Karst Groundwater

By Tom Aley | March 2, 2000
From Missouri Conservationist: Mar 2000

Karst groundwater is an incredible natural resource. It provides drinking water for many of our residents, beautiful springs to feed our waterways and habitat for many unusual underground species. The quality of the groundwater is dependent upon how we use the land and how well we protect the quality of groundwater recharge.An old adage is that whatever goes up comes down. In karst areas, whatever goes down, comes up--up through a cave, a spring or a well.

About three quarters of the water that reaches the major rivers in Missouri's karstlands has passed through groundwater systems for at least some distance. Karst is a landscape where underground water follows dissolved out channels in the rock. Sinkholes, springs, and caves are among the common features in karst areas.

Water that moves from the surface into the groundwater system is called groundwater recharge. Groundwater recharge replenishes wells, cave streams and springs.

Some groundwater recharge seeps and oozes through the subsurface, and in so doing receives fairly good natural cleansing. Other groundwater recharge occurs through a vast network of localized openings that are able to rapidly transport both water and contaminants. Water that seeps and oozes through the subsurface is called diffuse recharge. That which flows through localized openings is called discrete recharge.

From 1966 to 1973 I directed a watershed study for the U.S. Forest Service on Hurricane Creek south of Winona. Surface flow from Hurricane Creek enters the Eleven Point River while most of the underground flow from this basin discharges from Big Spring and flows into the Current River south of Van Buren.

We found that only about 25 percent of groundwater recharge was diffuse recharge; the remaining 75 percent was discrete recharge and was, therefore, not effectively cleansed.

Natural cleansing can be a misleading term--in many cases the underground openings are larger than the bacteria or parasites that cause waterborne illnesses in people and animals. Some discrete recharge zones cannot even filter out large materials such as acorns, walnuts, cans and pieces of styrofoam.

Sinkholes, which are depressions in the land's surface that have underground drainage, are abundant in some Missouri karst areas and rare in others. Their shapes are variable and range from bowl-shaped or elongated depressions to steep-sided natural funnels that may lead directly into cave passages and underground streams. Regardless of their shape, all sinkholes provide a direct connection between surface water and groundwater.

In a study in southern St. Louis County we simulated intense rainfall by using fire hydrants and hoses to introduce a 30-minute pulse of water and a harmless dye into a sinkhole. We then measured the response in the cave stream. We tested six sinkholes this way, and each accepted water faster than we could deliver it with a fire hose.

The recharge water from each sinkhole increased water levels at the sampling station on the cave stream in 45 minutes or less, even though the underground travel distances were as much as a quarter mile. Intense thunderstorms in the area would add water to all sinkholes at once, and would substantially increase the travel rates over what we observed in our study.

The experiment shows how rapidly caves and springs respond to surface runoff that enters the groundwater system through discrete recharge zones. Because intense rainstorms can cause rapid and lethal flash floods on cave streams, and water may totally fill passages, people should give careful consideration to the weather before entering caves.

Although sinkholes are easy to identify, other discrete recharge zones show little or no surface evidence of the direct connections between the surface and the groundwater system. If septic fields intercept such discrete recharge zones they contaminate the groundwater. In a karst area, the fact that "everything goes away" does not necessarily mean that a septic field system is adequately treating the sewage.

Losing streams are also discrete recharge zones. A losing stream is a surface stream that contributes water to the karst groundwater system in localized areas. In the Ozarks, almost every stream that lacks year-round flow has losing stream segments.

Losing stream valleys are important groundwater recharge zones in the Ozarks. Although valley areas represent about 10 percent of the land area, they are responsible for about 40 percent of the groundwater recharge to karst groundwater systems. Protection of water quality in these valleys is critical for protection of groundwater quality in wells and springs.

The typical losing stream in the Ozarks is a dry gravel stream bed, except for a few days or weeks after major rainfall. Many of the losing stream segments were once sinkholes that were filled with coarse stream gravel washed in during storm flows. Losing stream segments can move a few gallons to a few hundreds of gallons per minute of water from the surface stream into the groundwater system.

One losing stream segment we studied made slurping sounds as it transported over a million gallons per day of poorly treated industrial sewage into the regional karst groundwater system. Contaminants sinking through this stream polluted rural wells and springs in a 60 square mile area.

Runoff from pasture lands can also degrade water quality via losing streams. Strips of ungrazed vegetation along stream channels, and especially along the normally dry losing stream channels, remove many contaminants from storm water. Even a vegetative strip as narrow as 50 feet wide along each bank of a losing stream channel is valuable in protecting groundwater quality.

Suburban and urban developments increase the quantity and decrease the quality of storm runoff water that reaches losing streams. When a storm hits, the vast majority of the contaminants are concentrated in the first quarter to half inch of storm runoff water. This "first flush" storm water runoff, often similar in quality to raw city sewage, enters our karst groundwater supplies through sinkholes or losing streams.

Developers often construct basins to detain storm water and reduce flash flooding in urban and suburban areas. However, most of these basins do not provide treatment to improve the quality of the detained water, and in some cases these detention basins have been located on top of losing stream segments. Improved storm water management approaches for urban and suburban activities are needed in Missouri's karst areas. Karst groundwater quality cannot be protected without ensuring good quality recharge water.

Karst groundwater systems provide habitat for many fascinating animals. Except for caves with large bat populations, most of the food for cave animals is washed in through discrete recharge zones. As a result, cave fauna is commonly concentrated around the points where these discrete recharge zones connect to caves.

A fascinating characteristic of karst groundwater movement is the great speed with which much of the water moves. Karst groundwater travel rates in Missouri are often in the range of a mile per day. In non-karst areas, groundwater travel rates are commonly only a few feet per year.

The longest distance groundwater trace in the United States was from a losing stream segment of the Eleven Point River to Big Spring. Big Spring is 39.5 miles away and in the Current River basin. The dye introduced in the losing stream was first detected at Big Spring 16 days later.

Not all groundwater moves at the same speed. As a result, a contaminant may reach some springs or wells within a few hours or days of the time it entered the groundwater system, while the contaminant from the same source may not reach other wells for weeks or months. Travel times also depend upon characteristics of the contaminants. The net result is that cleaning up groundwater, including karst groundwater, is routinely slow, difficult, expensive and seldom totally successful. As with most natural systems, damage is easy, fixing is difficult and prevention is best.

Another unique feature of karst groundwater systems is that water entering the subsurface at a single point may sometimes discharge from multiple springs and wells. The discharge points are sometimes in different stream or river basins and may be separated from one another by a number of miles. Such complex flow systems help explain the relatively large ranges for some aquatic cave species. The range of Ozark cavefish, for example extends from Springfield southwestward into northwest Arkansas and northeast Oklahoma

This Issue's Staff

Editor - Tom Cwynar
Managing Editor - Jim Auckley
Art Editor - Dickson Stauffer
Designer - Tracy Ritter
Artist - Dave Besenger
Artist - Mark Raithel
Photographer - Jim Rathert
Photographer - Cliff White
Staff Writer - Jim Low
Staff Writer - Joan McKee
Composition - Libby Bode Block
Circulation - Bertha Bainer