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NATURAL HERITAGE

Living With Sinkholes

graphic of Living with Sinkholes brochure

Introduction

Sinkholes are natural depressions on the land surface that are shaped like a bowl or cone. They are common in regions of karst, where mildly acidic groundwater has dissolved rock such as limestone, dolostone, marble, or gypsum.

Karstlands - characterized by sinkholes, sinking streams, springs, caves, and solution valleys–form where surface water enters the ground and migrates downward through solutionally enlarged openings to conduits, such as caves. These avenues may not be traversable by human explorers; however, water that infiltrates at sinkholes may easily make its way into a well-developed drainage system.

Groundwater Quality

The most important current and future environmental issue with respect to karst is the sensitivity of karst aquifers to groundwater contamination. The effect of man on karst is most severe in cases where polluted surface waters enter karst aquifers. This problem is universal among all karst regions in the United States that underlie populated areas. The country's karstic groundwater problems are accelerated with the advent of (1) expanding urbanization, (2) misuse and improper disposal of environmentally hazardous chemicals, (3) shortage of suitable repositories for toxic waste (both household and industrial),and (4) ineffective public education on waste disposal and the sensitivity of the karstic groundwater system.

There is a general lack of public understanding of groundwater behavior, particularly in karst. Karstic aquifers can not filter contaminated groundwater sufficiently to render it potable at a discharge sites. Water travels rapidly through solutional conduits because recharge points are directly connected to discharge points. Not only does the "garbage in, garbage out" principal apply to karstic groundwater, but contaminants move rapidly and with relatively little dilution in this type of terrane.

Because sinkholes are natural holes in the ground surface, they have been inviting sites for dumping of trash. The number of active and inactive sinkhole dumps in karst regions is staggering. It is conceivable that each county with karst has hundreds of sinkhole dumps. The profusion of these dumps is the result of (1) the absence of a refuse-removal service in rural areas and the expense and inconvenience of trash haulage, (2) the convenient proximity of sinkholes, and (3) a lack of appreciation of the role of sinkholes in the karstic groundwater system.

Sinkholes are natural funnels that conveyed toxic substances directly into the karstic plumbing system. In many cases, chemicals may be transmitted directly to domestic wells in a matter of the few hours. Thoughtless disposal of game or farm animal carcasses into sinkholes (a common practice) can contaminate the well water of the landowner and even his neighbors.

graphic of sinkhole with trashSinkhole dumping is only one way of contaminating a karstic groundwater supply. Fertilizers, herbicides, and pesticides applied to fields overlying carbonate rock can enter the aquifer through diffuse infiltration and contaminate springs and wells. Improper siding of municipal landfills on or near karst allows leakage or runoff from these landfills to easily contaminate karst waters. Chemicals introduced in this fashion may include many of the most hazardous, including hydrocarbons, heavy metals, PCBs, and others. Additionally, leaky septic systems or sewage lines and effluent from feed lots or faulty sewage treatment facilities introduced coliform bacteria and other disease causing organisms into the karst system.

Many of today's streams, including those in rural areas, are polluted. Commonly, surface streams in karst terrane readily lose water through their beds. Contaminated surface waters entering carbonate rocks may carry toxic substances to subsurface streams. Accidental chemical spills and runoff from highways salted in winter to prevent freezing are just two examples of contamination along transportation corridors. Effluent from commercial and industrial operations along such corridors is also a problem.

A Delicate Ecosystem

Caves contain fragile organisms that have evolved in the natural underground environment. Most people think of bats as the most common creature of caves. On the contrary, there is an amazing variety of cave life. Because these animals are highly adapted to stable ecological surroundings, they are particularly sensitive to disturbances, especially the introduction of foreign substances into the groundwater flowing through caves. Even "clean" fill, such as brush, hay, sawdust, or dirt, may lead to chemical imbalances in the karstic groundwater that adversely affect the ecosystem. This is largely due to the rapid decay of organic matter and consumption of oxygen. Aside from maintaining adequate water resources, it is in the interest of conservation of species endemic to caves that man be concerned with clean groundwater in karst regions.

Subsidence and Ground Instability

Occasionally the land surface in karst regions may collapse. Most of these events are triggered by man's activities in the karstic environment. Excessive pumping of groundwater from karstic aquifers may rapidly lower the water table and calls a sudden loss of buoyant forces that stabilize the roofs of cavernous openings. Man-induced changes in surface water flow and infiltration also may cause collapse. Most sinkholes that form suddenly occur where soil that overlies bedrock collapses into the pre-existing void. Sinkholes are subsidence or collapse features that form at points of local instability. Their presence indicates that additional sinkholes may develop in the future. Man-made structures in the vicinity of sinkholes are at risk for structural damage unless they have been adequately designed.In populated areas sinkholes are viewed as unwanted holes in the ground. Consequently, there is a great desire to fill them in order to level the ground. However, naturally developed paths of infiltration are often blocked, leading to potential ponding and flooding on the fill. Secondly, fill materials may be sapped into the subsurface and subsidence may occur. These disturbances easily impact any structures built on the fill. Additionally, the increased weight of water, fill, and structures upon the cavernous bedrock could cause future collapse or subsidence.

Environmental Problems

Mankind has only recently become aware of how environmentally sensitive karstlands can be. Sinkholes, in particular, pose several problems that ultimately affect groundwater in karstic terranes and delicate cave ecosystems. Environmental concerns included: (1) introduction of contaminants and pollutants into the groundwater, (2) catastrophic collapse and gradual subsidence of the land surface, and (3) flooding during or following intense storms. In fact, stresses induced by mankind in karstic terrane result in environmentalproblems that are much more acute than those that would occur in terranes underlain by other types of rock.Much of karstlands of the United States lies in rural regions where environmental impacts are generally limited to those imposed by agricultural practices and highways. However, urbanization is increasingly affecting many karst areas, resulting in severe karst-related environmental problems. Karstic terrane, particularly that of moderate to high sinkhole density, thus imposes constraints on land use. Mismanagement of karstlands, whether through unsupervised development, poor farming practices, improper waste disposal, or other means, will often damage groundwater resources, cave ecosystems, or man-made structures built on karst.

Proper management of karstlands requires proper management of sinkholes, because sinkholes and caves are components of integrated groundwater systems.

Sinkhole Management

Sinkholes serve as discrete points of recharge to the karstic aquifer and care must be taken to prevent the introduction of any toxic substances into them. Common sources of contamination include runoff containing chemical and agricultural waste from both urban and rural areas, accidental spills of hazardous materials, and dumping of waste directly into sinkholes. Funneling of runoff through sinkholes transmits contaminants into the groundwater. Appropriate sinkhole management must include assessment of the vulnerability of the sinkholes and karst system and maintenance of their natural integrity.

Establish a Natural Buffer Zone

A good conservation practice would be to establish natural buffer zones around sinkholes in order to maintain the quantity and quality of recharge entering the aquifer. Conditions, such as fractures in the bedrock, size of drainage area, and proximity to sources of contamination, should be considered when establishing the level protection that is needed.What if your sinkhole is already modify or polluted? Removal of trash and restoration of original contours around sinkholes and cave entrances have been successful. Although worthwhile, such recovery requires considerable effort and time for the remaining pollutants to be flushed through the system.

Need More Information?

Individuals or groups desiring more information should contact the Virginia Cave Board, the Department of Conservation and Recreation, Division of Natural Heritage, the National Speleological Society, or the American Cave Conservation Association.