and politicians about the effects of regulations on the aquifers, and the consequences of subsidence.
The USGS 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 Groundwater withdrawn from the Chicot, Evangeline and Jasper aquifers, within the Gulf Coast aquifer system, has been the primary source of water for these uses since the early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.
Reports on groundwater monitoring are issued each year, but this annual report marks 40 years of long-term monitoring of groundwater levels and land-surface subsidence in the region.
The report specifically examines the rate of changes in the Chicot, Evangeline and Jasper aquifers as well as compaction in the Chicot and Evangeline aquifers. The agency found, through its routine monitoring, that water levels in the Chicot and Evangeline aquifers have and continue to rise since 1977, for example. The Jasper aquifer water levels have declined as a result of development in the area.
While water levels were roughly 65 feet lower in some areas, other areas saw increases by a similar amount.
The Harris-Galveston Subsidence District was established by the Texas Legislature in 1975 to reduce or minimize further groundwater subsidence. Its first groundwater regulatory plan was drafted in 1976. Data collection by the USGS followed.
Compaction levels have been studied by the USGS across 11 sites, where extensometers were either activated or installed between 1973 and 1980. Some of the land compaction is notable. Monitoring equipment in Addicks, Texas, next to Interstate 10, has measured as much as 3.7 feet of subsidence since 1974.
The USGS explains the importance of understanding subsidence and compaction, noting:
Compaction of the silt and clay layers reduces the ability to store water in aquifers, and can cause long-term environmental effects such as damage to roadways, bridges, and building foundations […] [It] can partially or completely submerge land, collapse water well casings, disrupt collector drains and irrigation ditches, and alter the flow of creeks and bayous such that there may be an increase in frequency and severity of flooding in affected areas.
The potential for flooding is more pronounced in coastal areas.
No Bouncing Back
Although data collection began fairly recently, people in the area started withdrawing groundwater in 1836. These withdrawals have resulted in significant compaction. An area of about 3,200 square miles has subsided more than a foot, and some areas have seen subsidence of as much as 13 feet.
Subsidence can be stopped but the land will never ‘bounce back.’ Once the water is removed and the clay and soil compacts, it is incapable of springing back like a sponge. The best we can do is prevent further subsidence from taking place.
The USGS has monitored water levels in the Houston-Galveston region since the 1930s. It has published the data annually since 1977. Roughly 690 water wells throughout an 11-county area are monitored to collect data on groundwater levels.
The Houston-Galveston region consists of Harris, Galveston, Fort Bend, Montgomery, Brazoria, Chambers, Liberty, San Jacinto, Walker, Grimes, and Waller Counties. Agencies in those counties cooperating with the federal agency include the Harris-Galveston Subsidence District, City of Houston, Fort Bend Subsidence District, Lone Star Groundwater Conservation District, and the Brazoria County Groundwater Conservation District.
The new report, Water-Level Altitudes 2016 and Water-Level Changes in the Chicot, Evangeline, and Jasper Aquifers and Compaction 1973-2015 in the Chicot and Evangeline Aquifers, Houston-Galveston Region, Texas, includes maps depicting water-level altitudes, short and long-term water-level changes and aquifer compaction.