Karst

By Dave Sligh

If you care about the quality of the James River, you should know the word "karst," so here's a quick lesson in its meaning and importance. Geologists speak of "karst terrain" or "karst geology" to describe areas where the most prominent bedrock consists of sedimentary rocks, like limestone or other rocks that are easily erodible. These types of rock formations are found throughout the world and are expecially prominent in the western parts of Virginia, from the Blue Ridge mountains to the ridge and valley region to the Appalachian plateau. The map below shows the Virginia counties where most of our karst is found.

Those counties highlighted include Highland, Bath, Alleghany, Augusta, Rockbridge, Botetourt, Craig, Roanoke, and Giles Counties - all of which lie partly within the James River's drainage basin. These areas have features like caves and sinkholes and provide great reservoirs for groundwater, often allowing very productive wells. The land can be honeycombed so that water can move swiftly and in large amounts through natural underground conduits.

The great environmental concern with karst is that water, and the pollutants it carries, can reach the groundwater almost immediately, if it enters a sinkhole or comes in contact with bedrock that shows through thin soil layers. Pesticides and herbicides, animal waste, commercial fertilizer, sewage sludge, polluted stormwater runoff, water from home drainfields, and other pollutants can, and in many cases have, polluted drinking wells in many cases and caused serious human health problems.

Further, the pollutants don't just reach the groundwater but, because streams are so closely connected with surface streams, can end up in creeks and rivers in minutes or hours, as opposed to months or years that groundwater takes to migrate and be treated in other areas. It can also be very difficult to follow these connections, especially because the drainage patterns of water on the land may not be the same. Studies show that pollution on the ground or in streams in one watershed can later appear in other watersheds, linking pollution sources and streams that could not be predicted by looking at the land surface.