In the simplest of terms, land subsidence is the sinking of the earth’s surface due to subsurface movement. Aquifer compaction from groundwater withdrawal, oil and gas extraction, underground mining, sinkholes, drainage of organic soils, and natural depositional compaction are the most common causes of subsidence in the United States. More specifically for the greater Houston-Galveston region, land subsidence is the decrease in land-surface elevation caused by aquifer compaction due to abundant, long-term groundwater withdrawals.
In the greater Houston-Galveston region, groundwater is pumped primarily from the Chicot and Evangeline aquifers as a source of freshwater. The geology of these aquifers includes discontinuous layers of gravel, sand, silt, and clay. The extraction of large volumes of groundwater causes clays to depressurize and compact. Aquifer compaction affects overlying stratigraphic units all the way to the land surface and is observed as subsidence.
Illustration of subsidence in a Gulf Coast aquifer as a result of groundwater withdrawals producing a decrease in the potentiometric surface (the groundwater level). Source: Kasmarek, M.C., Ramage, J.K., and Johnson, M.R., 2016, 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: U.S. Geological Survey Scientific Investigations Map 3365, pamphlet, 16 sheets, scale 1:100,000, http://dx.doi.org/10.3133/sim3365.
HGSD’s goal is to regulate groundwater use in volumes that will not deplete the aquifers in order to prevent subsidence and ensure future water availability. Since 1975, HGSD has regulated the use of groundwater and strongly encourages water users to shift from groundwater to other sources of water, such as treated surface water, through disincentive fees. Additional information on the HGSD’s regulatory program is provided in the Regulatory Plan, which works in conjunction with the District’s well permitting program. As a result of HGSD rules, groundwater use has reduced dramatically in the areas along the coast and Houston Ship Channel, and consequently subsidence in those areas is negligible.
Subsidence can be reduced, stopped in some cases, when the pumping of groundwater is reduced. However, the conversion from groundwater to alternative sources of water (surface water, reclaimed water, etc.) is not as simple. Many of the cities, industries, and others in Regulatory Areas 1 and 2 (the coastal areas) converted in the 1990s to surface water, at considerable costs. Conversion to alternative supplies is an ongoing process for Regulatory Area 3, with full conversion required to be completed by 2035.
Considerable supplies of surface water are available through the development of Lake Livingston on the Trinity River, Lake Houston and Lake Conroe on the San Jacinto River, and the Brazos River. The HGSD reviews the availability of freshwater resources as part of the science and research program. An alternative water supply assessment is currently underway as part of the Joint Regulatory Plan Review, and will include an assessment of aquifer storage and recovery, brackish groundwater and sea water desalination as alternative water supply options.
Flooding is a serious consequence of subsidence especially in the Houston-Galveston area as much of the land is either within coastal zones or has limited green space to absorb surficial water. Other impacts of subsidence include localized faulting and infrastructure damage.
A perfect example of the effects of subsidence in coastal areas is the Brownwood Subdivision located in the City of Baytown. The Brownwood subdivision originally had over 400 homes built in 1940s. As a result of large volumes of groundwater withdrawal, the area experienced as much as 10 ft of subsidence that resulted in some homes being inundated and others experiencing frequent flooding. Brownwood is now mostly underwater and has been turned into the Baytown Nature Center that provides habitat for native plants and animals. The Houston History Magazine wrote an article in March 2019 that includes links to original video footage of the site and interviews with the residents of Brownwood. .
Brownwood Subdivision in 1944 to 2002. Slide the arrows to see the impact of subsidence.