Land subsidence is sinking of the land surface, caused by compressing the many layers of clay beneath the surface. When we pump large amounts of groundwater from the aquifers beneath us, we pull water out of the many layers of clay, which can cause the clay to compact. Some natural land subsidence also occurs over long periods of time, due to natural settling.
Subsidence can be an important issue in certain areas such as the Gulf Coast region. People who live in the lower lying coastal areas (i.e., lower elevations and flatter topography) have much more reason to be concerned with subsidence than those who reside further inland in areas of higher elevations with more topographic relief. One of the primary concerns of subsidence in the coastal regions is flooding due to storm surges associated with hurricanes, where permanent seawater inundation and periodic storm water surge can dramatically impact property.
Subsidence is known to have occurred in the southern part of Montgomery County based on monitoring and long-term surveying of land surface. Since the mid 1970’s, the USGS and the Harris-Galveston Subsidence District have conducted scientific investigations and subsidence monitoring for over 40 years. These efforts have identified and documented the susceptibility of land surface subsidence due to the production of groundwater in the Chicot and Evangeline aquifers.
The Lone Star Groundwater Conservation District in 2012 purchased six Periodically Accessible Monitors (PAMs), which are specifically designed to detect subsidence. These units operate in conjunction with the two that have been monitoring land levels in Montgomery County for more than 10 years.
Regular, precise, automated readings are taken of the elevation of the ground at locations throughout the county. The data is temporarily stored in an on-site monitor and periodically uploaded to computers at the Harris Galveston Subsidence District.
The live data from the two older PAM units may be viewed by clicking on the Subsidence Monitoring Network Map (If you have Google Earth installed on your computer, simply click on the .kml file, and Google Earth will automatically open, showing you the network map.). Lone Star GCD is working with the Subsidence District to make the newer stations available online. You can also view water levels by visiting the Lone Star Static Well Level Monitoring Network Map.
Harris-Galveston Subsidence District’s (HGSD) Presentation on the Investigation of Brackish Resources in the Gulf Coast Aquifer and Determination of the Potential Subsidence Risk from Resource Development (Sept. 11, 2018)