Why are karst areas naturally highly vulnerable to pollution?
Water can move rapidly through fissures widened by solution.
Sinking streams provide direct water entry points to groundwater, with little or no filtration or attenuation of contaminants.
Solution hollows or dolines may also provide direct entry routes through vertical shafts.
The characteristic soil cover over karst limestone is very thin, maybe only a few centimetres deep, and so provides little protection.
Possible entry routes for contaminants to enter karst groundwater (John Gunn)
What sources of contamination are there?
Agricultural sources of pollution include badly stored farm wastes (e.g. unlined manure pits and slurry lagoons, silage clamps with cracked bases or no effluent collection system), unmanaged farmyard runoff, and sheep dip disposed of in soakholes or swallow holes. All farm effluents and dirty water should be collected and spread on land, and the rate of application should be discussed with an agricultural advisor. Excessive manure spreading, or spreading on thin soils, may cause pollution of groundwater with a range of substances.
Septic tanks may pose a risk, especially if soakholes are used for effluent disposal. Where the subsoil is very shallow, artificial percolation areas or 'package' domestic sewage treatment systems may be necessary.
Natural depressions, caves and quarries in karst limestones have often been used for disposal of wastes. Dumping of household rubbish, dead animals, farm wastes and rotten vegetables into such places is a common problem. At a larger scale, landfills in karst areas, particularly in old quarries, will cause pollution if the leachate is not properly contained. Environmental Protection Agency guidelines now prohibit landfill sites on regionally important karst aquifers.
Leaking fuel storage tanks may also cause serious water pollution, whether above ground (e.g. domestic heating oil tanks) or underground (e.g. at petrol stations).
Potential pollution hazards in rural karst areas. Pollutants move rapidly from the land surface through fractures and conduits in the limestone down to the water table. In the process groundwater gets polluted and wells may be affected (Catherine Coxon)
Types of pollutant
The pollutants found in water supplies in karst areas vary depending on the local sources of contamination. The most common problem is contamination by micro-organisms from human or animal wastes.
Farm wastes and sewage can also give rise to chemical pollutants including nitrate and ammonia. In some cases, an unusually high level of a chemical constituent (e.g. chloride, potassium), though not in itself harmful, may indicate a pollution problem. Excessive iron and manganese causing taste or staining problems, may occur naturally, but may also be released from the soil or rock into water supplies by pollution, particularly by silage effluent pollution. Sheep dip which is incorrectly disposed of may pollute water supplies with phenols and a range of potentially harmful pesticides. Leaking fuel tanks can contaminate groundwater with a range of hydrocarbons which will cause taste and odour problems and may pose a health risk, even at very low concentrations.
Removing the risk of microbial pollution in vulnerable karstic areas requires two complementary approaches. It is usually impossible to exclude all faecal micro-organisms, so the water supply must be treated to remove them, e.g. by filtration and treatment with chlorine, ozone or ultra-violet radiation. However, these treatment systems may fail occasionally (e.g. inadequate dosage of chlorine, suspended sediment interfering with ultra-violet treatment), so it is important to keep the raw water supply as uncontaminated as possible by controlling sources of faecal pollution. Prevention is better than cure!
Karst groundwater is particularly prone to pollution by faecal micro-organisms (bacteria, viruses or protozoans from human or animal waste) because the solutionally widened fissures and cave passages provide ready access routes. Contaminants can travel widely in very short times, with no purification. Faecal coliform bacteria, including E. coli, are used as indicator organisms when testing water supplies because they occur in large numbers in human and animal waste. They are not necessarily harmful themselves (although some strains of E. coli are dangerous) but indicate that pathogenic (disease-causing) micro-organisms, which are more difficult to detect, might be present in the water.
Sinkhole infilled with rubbish
Construction of a bungalow and septic tank in an area where
karstic limestones are at ground surface (Catherine Coxon)
Dealing with microbial pollution in water supplies
- Protect the water resource by controlling pollution sources (e.g. septic tank systems, farm wastes)
- Treat the water supply to ensure absence of faecal micro-organisms