By Bill Ward
Published in The Boerne Star on August 27, 2004
“Native Plants”, the quarterly publication of the Lady Bird Johnson Wildflower Center, recently had an article by Katherine Ellison on how land stewardship in the Catskill Mountains affects the quality and quantity of drinking water in New York City. The determining factor in the amount and purity of the drinking water is the health of the natural native vegetation in this upstate New York watershed.
In a healthy watershed, trees and creek-side plants cleanse surface water as it flows through roots and soil. Diverse natural vegetation with its expansive root system underground helps store and stabilize water supplies. Where development has stripped away much of the vegetation, the chances of polluting surface water are greater. Also in these de-vegetated landscapes, runoff of rainfall and snowmelt is increased, and the frequency of flooding is greater.
This New York example concerns surface water, but the same principles are relevant to the quality and quantity of groundwater in the Hill Country. A healthy and diverse native-plant population helps ensure that a watershed will supply clean and abundant water to both surface streams and underground reservoirs (aquifers).
“Watershed” generally is used to signify the region drained by, or contributing water to, a stream, lake, or other body of water. In the Hill Country we are commonly more concerned about ground water than surface water. Instead of watersheds, we usually are more interested in recharge areas. “Recharge area” or “catchment area” is a geographic area where surface water infiltrates into the subsurface and eventually reaches the zone saturated with groundwater in one or more aquifers.
As many studies and models of surface hydrology have shown, much more rainfall is absorbed into the ground where the landscape is well vegetated with a diversity of plants than in a poorly vegetated terrain. Taking away the natural vegetation for construction and paving can increase runoff by hundreds of times. In this setting there is little chance for pollutants in runoff to be reduced by cleansing in root systems, and the quality of stream water and ground water can be damaged.
In limestone terrains such as the Boerne area, there are areas where caves, sink holes, and solution-enlarged fractures allow surface waters to rapidly descend to the water table. If these same areas are poorly vegetated because of overgrazing or development, then it seems likely that polluted water may be reaching an aquifer.
The vulnerability of Trinity Aquifer ground water to contamination varies from place to place depending on the efficiency of aquifer recharge in a given area. Not all Trinity limestones transmit ground water with the same efficiency. Proof of this can be seen in Cibolo Creek south of Boerne where it flows across the outcrop of the lower Glen Rose Formation. The creek always is flowing through the Cibolo Nature Center, but downstream in Fair Oaks Ranch it commonly is dry.
As Cibolo Creek flows southward from Boerne, it cuts down into progressively lower layers of the Glen Rose limestone. For two miles or more downstream from Boerne, the creek flows across Glen Rose rocks which do not easily absorb the surface water. In other words, this is an area of low recharge.
Eventually Cibolo Creek crosses a thick unit of a type of limestone that is prone to develop cavernous pore systems. These large pores easily transmit water, and some of the stream flow is lost into the rock. This limestone layer also is cut by near-vertical fractures which have been widened by dissolution of the wall rock. These fractures also easily transmit stream water into the subsurface. This is indeed a recharge area where, during times of normal rainfall, the water in Cibolo Creek flows into an underground aquifer.
Different limestone layers in the Trinity aquifers are so variable in porosity and permeability and thickness that recharge is difficult to predict and to quantify. Surface recharge may be high in a few areas, but it is low across much of the outcrop. Factoring in the great loss of rainwater to evapotranspіration gives low recharge percentages for almost any area.
In addition, not enough is known about the distribution and density of fracture systems in this area and their influence on recharge. How. where, and if recharge of Trinity aquifers takes place needs more study.
Studies by the Water Resources Division of the US Geological Survey in San Antonio have shown vulnerability of contamination of Trinity and Edwards Aquifer ground water in northern Bexar County depends on variables such as (1) the type of rock at the surface, (2) presence or absence of faults, fractures, near-surface caves, and sinkholes, (3) slope of the land, and (4) soil type. Perhaps the type and health of native vegetative cover also should be considered a controlling variable.
