Water is the source of life, and karst water provides a source of drinking water for about a quarter of the world's population. Among about 20% of the population in China, nearly 30 large and medium-sized cities use karst water as the main source of water supply. It can be seen that the quality of karst water is directly related to the health and social and economic development of the general public.
Under natural conditions, karst water is mostly weakly alkaline, and weak alkaline water is good water?
Influenced by the lithology of the carbonate strata, the natural groundwater in the karst area is weakly alkaline. The main mineral components of carbonate rock are calcite and dolomite. Under the action of dissolved carbon dioxide in water, carbonate rock dissolves to form weakly alkaline karst water rich in Ca2+ and HCO3-.
The content of Ca2+ and HCO3- in natural karst water is high. When boiling water, a layer of white scale, ie calcium carbonate deposit, often appears on the bottom of the pot. This is because the temperature of the water rises, the dissolved carbon dioxide in the water escapes, the solubility of calcium carbonate decreases, and precipitation occurs. Therefore, karst water should not be directly eaten by drinking, and it is beneficial to be healthy after drinking. Otherwise, it is prone to kidney stones, which is also the most common disease in karst area. It can be seen that “weak alkaline water is good for health†is not scientifically based.
As an important part of water resources, karst water quality directly affects the social and economic development of karst areas through human health, water environment health, and economic costs. However, many people often confuse water quality with water pollution. A correct understanding of water quality and water pollution is the key to avoiding misunderstanding.
The quality of water is based on the existing water quality standards and the classification of water environment quality for the purpose of human health, such as “surface water quality standardâ€, “groundwater quality standardâ€, “standard of drinking water hygiene†and “water quality of urban water supplyâ€. Standards, etc., express the quality of water and its applicability.
Water pollution refers to the phenomenon that water quality develops in the direction of deterioration under the influence of human activities. Regarding water pollution, there is currently no unified evaluation standard. At present, it is common to use the control value, that is, the water quality with no obvious pollution source as the standard, and the change trend of water quality is expressed.
It can be seen that the water quality and water pollution have different meanings and the evaluation criteria are inconsistent and cannot be confused. Poor water quality is not necessarily contaminated, it may be caused by the original geological environment. For example, the iron and magnesium in the water in most karst areas are high, mainly due to the high content of iron and magnesium in the aquifer rocks. The water is polluted and not necessarily not drinkable. For example, the manganese content in the groundwater is originally less than 0.05mg/L. After the manganese mine discharges, the manganese content in the groundwater reaches 0.09mg/L. We think that the groundwater is industrially polluted, but Since the manganese content in the groundwater has not exceeded the groundwater class III water quality standard limit of 0.1 mg/L, the groundwater is still a qualified source of drinking water. It can be seen that the unqualified drinking water can only be defined when the concentration of the contaminated component exceeds the corresponding water quality standard.
In recent years, the survey shows that the quality of karst water in the southwestern China is generally good, but the local water quality is poor, especially the quality of groundwater in the suburbs of the city is deteriorating. It can be directly used as drinking water source, accounting for 86.1%. After treatment, it can be used as drinking water source, accounting for 10.0%, and should not be used as drinking water source, accounting for 3.9%.
Surface water and groundwater in karst areas often change frequently, especially in the karst areas of southern China, which is extremely common. Surface water is susceptible to pollution due to exposure to the surface, so once the surface karst water is contaminated, the underground karst water will also be contaminated.
There are many ways of water pollution in karst areas, which makes karst water pollution difficult to identify and control. The sub-topography such as depressions, falling water caves, water-dissipation caves, shafts, karst collapse pits, etc., which are widely developed on the surface of karst areas, are the main channels through which pollutants directly enter the underground and pollute the groundwater.
However, there are various types of geomorphological combinations in karst areas. Different geomorphological types and different pollution source locations can form different characteristics of pollution, with different pollution pathways and pollution mechanisms, such as secondary pollution sources in karst areas, slow-release pollution, meteorological intermittent type. Pollution, irrigation pollution, etc.
With the development of urbanization, the underground rivers in the suburbs of some urban cities are becoming “urban sewersâ€. For example, the underground river of Laolong Cave in Nanshan, Chongqing, the underground river of Xiangshui Cave in Kaiyang, Guizhou, and the underground river in Danli Lake in southwest of Guangxi... In these underground “waterway†underground rivers, new types of organic pollutants such as pesticides, antibiotics and hormones are coming. The richer, the serious threat to the water environment and ecological security.
The principle of prevention and control of karst water pollution is prevention and prevention. First of all, it is necessary to reduce the pollution sources, intercept the pollution, and control the way the pollutants enter the karst water system from the source. In the control of pollution sources, the most important thing is not to discharge pollutants into the sinking holes, depressions, etc.; such sensitive areas must be avoided in the selection process of pollutant dumping sites. Secondly, it is to establish a suitable karst water source protection area on the basis of scientific analysis and research, which is an important part of the prevention and control measures of karst water pollution. Finally, it is to carry out water ecological management, make full use of karst wetlands (including constructed wetlands and natural wetlands) or PRB technology to carry out contaminated karst water treatment. Due to the special structure of the karst system, once the karst water is polluted, it is extremely difficult to treat. At present, there is no successful case of karst water pollution control at home and abroad.
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