Know Your Aquifer

Water - a natural resource essential to all life - flows over, under, and above the Earth’s surface. We rely on water for many needs, such as drinking, cooking, irrigation of crops, industrial, and recreational uses. The primary sources of water supply for Texans are groundwater and surface water. In 2018, Texans used approximately 7.96 million acre-feet of groundwater (TWDB), was 56% groundwater, 42% surface water, and 2% comes from reuse ( TWDB ). The Gulf Coast Aquifer system in Texas provides groundwater to more than 50 counties stretching from the Sabine River to the Rio Grande, including Harris and Montgomery and their surrounding counties. The Gulf Coast Aquifer system is comprised of three aquifers: the Chicot, Evangeline, and Jasper. These counties are also served by surface water from Lake Livingston, Lake Houston, and Lake Conroe reservoirs. 

In Texas, surface water is largely owned by the state with small exceptions for domestic and livestock use - though water users hold permits to withdraw surface water. Groundwater rights are generally held by the person who owns the land above the water resource. The groundwater is allowed to be used by landowners who drill a well to access it, with exceptions. No user may pump groundwater to a point of depriving others access to groundwater, pump without permission if they are located in a groundwater conservation district, be wasteful with water, or cause harmful subsidence.

Ensuring we understand what an aquifer is, how aquifers are used, how we get drinking water from aquifers, and what entities govern their usage is critical to managing water resources sustainably. The Know Your Aquifer Story Map provides information on these critical issues. An interactive Groundwater Dashboard Frequently Asked Questions (FAQ) section at the end allows the exploration of data and information to answer additional questions you may have.

The  Regional Groundwater Science Partnership  is providing this resource to improve understanding of the relationship between groundwater, surface water, and subsidence within the Gulf Coast Aquifer system.

The hydrologic cycle, or the water cycle, is the movement of water between the atmosphere, Earth’s surface, and the ground. The water cycle begins when water, driven largely by the sun’s heat, evaporates from surface water bodies such as lakes and oceans. As the evaporated water rises into the atmosphere and cools, it condenses into clouds. These condensed water droplets fall to the Earth’s surface as precipitation in the form of rain and, on a rare day in Texas, snow or sleet. Upon reaching Earth, precipitation can flow into surface bodies of water, return to Earth through evapotranspiration, or  infiltrate into the ground. Some water flows over the surface of the land and gathers in rivers, streams, and lakes. Surface water can eventually trickle through permeable (easy to flow through) sediment and rock deposits to gather in aquifers as groundwater. The water may remain in the ground for decades, centuries, or millennia before resurfacing. Groundwater naturally flows to the surface in some areas, providing supply for streams in Texas. Interactions between groundwater and surface water are important to the hydrologic system.

“The hydrologic cycle, or the water cycle – is the movement of water between the atmosphere, Earth’s surface, and the ground.”  

Water can be classified into two primary sources based on where we find it: surface water or groundwater. Much like the names imply, surface water is water that exists on the Earth’s surface, and groundwater is water we find underground. Surface water can be in the form of streams, bayous, rivers, lakes, reservoirs, estuaries and oceans - really any water that a person can see above ground. Water supply reservoirs are purposefully used to manage surface water. The surface  water reservoirs in our area are Lake Conroe, Lake Houston, and Lake Livingston ( Drinking Water Operations | City of Houston - Houston Public Works ). 

Groundwater is found in gaps between soil and rocks in the ground. The reservoirs where groundwater is stored are called aquifers and are composed of permeable sediments like sand, gravel, and rock formations with porosity and permeability. Aquifers hold water much like a sponge - water can be soaked up to fill pore spaces, but it can also flow freely. Aquifers with more total space in its pores have a higher porosity.

Aquifers are underground formations that hold water within gaps between rock particles, like sand, silt, and gravel ( USGS ). An aquifer system is a large storehouse of water and can consist of many aquifers. Aquifers can be made up of different types of soil and rocks, and their differences impacts the aquifer’s ability to transport and store water. For example, the Edwards Aquifer in Central Texas is a karst limestone formation made up of rocks with large holes (like swiss cheese). Water flows much more quickly through these rocks than the rocks in most other Texas aquifers. Groundwater can take years, decades, or millennia to trickle from the surface and move through the rocks and soil of an aquifer. Time is essential for recharge to occur because precipitation falling on Earth’s surface has to trickle through porous spaces of the ground to enter an aquifer as groundwater.  

There are confined and unconfined aquifers. Unconfined aquifers are close to the surface without a confining layer of solid material above them. Confined aquifers are beneath an impermeable layer that traps the groundwater under pressure. In some locations  confined aquifers can exist below unconfined aquifers. Groundwater that comes from a confined aquifer is called artesian water. Some wells drilled into these aquifers can flow freely even without a pump because of the pressure from the confined aquifer.

The Gulf Coast Aquifer is an aquifer that stores groundwater. According to the  Texas Water Development Board , the Gulf Coast Aquifer has an area of 41,970 square miles and serves 56 counties. The US Environmental Protection Agency ( EPA ) estimates that the Gulf Coast Aquifer supplies these counties with nearly 1.4 million acre-feet of water per year which equates to about 456 trillion gallons -enough to fill 691,200 Olympic-sized swimming pools or enough to fill Lake Conroe approximately 3.4 times!

The Gulf Coast Aquifer system is comprised of three aquifers: The Chicot, Evangeline, and Jasper. The Gulf Coast Aquifer also includes the Burkeville confining unit, the layer that separates the Evangeline and Jasper aquifers, confining the Jasper and the Catahoula Sandstone confining unit, a very deep bottom-layer (basal) hydrogeologic unit. A cross section reveals that the aquifers vary in thickness. The approximate downdip limit of freshwater is around 3,000 feet. This limit means that in the Jasper and Evangeline aquifers, the water beyond this depth becomes more brackish. Brackish water is somewhat saltier than fresh water but not as salty as seawater. Currently, there are thousands of groundwater wells in the area that spans the Chicot, Evangeline, and Jasper Aquifers, and it is expected that more will be drilled in the future.

Within an aquifer system, each sub-aquifer can have a different recharge rate because of its distance from the surface and the types of rocks and soil above it. Where all three aquifers are present, the Chicot and the Evangeline are the shallowest, so water from the surface reaches them first. The Jasper is the deepest of the three aquifers, and there is a layer of low permeability sediments between the Evangeline and the Jasper Aquifers (the Burkeville confining unit) that increases the distance and time required for water to flow between the Jasper and overlying Evangeline Aquifer. In addition, recharge rates are impacted by land cover; recharge zones (areas of the aquifer where water can infiltrate through the permeable rock and sediment) vary spatially.

A drought can limit how much water reaches the aquifers from the surface. The decreased recharge rate can mean lower aquifer water levels – the rate at which the aquifer water levels are impacted will vary based on the recharge rate. During periods of drought, people increase water withdrawals to compensate for lower precipitation and water availability. The lowering of the artesian pressure in aquifers resulting from pumping can lead to subsidence.

When Earth’s land surface begins to sink in elevation, we refer to it as land subsidence. Subsidence can occur due to natural or human processes and is most often caused by the removal of subsurface water, oil, natural gas, or mineral resources. Normally, water under pressure in artesian aquifers fills in the pores, or the spaces between rock particles, which helps support ground layers. As water is withdrawn from the pores and the water pressure is reduced, support lessens for the clay layers that occur between the aquifer sand layers and the clays. As the subsurface spaces between rock particles diminishes, layers begin to compact, resulting in subsidence. Compaction occurs in fine grained clay and sand under the Earth’s surface when water is removed. Pumping can be less than or equal to recharge and compaction in the empty pore spaces can still occur. Compaction of the layers of clay and sand particles due to groundwater extraction can lead to subsidence.

When too much subsidence occurs, faults may appear on the surface, homes and buildings can shift causing foundations to crack, or the risk of flooding can increase in low-lying areas. Additionally, subsidence puts a strain on infrastructure such as roads and bridges, limits the aquifer’s ability to hold water in the future, and may cause issues with well integrity  (USGS TXWSC - Gulf Coast Aquifer Subsidence) . Subsidence is important to address early as compaction in aquifer clay layers is often irreversible. If the spaces in the soils shrink through compaction, it is not possible for the water to refill those areas, ultimately reducing the capacity of the aquifer to serve as a water supply reservoir.

In the next 50 years, surface water is projected to be depleted by 1% and groundwater is projected to be depleted by 24% ( TWDB ). Entities like Groundwater Conservation Districts are charged with managing and maintaining groundwater supplies for the future and with educating their respective communities about the groundwater in their area.

“In 2015, approximately 90% of groundwater used by Texans was withdrawn from areas managed by GCDs.”

GCDs are often limited to specific counties, while aquifers span multiple county boundaries. GCDs meet with other districts in designated Ground Water Management Areas (GMAs) to set targets (Desired Future Conditions) for groundwater supply in their shared aquifers. There are 16 GMAs in Texas; the Houston-Galveston Region is in GMA14, outlined in the map

 The Texas Legislature established the Lone Star Groundwater Conservation District (LSGCD) in Montgomery County in 2001. LSGCD develops and implements management strategies for both conservation and use of groundwater resources. LSGCD is part of the  Ground Water Management Area 14  (GMA14). GMA14 includes the groundwater conservation districts of Bluebonnet, Brazoria County, Lone Star, Lower Trinity and Southeast Texas as well as the Harris-Galveston and Fort Bend Subsidence Districts, and covers the Gulf Coast and Carrizo-Wilcox aquifers LGSCD and GMA14 entities serve as the go-to for the Montgomery County community for information about current conditions and desired future conditions.

"The work of the  Regional Groundwater Research Partnership  mission is to make complex science accessible to stakeholders to promote sustainability of our region's groundwater resources for future generations."