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Karst Notebook-Sinkholes

A large portion of Hays County is heavily fractured from the extreme pressures and stress generated when the Edwards Plateau was uplifted about 40 million years ago. The fractured limestone is in part responsible for a landscape rich with sinkholes. A sinkhole is considered the most fundamental structure of karst terrain and is common throughout Hays County. They are formed by naturally acidic rainwater entering the limestone bedrock via a network of fractures or cracks. Over long time periods, the cracks widen and expand as more limestone is dissolved, thus creating a channel from the surface to the aquifer.

Sinkholes, springs and caves have similar underground structures with the difference being the size of the solution cavities or the elevation of the water table. The illustrations below show the basic differences between them. Click on the images to enlarge and view in Lightbox. Click or swipe to scroll through the images.

Sinkholes have a significant impact on the regional aquifers. They allow rainwater to be efficiently collected and routed to the aquifer. Sinkholes are the source of an important re-charge system to both the Cow Creek Aquifer in the Wimberley Valley and the Edwards Aquifer in San Macros. These aquifers supply source water to some of our most iconic water attractions. This includes Spring Lake flowing water from the Edwards Aquifer and the Cow Creek Aquifer in Wimberley. The Cow Creek Aquifer is the primary source of drinking water for Wimberley and supplies source water for Jacobs Well, Blue Hole and Pleasant Valley Springs.

The Environmental Risk From Sinkholes

Being a gateway to the aquifer, sinkholes elevate the risk of environmental damage from surface spills or other types of contamination. Sinkholes can quickly transfer pollution directly into the aquifers and ultimately at our much-loved springs. Some studies note that sinkholes increase water flow through the aquifer which may result in faster dilution of contaminantes; however, in Hays County increased pumping and the effect of droughts are responsible for lowering the water table and flow within the aquifers. For example, Jacob’s Well which once flowed year round, now experiences short periods of no flow. During these periods, it acts more like a sinkhole than a spring.

Want to know more about Karst? Click on link to read my article: Karst-The Role of Water in Shaping the Hill Country.

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