Many areas in the United States undergo problems with land subsidence. However, when all areas are considered throughout the country, the Greater Houston area in the Gulf Coast Aquifer is viewed as the metropolitan area that has been most adversely affected by the previously stated problem (Coplin and Galloway, 2009). Some of the counties within the area that is plagued by the problem of land subsidence are Harris, Galveston, Fort Bend, Jasper, and Wharton (Texas Water Development Board). The primary cause of land subsidence in that area has been attributed to groundwater pumpage most of which has been used for the municipal supplies, commercial and industrial usage, and irrigation (Kasmarek et al., 2012).

Figure depicting a map of Harris and surrounding counties showing the boundaries of the subsidence districts (Michel, 2006

Subsidence cause

Coupled with the rapid pumping rate of groundwater, the beds of clay found in the aquifer system are major contributing factors for the persistent land subsidence problem that occurs in the Houston-Galveston counties. Generally during pumpage, the potentiometric surface declines and the hydraulic pressure on the sediments decrease. The lowering of the hydraulic pressure and the weight exerted by the overlying sediments cause the compaction of the dewatered sediments. Because sands possess relatively large pore spaces, greater hydraulic conductivities, and generally rounded grains, they are dewatered first and compact only slightly when pumping rates are low. However, when pumping rates are increased and continued over long periods, the water removed from the sands are accelerated and eventually water is withdrawn from less transmissive layers like clays. Since clays are sheet silicates, their individual grains are flattened. As they become dewatered, the burden from the overlying sediments increase, and the clays readily compress which leads to a reduction in porosity and thickness of the layers. A collapse of the ground surface eventually ensues because the weight of the compacted overlying rock material would not be able to be supported when the sediments in the aquifer experiences the vertical load on its skeletal matrix (Davidson and Mace, 2006).

Figure depicting the mechanism of subsidence in an aquifer composed of sand, gravel, clay, and silt (Kasmarek et al., 2012)

In the case of the Gulf Coast Aquifer system, particularly in the Houston-Galveston region, the water levels have drastically declined within two aquifers, the Chicot and Evangeline. Estimates from 1943 to 1977 show that the Chicot aquifer experienced water-level declines of as much as 200 feet, while the Evangeline aquifer recorded approximate reductions in the region of 300 feet. Because of the pumpage of large quantities of groundwater from the aquifer throughout most of the 20^th century, water was ultimately removed from the layers of clays after the sand layers were exhausted of their resources. The clays then compacted due to the reduced internal pressure and the weight from the overburden above the clay layers, eventually resulting in land-surface subsidence (Michel, 2006).

Subsidence history

The history of subsidence in the Houston-Galveston area has been well documented. The first known records were done by Wallace Pratt and Douglas Johnson in 1927. Over an eight year period from 1917 to 1925, both men noted roughly three feet of subsidence in the area. In the late 1950s and early 1960s, residents of Brownwood believed that the water surrounding their living area was rising; however, they were deceived because in actuality the land was sinking. Until 1961, after the passing of Hurricane Carla, many areas were flooded which brought attention to the problem of subsidence in the Greater Houston area.  In addition, during the 1960s, industries along the coast of the Houston Ship Channel observed that the ships they serviced in the Channel were becoming increasingly higher than the docks and loading facilities. By the late 1970s, land subsidence was about five feet in the Texas City area, greater than nine feet along the Houston Ship Channel, and as much as ten feet in the general Houston-Galveston region (Michel, 2006). Furthermore, approximately 3,200 mi² of the 11,000-mi² geographic area had subsided more than one foot (Kasmarek et al., 2012).

Figure showing subsidence from 1906 to 2000 in the Houston-Galveston area (hgsubsidence.org) 

Subsidence effects

Generally, regional land subsidence is not quite obvious, but in some areas drastic changes result which then may have lasting effects on the surrounding environment. In some cases, as much as ten feet of subsidence has shifted the position of the coastline, changed the distribution of wetlands and aquatic vegetation, and affected recreational activities and tourism contributions to the area’s economy. Additionally, land subsidence has increased the effects of coastal flooding. A prime example of is the San Jacinto Battleground State Historical Park. About 100 acres of the historical site is now partly submerged underwater due to subsidence (Coplin and Galloway, 2009). Another pertinent example is in the Brownwood subdivision of Baytown where over 2.5 m of subsidence had been measured. The area was later inundated by subsidence-related flooding which has led to the abandoning of most of the homes and relocation of the town’s residents (Thompson and Neighbors, 1986).

Figure showing the effects of flooding in Houston due to subsidence (Coplin and Galloway, 2009)

Figure showing an abandoned house in the Brownwood subdivision (Coplin and Galloway, 2009)

Figure showing coastal subsidence allows shorelines to move landward causing the demise of some coastal woodlands (Coplin and Galloway, 2009)

Figure showing evidence of subsidence around a well in Baytown in the Harris-Galveston Subsidence District (hgsubsidence.org)

Associated fault movement

The withdrawal of vast amounts of groundwater resulting to dramatic declines in artesian pressure, not only caused in subsidence, but it also accelerated fault movement in an area that is riddled with faults. The Houston-Galveston area possesses over 321 km active faults (Thompson and Neighbors, 1986). Some of them have no topographic escarpments but control drainage patterns (Kreitler, 1977). The multitude of faults that exist in the Houston-Galveston region are both regional-scale slowly sliding of the land mass towards the Gulf of Mexico and local structures associated with oil fields (Coplin and Galloway, 2009 from Holzshuh, 1991). Groundwater removal activates these faults by the resulting compaction of the aquifer (Kreitler, 1977). The monitoring of fault creep, water levels, and land subsidence has indicated a clear cause-and-effect relationship which has led to the conclusion that fault movement is caused by water-level decline and the resulting land subsidence. In the 1970s, the use of imported surface water and a reduction in water pumpage resulted in a period of water-level recovery in the Houston area. The result of those steps led to a stoppage or reduction of fault creep in the area of water-level recovery while fault creep continued in the areas of ongoing water-level decline (Coplin and Galloway, 2009).

Figure showing a house in Baytown near Brownwood damaged by fault creep (Coplin and Galloway, 2009)

Solutions to the subsidence problem

The reduction in the rates of subsidence in the Houston-Galveston area can be attributed to the combined efforts of groundwater regulation and switching water exploitation from groundwater to a greater reliance on surface water. In 1975, the Texas State Legislature established the Harris–Galveston Coastal Subsidence District. The District’s area encompassed approximately 5,620 km² (Thompson and Neighbors, 1986). The purpose of the District was to regulate and reduce groundwater withdrawals within its confines to slow the rate of land-surface subsidence and the increased flooding that inadvertently followed (Michel, 2006). The District had no taxing authority but collected a fee from each well owner who was issued a permit for the drilling and operation of wells with an inside casing diameter greater than 12.7 cm. To ensure accurate accounting of the amount of groundwater withdrawn by a particular well, the District required each well to have a meter which was inspected annually by a district member. If it appeared that an individual violated the terms of the permit or failed to comply with the law, the District had the authority to file suit or possibly recover monetary penalties (Thompson and Neighbors, 1986). In the years following, the Texas State Legislature established the Fort Bend Subsidence District in 1989 and the Lone Star Groundwater Conservation District in 2001. The purpose of the additional Districts was to regulate groundwater withdrawals in Fort Bend and Montgomery Counties, respectively. Later in 2003, the Brazoria County Groundwater Conservation District was established, and it was tasked with the responsibility to maintain the quality and availability of the county’s groundwater resource for current users and future generations (Kasmarek et al., 2012). Additionally, in the mid-1970s, surface water started to be utilized as a substitute for groundwater. For example, Galveston County began converting from groundwater to surface water from the Brazos River through a series of over-land canals that brought water to the Texas City population and industries. The cities of Baytown, Houston, Pasadena, and others converted most of their groundwater use to surface water from the San Jacinto and the Trinity River. Both the regulations and the switch to a greater reliance on surface water reduced the dependence and ultimately the withdrawals of groundwater considerably (Michel, 2006).

Subsidence Monitoring

The main methods employed by the District to measure subsidence were borehole extensometers and precision leveling. Later, the use of global positioning satellite system was added to its arsenal. A borehole extensometer is a monitor well specifically designed to record changes in vertical displacement between the land surface and the bottom of the well which monitored the total amount of subsidence. Initially, thirteen extensometers ranging in depth from 130 to 915 meters were installed. The extensometers were strategically positioned. Seven of the extensometers were installed rather shallowly to relate compaction to depth. The remaining six of the extensometers were constructed to depths below which artesian pressures were not declining and compaction was not occurring. Therefore, the total subsidence could be quantified. In addition, over the years, multiple land surface leveling surveys have been done in the Houston-Galveston area using specific extensometers as points of reference. Precision leveling determined the surface elevations then the results were used as a means of comparison with previous years to ascertain the extent of the subsidence problem.  During the 1986 leveling program, the District began using the global positioning system (GPS). The system proved useful for traversing long distances, obtaining subsidence data in areas where leveling lines or extensometers were not present (Thompson and Neighbors, 1986).

Figure depicting the cross-sectional perspective of a borehole extensometer (Kasmarek et al., 2012)

  Figure showing a USGS hydrologist measuring levels at an extensometer site (Coplin and Galloway, 2009)            

Flow Path Model

Contamination Issues

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