GMA 14 Includes Subsidence Metric for the Proposed Desired Future Conditions of the Upper Gulf Coast Aquifer
The Groundwater Management Area (GMA) 14 Water Planning Group, which the Subsidence District participates in as an interlocal partner, approved proactive measures to prevent regional subsidence and flooding in the Gulf Coast region.
The proposed desired future condition (DFC), which member groundwater conservation districts unanimously voted to consider, sets limits on how far the aquifer water level can be drawn down in the Gulf Coast aquifer. It also includes limits on subsidence, or the sinking of land surface.
“The adoption of no more than one additional foot of subsidence, on average, in the proposed DFC is a good first step in recognizing the impacts of unregulated groundwater withdrawal on the Gulf Coast aquifer in this area,” General Manager of the Subsidence District Michael Turco said.
“The metrics proposed by the GMA 14 are not only reasonable and attainable, but also necessary for our region’s resiliency and preventing subsidence and flooding.”
The proposed DFC will now go through a 90-day public comment period before the GMA 14 votes to adopt it.
“Over time we’ve seen the effect subsidence has on coastal areas and increasingly how it’s also affecting communities and counties further in-land, as far north as Montgomery County,” Turco explained. “Protecting property owners and the public from the dangers of subsidence – including flooding – is a top priority of the Subsidence District.”
The Role of the GMA 14 in the Region
The Groundwater Management Areas, overseen by the Texas Water Development Board, provide for the conservation, preservation, protection, recharging, and prevention of waste of the groundwater, and to control subsidence caused by withdrawal of water from those groundwater reservoirs.
Made up of Groundwater Conservation Districts, these regional groups are charged with developing the DFC for their aquifers and resulting modeled available groundwater (MAG) for the counties within their area. Ultimately, the MAG serves as a guide which can be used by Groundwater Conservation Districts to set reasonable regulation on groundwater withdrawal.
Introducing Subsidence as a Metric
The GMA 14 considered three scenarios for determining the DFC, all of which included a subsidence metric. While subsidence has always been one of the nine factors the GMA 14 is statutorily required to consider as part of the planning process, this is the first time it’s been introduced as a consideration in the DFC.
Entities in Harris and Galveston Counties, following Subsidence District regulations, have switched from using groundwater as their primary water source to alternative sources that do not contribute to subsidence, such as treated surface water.
In areas that have completed this conversion process, the rate of subsidence has been significantly reduced. Where groundwater use has been cut to 20% of the total water demand or less, subsidence rates were reduced from over one foot per decade to nearly stable.
“As an inter-local participant in the GMA 14 planning process, we look forward to working with member districts to monitor subsidence, aquifer levels and will provide technical assistance along the way as we work together to prevent future subsidence in the region,” Turco shared.
Study Links Groundwater Extraction to Subsidence
A recent Houston Advanced Research Center (HARC) Groundwater Report confirmed the link between groundwater extraction and subsidence in Montgomery County, and emphasized that continued and expanded monitoring is necessary to understanding the long-term impacts of groundwater extraction on the region.
“Adoption of the new DFC will be especially important for the rapidly growing areas adjacent to the Subsidence District,” Turco continued. “For example, we’ve already seen evidence of local surface water conversion significantly lowering subsidence rates in southern Montgomery County as a result of conversion from groundwater to surface water in 2015. Adopting the new DFC should help alleviate future subsidence risks and flooding.”
Early next year, the Subsidence District is expected to complete a study on flooding impacts related to subsidence in the Spring Creek watershed, which borders Harris and Montgomery Counties. This study will provide an important perspective on the benefits of future groundwater conversion within the District and groundwater management decisions in Montgomery County.
The District has also initiated a study to examine new information from Interferometric Synthetic Aperture Radar (InSAR) satellite data that can be used to fill in the gaps of the current subsidence monitoring network.
About the Harris-Galveston Subsidence District
The Subsidence District was created in 1975 by the Texas Legislature with the mandate to cease and prevent subsidence within Harris and Galveston Counties by regulating the use of groundwater. Subsidence has been a long-standing problem in Harris, Galveston, and surrounding counties, with total subsidence near Galveston Bay reaching over 10 feet. Subsidence can contribute to flooding, infrastructure damage, and faulting.
Since 1975, groundwater regulation implemented by the District has resulted in increased aquifer water levels and slowing or cessation of subsidence in regulatory areas closest to the Gulf of Mexico. Regional water authorities and the City of Houston are working together to develop the necessary water infrastructure to facilitate the District’s regulatory plan which mandates continued groundwater use reductions in Northern and Western Harris County where by 2035, groundwater will comprise only 20% of the total water demand of Harris County.