Houston, a major U.S. city on the low-lying Texas coastal plain, is experiencing ground subsidence, where its land surface is gradually sinking. This ongoing issue has garnered significant attention due to its potential implications for the region. This article explores what ground subsidence entails, the primary factors contributing to Houston’s sinking, its consequences, and the strategies being implemented to mitigate further land loss.
What is Ground Subsidence?
Ground subsidence refers to the gradual lowering or settling of the Earth’s surface. While natural phenomena can cause it, human activities often exacerbate or accelerate subsidence. In the Houston-Galveston region, land subsidence is largely defined as the decrease in land-surface elevation resulting from the compaction of fine-grained aquifer sediments, such as silts and clays, caused by extensive groundwater withdrawals. Once these sediments compact, the process is largely irreversible, meaning the land does not rebound even if groundwater levels rise.
Key Reasons Houston is Sinking
The primary driver of land subsidence in the Houston area has been the extensive withdrawal of groundwater from its underlying aquifers. Since the early 1900s, the Chicot and Evangeline aquifers, part of the Gulf Coast aquifer system, have served as the main source of water for municipal, industrial, and commercial use in the greater Houston region. As water is pumped out of these aquifers, the pressure supporting the overlying soil layers decreases. This reduction in pressure causes the clay and silt layers within the aquifer system to compact irreversibly, leading to the measurable sinking of the ground surface. By 1979, some areas in the Houston-Galveston region had subsided by as much as 10 feet due to these withdrawals.
Historically, oil and gas extraction also contributed to localized subsidence, particularly in areas like the Goose Creek Oil Field near Baytown in the early 1900s. While oil and gas withdrawal can cause localized sinking, its overall contribution to regional subsidence in Houston has been found to be relatively small compared to the effects of groundwater pumping. Groundwater withdrawal remains the predominant cause, with some areas experiencing up to 9 cm of subsidence between 2016 and 2020, equivalent to 2 cm per year.
Impacts on the Houston Area
The sinking land in Houston has several significant consequences for the region and its inhabitants. A major impact is the increased risk and severity of flooding. As the land subsides, areas move closer to sea level, compromising natural drainage and making them more vulnerable to heavy rainfall events and storm surges from hurricanes. The Brownwood Subdivision in Baytown, for instance, has largely been submerged and converted into a nature area due to subsidence. Uneven subsidence can also trigger movement along existing faults, leading to surface offsets and damage to property.
Land subsidence also causes considerable damage to public and private infrastructure. Structures like building foundations, roads, bridges, and pipelines can crack or become misaligned due to uneven settling of the ground. This damage often requires costly repairs and can pose safety hazards. The total economic costs associated with subsidence damage and mitigation in the greater Houston area are substantial, potentially reaching billions of dollars.
Addressing the Subsidence Challenge
To combat land subsidence, significant efforts have been made to regulate groundwater pumping and promote alternative water sources in the Houston area. A key player in these mitigation strategies is the Harris-Galveston Subsidence District (HGSD), established by the Texas Legislature in 1975. The HGSD’s mission is to manage groundwater withdrawal and enforce regulations to prevent further land subsidence within Harris and Galveston counties. This district was the first of its kind in the United States, created in response to growing concerns about subsidence’s impact on the region.
The HGSD has implemented regulatory plans that mandate a reduction in groundwater use by encouraging a shift to surface water sources. For example, areas along the Houston Ship Channel were required to convert to surface water supplied from reservoirs like Lake Livingston, which significantly improved subsidence rates in those coastal regions. Current mandates require water providers in specific regulatory areas to reduce groundwater use to 60 percent of total water demand by 2025 and 80 percent by 2035. These conversions involve developing new infrastructure to deliver surface water from sources such as Lake Houston, Lake Conroe, and the Trinity River. The HGSD also conducts scientific research and water conservation programs to ensure a sustainable water future and continued reduction in subsidence rates.