What, exactly, are aquifers?

At its most basic, an aquifer is a layer of rock, sand, gravel, or soil that can store and transmit groundwater. 68% of the world’s freshwater is tied up on glaciers and polar icecaps (mostly in Antarctica), 30% is underground, and only about 2% is found in surface water – rivers, streams, and lakes. Therefore, groundwater contained primarily in aquifers, is the single most important source of freshwater – both to support ecosystems and to support human populations. Even setting aside ecological considerations (which I honestly prefer not to do), it should be obvious that protecting and conserving this resource is imperative for the long-term health of human populations, especially as global human populations continue to grow.

Anyway, back to what makes up an aquifer. Aquifers are permeable underground layers of rock, gravel, or sand that both holds and allows for the flow of groundwater. Two geologic features of these layers are of vital importance for understanding the way that water moves – the porosity (available spaces in the layer) and permeability (how easily water moves through the layer). In most cases, what contains an aquifer is that these permeable layers lie above or between a layer or layers that is impermeable (usually shale or clay layers).

One general way to classify aquifers is whether or not they are unconfined or confined. An unconfined aquifer lies on top of an impermeable layer, and is open to the surface with no impermeable layer above the aquifer. In these aquifers, water can seep directly into the aquifer from the surface – either from rainfall or from rivers and streams. The upper boundary of an unconfined aquifer is the water table. Unconfined aquifers can be shallow (as in parts of the Edwards Plateau) or deep (as is seen in the Texas Panhandle’s Ogallala Aquifer). On the other hand, a confined aquifer lies between two impermeable layers – one below the aquifer, and one between the aquifer and the surface. These layers trap the water between them, and the weight of the surface rocks and impermeable layer above the aquifer creates hydrologic pressure on the aquifer. Springs and wells that connect to confined aquifers often
flow naturally without pumping due to this pressure.

Aquifers can also be generally categorized based on their recharge rate. Some aquifers contain ancient water that was trapped underground thousands or even millions of years ago, and that don’t naturally recharge under current climate conditions. This is the case for many aquifers in desert regions, such as the some of the sandstone aquifers in Africa. Although the Ogallala Aquifer of the Great Plains (including the Texas Panhandle) does recharge naturally due to rainfall, it does so very slowly, and much of its water accumulated long ago during wetter climate conditions than are seen today. This aquifer doesn’t quite qualify as a fossil aquifer (because it does recharge) but it comes close. Renewable or rechargeable aquifers are those that recharge readily and quickly with rainfall. Our Edwards/Edwards- Trinity aquifer in Kinney County is this sort of aquifer.

In Texas, there are a variety of different Aquifers. The largest aquifer in North America is the Ogallala Aquifer that extends from South Dakota south to the Texas Panhandle. It is an unconfined aquifer made up mostly of sands and gravels lying above an impermeable layer of clay. Because it is unconfined, it does recharge with rainfall, but this recharge is typically very slow owing to both the depth of the aquifer and to low levels of precipitation in the region. In many areas, withdrawal rates far exceed recharge rates, and water levels are declining in many areas.

Another important aquifer in Texas is the Carrizo-Wilcox Aquifer that extends from northeast Texas (approximately Tyler) southwest to Laredo. The Carrizo-Wilcox is a sand aquifer, and in some areas, it is unconfined (sands exposed to the surface) while in other areas it is confined (sand layers lie underground between two clay layers). This aquifer does recharge when rains fall on the unconfined sections of the aquifer. However, withdrawal rates may exceed recharge in some areas, particularly the drier parts of the aquifer region (e.g. Laredo).

The most important aquifer system to most Texans is the Edwards (and Edwards-associated) Aquifer. Depending on how you define it’s sections (e.g. “Edwards proper” vs “Edwards-Trinity”), it extends from Bell County in the northeast, south to San Antonio and west to Kinney County – with the interconnected Edwards-Trinity (plateau) section covering most of the Edwards and Stockton Plateaus (for those interested in splitting hairs on the different regions of the what is the Edwards and Edwards-Trinity aquifers, see Robert Mace’s discussion of these aquifers here:
https://sosecretoccultandconcealed.com/2024/11/22/the-edwards-aquifersssss-of-kinney-county/ ). 

In general, the Edwards Aquifer for most of its extent is a confined karst limestone aquifer made of porous and fractured limestone rocks trapped between impermeable layers underground. Because it is a karst aquifer, water is able to move rapidly through underground channels and caves. In the confined sections of this aquifer, springs formed where cracks allow water under hydraulic pressure to bubble to the surface – and water wells in this region may also flow under artesian pressure and reach the surface without pumping – a least when the aquifer volume is sufficient for pressures to drive water to the surface. The Edwards (and associated aquifers) also has a fairly well-defined recharge zone where the rock layers that make up the Edwards Aquifer are exposed on the surface. In these sections of the aquifer, it is unconfined, and rainfall in this region (and only in this region) is how the Edwards Aquifer recharges. The most important regions for recharge for much of the Edwards Aquifer is the Balcones Fault Zone – however, the Edwards and Edwards-Trinity Aquifers are made up of the same rocks, and the majority of the Edwards Plateau does serve as a recharge area for all of these aquifers to some extent. Mapping and modelling the exact extent of recharge, and how that rainfall in different regions impacts water levels and springs in any single location presents a challenge to hydrogeologists and geomorphologists (as many of us in Kinney County are well aware).

Finally, the last aquifer pertinent to Kinney County residents is the Austin Chalk Aquifer. While water from the Austin Chalk may be locally important, this aquifer is generally considered to be a minor aquifer. The Austin Chalk formation is made up mostly of porous, brittle chalk and limestone, and is heavily fractured. These fractures rather than pores in the rocks themselves are what allows groundwater to be stored and moved in this system. It is generally unconfined, meaning that it recharges from rainfall that falls directly onto the surface above it. It supplies water to domestic, livestock, and agricultural wells, but in most cases does not produce the same volumes of water as do major aquifers like the Edwards system as a whole. In areas where the Austin Chalk Aquifer and the Edwards (and associated) Aquifers lie in close proximity, they are generally not considered to be interconnected (but further study may change this interpretation). Increasingly, aquifers are a resource upon which all Texans depend, and with population growth – particularly along the I-35 corridor, but also of Laredo and Eagle Pass – demands on these resources are increasing. Without sound management of these resources, local needs and quality of life may well become depleted, to the detriment of everyone.