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Aquifers

Types of Aquifers

Aquifers are geological formations that can store, transmit and yield water to a well or spring. Texas aquifers are remarkably diverse in geologic structure, the amount of water they store, the amount of the water taken from them that can be replenished and the rate at which water moves through the aquifer.

There are two basic types of aquifers: confined and unconfined. The two kinds of aquifers respond differently to pumping.

Confined. A confined aquifer is basically a layer of water that is under pressure and is held between two layers of clay. The recharge area is limited to the land surface where the aquifer's geologic material is exposed to the land surface.

When a well is drilled into a confined aquifer, the water that is under pressure in it will rise in the well casing and may reach the surface. Wells with water flowing to the surface are often called free-flowing artesian wells. In most cases, wells drilled into artesian aquifers do not flow.

Unconfined. An unconfined aquifer is a layer of water that has a confining layer on the bottom and a layer of permeable soil above it. The recharge are is all of the land area above the unconfined aquifer. The water level in wells drilled into an unconfined aquifer will be at the same elevation as the water table. The water table will rise or fall in response to recharge and pumping.

Recharge

Generally, water percolates from the ground surface through an aquifer's recharge area. Surface water may come from rain or other precipitation. This water enters the aquifer by percolating down from the soil surface or through the permeable soil in the stream or river bed.

The rate at which an aquifer recharges varies greatly depending on the size of soil and rocks that are on and under the ground surface. Recharge also depends on the amount of rainfall and other precipitation in and near the area.

The Ogallala Aquifer, for example, is a huge aquifer underlying most of the Texas High Plains. Yet, it has limited recharge because of the area's lower rainfall and the clay soils that impede water percolation there.

In contrast, the Edward's Aquifer is highly rechargeable. It has karst limestone formations that allow rainfall to percolate down. It can be quickly replenished by rainfall. However, if much water is pumped from it, especially during drought, the water level in the aquifer can drop quickly.

Texas Aquifers

According to the Texas Water Development Board, nine aquifers supply about 97 percent of the groundwater used in Texas. The major aquifers have different annual pumping rates, recharge rates and projected safe annual availability rates. The other three percent is drawn from 20 minor aquifers.

1990
Pump Rates

1995
Pump Rates
Annual Recharge Projected Safe Annual Yields
Ogallala 5.55 6.22 0.30 3.81
Edwards (Balcones) 0.53 0.47 0.44 0.44
Edwards-Trinity 0.19 0.25 0.78 0.78
Carrizo-Wilcox 0.45 0.49 0.64 0.85
Trinity 0.19 0.19 0.10 0.11
Gulf Coast 1.23 1.15 1.23 1.23
Others 0.32 0.39 0.43 0.97
TOTAL 8.56 9.16 3.92 8.19

NOTES: Annual recharge rate equals the amount of precipitation and infiltration of surface water added to the aquifer each year. Projected safe annual yield equals annual recharge plus additional stored water that can be pumped without causing undue water quality and subsidence. Aquifers that have no storage can provide only the annual recharge rate. Others include the Seymour, Hueco-Mesilla Bolson and Cenozoic-Pecos Alluvium aquifers.

SOURCE: Mary Sager and Cyrus Reed, Texas Environmental Almanac, 2nd ed. 11 (Texas Center for Policy Studies, 2000.)

Overdrafting

Overdrafting occurs when water is withdrawn from an aquifer faster than it is recharges. It is a significant problem in Texas. The consequences of overdrafting include progressively higher water costs, land subsidence, water quality degradation and possible water depletion. Overdrafting can also harm springs and in-stream flows.

If overdrafting continues for long, or if the aquifer has limited or little recharge, the process is called mining. Mining has caused land subsidence in the Houston areas and has allowed saline water to intrude on the layers of fresh water near El Paso.

How much water do Texas aquifers provide each year?

According to the Texas Water Development Board, nine aquifers supply about 97 percent of the groundwater used in Texas. The major aquifers have different annual pumping rates, recharge rates and projected safe annual availability rates. The other three percent is drawn from 20 minor aquifers.

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