Streamline - Winter 2012 - (Page 11)
BY NANCY CARR, SOURCE WATER PROTECTION SPECIALIST
in the Karst Landscape
IN THE DISTANT geologic past, prior to the age of dinosaurs, an area including the present-day Great
Characterizing karst geology, the multiple springs, caves, sinkholes and disappearing streams are evidence of the action of acid on calcite over millions of years, and continuing today.
Valley and southwest Virginia was covered by a vast inland sea. This ocean extended west beyond the present-day Mississippi River, and was rich in minerals and creatures that came to play an integral role in the lives of today’s inhabitants. The mineral-rich water contained calcium, and the skeletons and shells of the primordial creatures were also calcium. As they died and many generations passed in the ancient sea ecosystem, the animal remains laden with calcium sank to the ocean floor and were deposited in layers with the minerals and other detritus. Eons passed and the sea floor rose, the inland sea disappeared and, during this process, the mass of deposited debris solidified into stone. We commonly see this limestone around us in the Shenandoah Valley and southwestern Virginia, as light gray outcrops in pastures and crushed rock used in roads. The mountains that border our Great Valley were created by the movement of the Earth’s crust plates pushing and sliding. As they moved and met resistance, great cracks formed, fissuring the limestone blocks and thereby affording paths for water to make its way down through the rock. Rainwater does not have a neutral pH. Falling water molecules combine with carbon dioxide to make carbonic acid, a slightly acidic liquid. As the rain and snow filter through soil to bedrock, they become more acidic. The limestone of our story, being composed largely of solidified calcium (containing sea creatures and minerals), dissolves readily in acid. (Try putting a seashell in a jar of vinegar.) The cracks in the bedrock of our Valley floors allow the acidic rain and snowmelt to penetrate the limestone, dissolving and wearing away channels that have led to two of our greatest assets. The fantastic caves and the many springs which formed in these karst landscapes are famous, and are the results of this process. Solution channels form a Swiss cheese-like mosaic underground, where water may run rapidly and sometimes without the filtering effects of sand or gravel. As erosion and dissolution continue, roofs and walls of the channels may collapse and form another common feature of this landscape. Sinkholes are often funnel-shaped depressions on the land’s surface, and may be a few feet across or larger than a football field. With the “drain hole” in the bottom of the funnel leading directly into the underground passageways, sinkholes quickly replenish groundwater when rain and snow enter the depression. In this respect, sinkholes are a valuable resource in the type of landscape called karst. In another respect, they present a problem for developers and water quality managers, including utilities that use groundwater. Characterizing karst geology, the multiple springs, caves, sinkholes and disappearing streams are evidence of the action of acid on calcite over millions of years, and continuing today. Here the groundwater is considered delicious and plentiful, but it is vulnerable to pollution by the nature of the geology. The topsoil covering the limestone bedrock is relatively thin in many areas, and the cracks in the limestone and the sinkholes don’t allow potential contaminants to be filtered or absorbed before they reach the groundwater. The collapse of solution channels to form new sinkholes can happen unexpectedly and inconveniently, as has happened beneath highways and under homes. An oil or chemical spill can reach drinking
Table of Contents for the Digital Edition of Streamline - Winter 2012
From the President
From the Executive Director
Groundwater in the Karst Landscape
There’s a Chill in the Air!
Chloramines in Question
Preventative (Planned) Maintenance II
What About Water Rates?
How to Get a Great Rate Analyst
Throwing My Loop
Do You Know What Your VRWA Benefi ts Are?
Welcoming New Members
Board Of Directors
Index To Advertisers/Ad.com
Streamline - Winter 2012