Karst aquifers provide a very important water resource in many parts of the Earth: karst terrains, in fact, are, by nature, lacking in surface waters, and all water circulation occurs underground.
However, these resources are very difficult to utilize and to protect. Karst aquifers, in fact, due to some of their characteristics, are particularly vulnerable to pollutants and excessive exploitation.
An excessive and uncontrolled utilization of the reserves of deep phreatic zones can be a hazard for these kinds of aquifers: deep waters, in fact, move very slowly and need years or decades to be substituted, so an excessive exploitation could endanger the utilization of the entire aquifer forever.
But it is mainly with regard to the propagation of polluting substances that karst aquifers seem particularly vulnerable.
In sand or gravel, where the speed of water is very slow, the result of the prolonged contact of water and rock is that the former gets depurated of possible pollutants because of a mechanical filter effect, the natural deterioration of some substances with time and because of the action of bacterial colonies living on the surface of the granules. These processes allow the aquifer to eliminate many pollutants, especially the organic ones, through a mechanism of auto-purification that helps to protect the aquifer from pollution.
In the uppermost zone of the karst aquifer, water flows fast, similar to surface water flow regimes and the effect of auto-purification is practically nil: whatever enters a karst aquifer generally exits unchanged at the spring, often a very short time later. On the contrary, in deep phreatic zones, where circulation is very slow, pollutants can collect and get stored and more and more concentrated. Subsequently, the particular mechanism of flood propagation, the piston flow, can provoke the instantaneous release, at high concentrations, of the possible polluting substance which might have accumulated slowly over the course of time. Often these episodes of instantaneous pollution seem inexplicable because no current source of pollution can be identified: small quantities of pollutants that are well tolerated in other kinds of aquifers, become potentially very dangerous in karst aquifers.
Unfortunately karst areas have another property that makes them even more vulnerable: the presence of a great amount of depressions, sinkholes, shaft and dolines in the catchment zone. These seem ideal for use as convenient dumping grounds, where useless things can be concealed, at times even highly dangerous material. Too often one forgets, or pretends not to know that by doing this the entire karst system gets polluted. Since the location of the springs is not always known in karst aquifers, the contamination produced in the catchment zone can pollute springs several kilometres away, at times even in adjacent valleys: at times the self-interest of those who live at higher altitudes can cause severe problems to unknowing inhabitants living in the valley below. Unfortunately, knowledge of karst aquifers is still so limited that a few years ago there was a proposal to use caves for the storage of toxic and radioactive waste!
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