By Bill Ward
Published in The Boerne Star on July 9, 2004
It is well publicized these days that several places in the Texas Hill Country are having problems with maintaining adequate supplies of ground water for human consumption. Many other places in the world have the same problem, because growing populations continually demand more ground water. Soon the demand exceeds the supply.
For many of us, the most important reason for promoting native plants is water conservation. Using native plants may enhance the quantity and protect the quality of water contained in our aquifers.
Once established, native plants in home landscapes require less water and less fertilizer. Less water on yard plants means our aquifers are tapped less often for non-essential purposes. Less fertilizer may mean than the groundwater is less contaminated by organic or inorganic chemicals seeping into the aquifer from the surface. In many places, restoration of native grasses and understory shrubbery will increase surface-water infiltration, thereby increasing the recharge of an aquifer.
I’ve noticed that most people who hear or read these reasons for advocating the use native vegetation nod their heads affirmatively. I’ve also noticed that many are not quite certain all this is true. Could plants really affect aquifers? And what exactly does “aquifer” mean? Everyone says the word, but there seem to be some misconceptions about its definition.
An aquifer is a body of rock from which appreciable amounts of water can be produced. Such a body of rock must have enough holes (pores) to hold substantial amounts of water. In addition, enough of the pores must be connected to allow water to be transmitted through the pore system. The water contained in rocks below the surface is called “ground water.” In other words, aquifers are rock layers with adequate porosity and permeability to allow appreciable amounts of ground water to flow through and from the rock.
Almost any rock can be an aquifer if it has connected pores. Even highly fractured granites are aquifers in some places. However, nearly all large aquifers are sedimentary rocks, either sandstone or carbonate rocks (limestone and dolostone). In this part of the Hill Country the aquifers are carbonate rocks.
Aquifers are known by their geologic names. For example, Cow Creek Aquifer is the Cow Creek Limestone. Trinity Aquifer refers to a geologic unit containing the Sycamore Formation, Hammett Shale, Cow Creek Limestone, Hensel Formation, and Glen Rose Limestone.
All of the Trinity Division formations were deposited during the early Cretaceous when the Boerne area lay on the northwestern margin of the ancestral Gulf of Mexico. In this area, the Cow Creek, Hensel, and Glen Rose were deposited in shallow seas, which fluctuated in depth through time.
Because of these environmental changes during deposition, the formations are composed of layers of several different kinds of carbonate rocks. Therefore, there is a good deal of vertical and horizontal heterogeneity in rock types within each formation. Some layers within these formations have good porosity and permeability; many do not.
Limestone and dolostone are much more susceptible to dissolution in natural water than are most other rocks. As ground water moves through the rock, some types of limestones are prone to develop cavernous porosity, while others may remain relatively impermeable, all according to original fossil content and depositional texture. Cracks and fractures in some carbonate rocks widen as ground water continues to dissolve the rock. The bigger the pores and wider the fractures, the more easily the rock transmits ground water.
It would be a mistake to think that ground water flows equally through a whole rock unit. For example, the entire thickness of the Cow Creek Limestone is unlikely to have the same porosity and permeability. Ground flow would be channeled through the layers with the largest pores and along dissolution-enlarged fracture systems. In some places most of the layers may have low porosity and permeability. The portion of the Cow Creek that is an aquifer probably varies from place to place.
Carbonate aquifers are complex systems of laterally and vertically heterogeneous pore systems. They are difficult to model hydrologically, and it is commonly difficult to predict their recharge. A great deal more needs to be known about where or whether various parts of the Trinity aquifer system are recharged. Are some aquifers being sucked dry faster than they can be recharged? There is some evidence that the answer is yes.
In limestone terrains such as the Texas Hill Country, many subsurface cave and fracture systems connect to dissolution pits and sink holes at surface. Much of the rainwater or other liquids flowing over the limestone goes underground through these dissolution features. Any contaminants in this surface water may go directly into the ground water.
The most precious natural resource in the Hill Country is ground water. Using native plants in landscaping is one way to help conserve the quantity and quality of this resource.
