That Sinking Feeling
May 23rd, 2018
Harris County and the seven surrounding counties (Brazoria, Chambers, Fort Bend,
Galveston, Liberty, Montgomery, and Waller) combined are larger than 32 of the individual states in the United States of America. Thus, it is not surprising that the greater Houston area has had many significant challenges that most states will not likely face.
What is 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, eliminating the ability to store water long-term in an aquifer.
Greater Houston is no stranger to subsidence, which was caused early last century by oil and gas withdrawal, and later predominantly by the over-reliance of groundwater. The fluids withdrawn from very shallow oil and gas fields allowed the clay layers to compact beneath the land surface.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. The area grew substantially on the basis of an already distributed groundwater source. Since the original two feet of subsidence from oil and gas withdrawal, the increasing population’s thirst for groundwater yielded as much as five times more subsidence.
Preventing further subsidence from groundwater extraction required critical thinking, so turn-of-the-century land-subsidence benchmarks were releveled in the 1940’s; not only did they show subsidence had been occurring, but provided a new point to measure from more diligently. With numerous studies linking groundwater withdrawal to subsidence — and ongoing measurements confirming those findings — groups of citizens began to work for a reduction in groundwater use in the late 1960’s.
By 1973, the City of Galveston had begun converting to surface water supplied from Lake Houston, and in May of 1975, the Texas Legislature created the Harris-Galveston Subsidence District (HGSD), the first of its kind in the United States. Authorized as a regulatory agency created to “end subsidence” and armed with the power to restrict groundwater withdrawals, the HGSD immediately went to work on a plan to positively impact the critical situation in the coastal areas. By 1976, the HGSD had begun the process of compiling hydrologic information on the characteristics of the Chicot and Evangeline aquifers, engineering planning information on water usage and water supply in Harris and Galveston counties, and implementing regulatory procedures associated with their first groundwater regulatory plan.
By converting industries on the Houston Ship Channel to surface water supplied from the Lake Livingston reservoir, subsidence in the Baytown-Pasadena area was dramatically improved. However, as subsidence was stabilizing in the coastal areas, groundwater levels in inland areas north and west of Houston were rapidly declining. In the Evangeline aquifer, measurements recorded a decline of more than 100 feet between 1977 and 1997.
Due to the increasing threat subsidence posed to these areas, the HGSD adopted a series of regulatory plans to reduce groundwater pumpage, and ultimately mandated, in their 1999 plan, a reduction to only 20% reliance on groundwater by 2030. It is HGSD’s goal that the same dramatic improvements will occur in these areas as were experienced south and east of Houston years ago.
As part of an ongoing effort to prevent further subsidence in the region, the Texas Legislature went on to create the Fort Bend Subsidence District in 1989, the Lone Star Groundwater Conservation District in 2001, and the Brazoria County Groundwater Conservation District in 2005. These Districts all have similar mandates to the HGSD and provide for the regulation of groundwater withdrawals in areas within their jurisdiction.