Several of the watersheds in Travis County also recharge our aquifers. In addition to our surface water resources, the aquifers are major sources of our drinking water. Because of the diverse geology of our area, the depth of aquifers that lie beneath us varies from only a few meters below the ground, to several hundred meters. Most of our aquifers are almost entirely dependent on rainfall to maintain their storage capacity.
Aquifers are generally defined as naturally-occurring underground units capable of absorbing and storing water. Aquifers have storage capacity in the fractured spaces between rock, gravel, or sand grains that characterize the geologic layer in which they are found. In central Texas, many of our aquifers are considered karst or karstic in nature. The term describes surface and subsurface areas dominated by limestone rock, typically heavily fractured with caves and sinkholes.
As long as the amount of water that is pumped from the aquifer is less than the amount of water that is recharged and absorbed into the aquifer, the level of the aquifer should remain full or even overflow, creating areas of seepage, springs, and the baseflow of rivers. If recharge is a lower rate than pumping, every time you pump water out of a groundwater well, the level of the aquifer (the water table) drops.
In urbanized areas such as much of Travis County, storm water destined to recharge our aquifers easily picks up pollutants as it flows across impervious areas and into the ground. Impervious surfaces also result in runoff moving downstream more rapidly, reducing or preventing natural processes that would otherwise result in runoff slowly percolating back into the ground. Urban aquifers can lose substantial recharge areas due to the increased impervious cover that covers up natural inlets. Therefore, transformation of land to urban uses can result in lower rates of groundwater recharge and an increased likelihood of the groundwater becoming elevated in pollutant concentration.
Travis County is underlain by significant groundwater aquifers that supply approximately 27,500 acre-feet of fresh water per year for domestic, agricultural, and industrial usage. Groundwater in Travis County emerges at springs and water courses providing critical habitat to biological communities that support endangered and other aquatic species. These underground freshwater sources of groundwater include:
- Barton Springs and Northern Segments of the Edwards Aquifer
- Trinity Group Aquifers
- Colorado River Alluvial Aquifer
The Trinity-Edwards Aquifer system is the dominant aquifer in Travis County. The updip Trinity Aquifer west of the Balcones Escarpment is generally fresh water while the downdip Trinity Aquifer is saline. The downdip and artesian portion of the Edwards Aquifer provides fresh water eastward until a “bad water line” beyond which saline water occurs. An important local aquifer, the Colorado River Alluvial Aquifer is located in eastern Travis County. A minor aquifer, the downdip Hickory Aquifer also exists in the western portions of the county. The Hickory in this location is saline. No freshwater aquifers are present in the southeast portion of the county.
The recharge of water into these aquifers is almost completely dependent upon rainfall and the flow of surface water in streams that pass over surface outcrops of these aquifers. Groundwater availability in Travis County is vulnerable to over-pumpage due to both continuing population growth and the occurrence of drought conditions. Southwestern Travis County has been formally designated by the State of Texas as a Priority Groundwater Management Area (PGMA) because this area experiences critical groundwater problems, including shortages of supply. Designation of a PGMA generally results in the creation of a Groundwater Conservancy District. Travis County adopted water availability rules in January, 2012, in an effort to initiate better groundwater management in the Hill Country PGMA, since the State of Texas process has not yet resulted in a GCD after greater than 25 years’ time.
