Texas Gulf Coast Groundwater and Land Subsidence

Over Forty Years of Research in the Houston-Galveston Region

In cooperation with Harris-Galveston Subsidence District, the City of Houston, Fort Bend County Subsidence District, Lone Star Groundwater Conservation District, and Brazoria County Groundwater Conservation District, the U.S. Geological Survey Texas Water Science Center Gulf Coast Program collects, processes, and interprets groundwater-level and aquifer-sediment-compaction data to understand the effects of groundwater withdrawal on land subsidence in the Houston-Galveston region of Texas.

Land Subsidence

Land subsidence is the gradual lowering of land-surface elevation. In the Houston-Galveston region, land subsidence is caused by compaction of fine-grained aquifer sediments (silts and clays) below the land surface due to groundwater withdrawals. Removing water from fine-grained aquifer sediments compresses the aquifer leaving less pore space available to store water resulting in the lowering (sinking or settling) of the land-surface. Most compaction that occurs as a result of groundwater withdrawals is irreversible; even if groundwater levels rise, compacted sediments and the associated land-surface lowering would remain as-is.

Consequences of land subsidence in the Houston-Galveston Region

• Reduces the ability to store water in an aquifer.
• Partially or completely submerges land.
• Collapses water well casings.
• Disrupts collector drains and irrigation ditches.
• Alters the flow of creeks and bayous which may increase the frequency and severity of flooding.
• Damages roadways, bridges, building foundations, and other infrastructure.

Groundwater Withdrawals

Aquifers

The primary aquifers in the Gulf Coast aquifer system in the Houston-Galveston region are the Chicot-Evangeline (undifferentiated) and Jasper, which are composed of laterally discontinuous deposits of gravel, sand, silt, and clay.

Groundwater withdrawn from the Chicot-Evangeline and Jasper aquifers has been the primary source of water for municipal supply, commercial and industrial use, and irrigation in the Houston-Galveston region since the early 1900s.

Prior to 1975, the withdrawal of groundwater from the Chicot-Evangeline aquifer was unregulated, and water levels in the aquifer were declining with associated depressuring, dewatering, and compaction resulting in subsidence.

By 1977, the withdrawals had resulted in water-level altitude declines of 350 feet below datum in the Chicot-Evangeline aquifer in southeastern Harris County, and correspondingly, by 1979, as much as 10 feet of subsidence had occurred in the Houston-Galveston region.

Regulatory Areas

To address the issues associated with land-surface subsidence and compaction, the 64th Texas State Legislature authorized the establishment of the Harris-Galveston Subsidence District to regulate and reduce groundwater withdrawals in Harris and Galveston Counties. Subsequent to establishing the Harris-Galveston Subsidence District, the Texas State Legislature established the Fort Bend Subsidence District and two groundwater conservation districts: Lone Star Groundwater Conservation District and, most recently, Brazoria County Groundwater Conservation District in the Houston-Galveston region to provide for the regulation of groundwater withdrawals in areas within their jurisdiction.

1975: Established by Texas State Legislature.

1976: First regulatory area (Area of Concentrated Emphasis, ACE) established.

1985: Divided into 8 regulatory areas by amount of groundwater reduction needed.

1992: Regulatory areas redivided into 7 areas.

1999: Regulatory areas converted into 3 primary areas.

2013: HGSD regulatory plan updated.

1989: Established by the Texas State Legislature.

2003: Regulatory areas divided into area A, which includes the Richmond-Rosenberg subarea, and area B.

2001: Established by the Texas State Legislature.

2005: Established by the Texas State Legislature.

Data Processing and Map Creation

Since 1977, the USGS Texas Water Science Gulf Coast Program has published regional groundwater maps for the Houston-Galveston Region, Texas. After collecting and evaluating groundwater-level data for quality control, data are incorporated into a geographic information system (GIS) and used for the construction of regional water-level altitude and regional water-level change maps.

Water-level altitude is calculated at each well by subtracting the water-level measurement (depth to water below land surface) from the referenced well land-surface altitude. The accuracy of the referenced well land-surface altitude data has improved through time and the most accurate land-surface altitude data available at the time of publication were used in the creation of each regional water-level altitude map. Up to 2020, regional water-level altitude maps were created by manually interpolating contours of water-level altitude data at observation wells in the Houston-Galveston Region. In 2021, kriging, a geostatistical interpolation method, was adopted as an effective alternative to manual contour interpolation.

Regional water-level change maps were constructed by interpreting the compiled water-level altitude data as the difference in water-level altitude at each well for the water-level data collection year and the historical year or as the difference in water-level altitude at each well for the water-level data collection year and the interpolated water-level altitude of the historic year from a modeled surface. Interpolated modeled water-level altitude surfaces were only used to compute differences when wells were destroyed or were not measured during the individual publication study period. Further information detailing the data processing and map creation for each published regional water-level altitude and regional water-level change contour map can be found in the individual year publication.