What Exactly is Karst?
Karst is a landscape formed from the dissolution of soluble rocks including limestone, dolomite and gypsum. It is characterized by sinkholes, caves, and underground drainage systems (Fig. 1). Nearly all surface karst features are formed by internal drainage, subsidence, and collapse triggered by the development of underlying caves. Rainwater becomes acidic as it comes in contact with carbon dioxide in the atmosphere and the soil. As it drains into fractures in the rock, the water begins to dissolve away the rock creating a network of passages. Over time, water flowing through the network continues to erode and enlarge the passages; this allows the plumbing system to transport increasingly larger amounts of water. This process of dissolution leads to the development of the caves, sinkholes, springs, and sinking streams typical of a karst landscape.
Why is Karst Important?
Dissolution associated with karst development in central Texas limestone has created a complex underground water flow network that includes caves large enough for humans to access. Rainwater travels through the network, controlled by the Balcones fault system, until it reaches the water table. The karstified limestone acts as an aquifer where water can be stored and later extracted by humans.
Two million people in central Texas get their drinking water from the karst aquifer known as the Edwards Aquifer. This resource is especially important for central Texas as the region becomes more urbanized. With a higher density of people, central Texas will face higher demand and increased pollution. Just like rainwater, pollutants can easily pass through the karstified limestone. Another difficulty is that streams and surface runoff entering the aquifer via sinkholes and caves bypass the natural filtration produced by seeping through soil and bedrock. This direct recharge quickly replenishes the water supply; however, it also leaves the aquifer particularly vulnerable to contamination.
The Edwards Aquifer
The Edwards Aquifer is a karst aquifer. It covers an area of approximately 4000 square miles stretching from Rio Grande River, near Del Rio, to the town of Salado in Bell County (Fig. 2).
On the surface the Edwards Aquifer is comprised of three areas: the catchment, recharge, and artesian zones. The catchment zone collects rainfall and directs it to the recharge zone. Surface runoff enters the Edwards Limestone through fractures, sinkholes, and sinking streams within the recharge zone. This area is considered an unconfined aquifer because water can enter and exit freely. The artesian zone is a confined section of the aquifer. The Del Rio Clay acts as a barrier, holding water in and preventing further recharge. In some places in this zone, groundwater is under pressure and is released in artesian springs.
Threats to the Edwards Aquifer
There are two main threats to the Edwards Aquifer: pollution and extraction. Urbanization concentrates pollution by collecting pollutants on impervious, or impenetrable, surfaces. Rain within the catchment and recharge zones washes pollutants into karst features and directly into the aquifer. Second, with increased population comes a higher demand for water. If the amount of extraction exceeds the amount of recharge, the water level within the aquifer will fall. If this occurs over a long period of time, the water table in the aquifer could be lowered substantially making existing wells unproductive, or allowing an influx of “bad” or saline water.
The Edwards Aquifer is home to over 15 endangered species and serves as a main water supply for 2 million people. It is essential for life in central Texas. Water conservation and contaminant transport regulation have become an essential part of protecting our aquifer.
In order to claim this Earthcache, e-mail answers to the following questions:
1. What type of karst feature do you find at GZ?
2. When standing at the entrance, how deep does the primary chamber appear to be? Assuming an erosion rate of 1 inch every 1000 years, how long would it have taken the primary chamber to develop? (approximate answers are fine).
3. Why do you think limestone makes such a good substance for the formation of karst topography?
4. What is a recharge zone?
Pics are not required, but always appreciated. Please don't post your answers in your log.
