What is Subsidence and why should we be concerned about it?
Webster’s defines subsidence as “to sink, to fall to the bottom; to settle.” Technically speaking, it is the reduction or decline in elevation of the land surface due to compressing the many underlying layers of soil. While it occurs slowly over long periods of time due to the natural compaction of soils, subsidence can be greatly accelerated by the withdrawal of groundwater from underground aquifers. In the Gulf Coast region, the withdrawal of oil and natural gas is also credited with contributing to subsidence.
The fluids withdrawn from very shallow oil and gas fields allowed the clay layers to compact beneath the land surface. Approximately two feet of subsidence resulted from early 20th century oil and gas withdrawal. The growth of greater Houston since the 1920’s demanded significant water supplies. The aquifers beneath land surface yielded amazing amounts of high quality water, and the area grew substantially on the basis of an available — and seemingly endless – source of groundwater. However, since the original two feet of subsidence from oil and gas withdrawal, the burgeoning population’s thirst for groundwater caused as much as five times more subsidence.
In the early 1970’s, groundwater pumpage was approaching 450 million gallons per day (mgd). Subsidence had resulted in elevation losses that threatened entire subdivisions with complete destruction from tidal flooding. With the water demands of the mushrooming population and the expanding petrochemical industry by the mid-1970’s, at least 6 feet of subsidence had occurred along an area between Baytown and Houston. The fate of the Brownwood subdivision of Baytown affords a particularly dramatic example of the dangers of coastal subsidence.
Brownwood was an upper income community of about 500 single-family houses constructed on wooded lots along Galveston Bay beginning in 1938. The area was originally 10 feet or less above sea level, but by 1978, more than 8 feet of subsidence had occurred. In July of 1979, 12 inches of rain fell on the subdivision and caused the flooding of 187 homes. In 1983, Hurricane Alicia struck a final blow, and all homes in Brownwood were abandoned. Today, the area is a swampy area best suited as home to waterfowl.
The Texas Legislature created the Harris-Galveston Subsidence District in 1975, to tackle the problem of subsidence. (The Fort Bend Subsidence District was created in 1989.) The District focused first on the coastal areas, which were most at risk due to the lower elevations. Cities and industries were largely cooperative and from 1976 to 1985, groundwater pumpage was reduced dramatically.
While groundwater reduction was producing significant results in Galveston County and southeastern Harris County, north and west Harris County began to subside at increasing rates due to the increased pumpage to serve the population growth.
Groundwater pumpage had increased significantly through the late 80’s and subsidence rates in northwest Harris County were beginning to equal the all time historic high rates from eastern Harris County — at one-tenth to one-quarter a foot per year.
The Jersey Village area suffered subsidence of 5 feet or more. On two successive years, extreme rainfalls caused major flooding throughout the subdivision…to the extent that some twice-flooded homes were purchased by FEMA.
In addition to Subsidence, the water levels in the water wells have declined significantly, endangering the supply of potable water. The US Geological Survey monitors what is happening to the water in the aquifers that supply the groundwater wells. The Jersey Village area, for example, has experienced a water level decline of up to 260 feet. The Champions/1960 and the F.M. 529/State Highway 6 areas have experienced declines which have affected both the quantity and quality of their water supplies.
Over the next 50 years, the greater Houston area is expected to more than double — essentially adding the current population of the City of Los Angeles to what is already the 4th largest city in the USA. If the increasing population were to rely on groundwater to quench its thirst, another 5 feet of subsidence would result in northwest Harris County by 2030.
The Subsidence District has demonstrated that reducing groundwater demand does in fact help halt subsidence. In some areas where reduction plans have been in place over the past several decades, the aquifers have recharged as well. Also critical, however – in both the short- and long-term – is the teaching and implementation of water conservation throughout our communities, neighborhoods, businesses, and households…all the way down to the youngest family members.
What is the Subsidence District’s groundwater reduction mandated timeline?
The Fort Bend Subsidence District has divided Fort Bend County into Regulatory Area A, R/R Sub- Area and Area B. For Area A, the area covered by the NFBWA, Cities of Sugar Land, Missouri City, and Stafford, the first milestone is to reduce groundwater pumpage by 30% by 2013; with a reduction in groundwater pumpage by 60% required by 2025. The NFBWA must submit its Groundwater Reduction Plan for certification by the Subsidence District by 2008 to avoid Disincentive Fees. (The Subsidence District points out that these conversion requirements are subject to change based on future Regulatory Plan re-evaluations.)