To sum up, groundwater pollution caused by liquid waste is one of the main environmental geological problems. To study its formation, change law and prevention measures, we must first understand the meaning of groundwater pollution.

At present, there is no unified definition of groundwater pollution at home and abroad. Professor matthess of Germany (G. Marthess, 1972) put forward: "Groundwater polluted by human activities refers to groundwater whose total dissolved solids and total suspended solids directly or indirectly exceed the maximum allowable concentration of domestic or international drinking water and industrial water standards; Natural groundwater, which is not affected by human activities, may also contain excessive components. In this case, according to the phenomenon that some components exceed the natural variation value, it can also be regarded as pollution. " Professor J.J.Fried (1975) thinks: "Pollution is the change of physical, chemical and biological characteristics of water, which usually restricts or hinders the utilization of groundwater in all aspects." American scholar D.W.Miller (1974) said in their paper: "Infection and polarization are synonyms, which refer to the deterioration of natural water quality to the extent that its applicability is destroyed by human activities; ..... The natural result of groundwater flowing through the aquifer will also increase the concentration of one or more components. This phenomenon is called mineralization. " In the book Groundwater published by R.A.Freeze and J.A. Nery 1979, it is said that "any dissolved substance that enters the water environment due to human activities, regardless of whether its concentration reaches the level of obvious deterioration of water quality, is called pollution, and the word pollution is used. From some of the arguments cited above, we can find some contradictory views. The main difference lies in the different understanding of pollution standards and pollution causes.

Under the influence of natural geological environment and human activities, some components in groundwater may be relatively rich or relatively deficient, which may produce unqualified water quality. If these two phenomena with different causes are collectively called "groundwater pollution", it is not rigorous in science, and it is not advisable from the practical point of view of groundwater resources protection. Because the former was formed in a long geological history, its appearance cannot be stopped; The latter was formed in a relatively short human history, and its appearance can be prevented.

Under the influence of human activities, the concentration of some components in groundwater always changes from small to large, and the actual pollution has already occurred before its concentration exceeds the standard. Therefore, it is actually unscientific to regard the concentration change after exceeding the standard as pollution, and it also loses the significance of prevention.

Therefore, we believe that the definition of groundwater pollution should be: under the influence of human activities, the phenomenon that groundwater quality changes in the direction of water quality deterioration is collectively called "groundwater pollution". No matter whether this phenomenon worsens the water quality to the extent that it affects the use, as long as this phenomenon occurs, it should be regarded as pollution. As for the phenomenon that some components of groundwater produced in the natural environment are relatively enriched and depleted, which makes the water quality worse, it should not be regarded as pollution, but should be called "natural anomaly".

Of course, in practical work, it is best to judge whether the groundwater is polluted and the degree of pollution based on the regional background value (or background value), and sometimes historical water quality data can be used, or water quality without obvious pollution sources can be used as the control value.

5.2.2 Groundwater pollution sources and pollutants

5.2.2. 1 groundwater pollution sources

Groundwater pollution sources can basically be divided into two categories, one is man-made pollution sources and the other is natural pollution sources.

Man-made pollution sources include: all kinds of liquid wastes, which are the most common and the largest in quantity, and the specific characteristics are described in the previous section 5. 1.2; Followed by solid wastes such as domestic garbage, industrial garbage, waste ore piles, tailings and sludge; There are still residual pesticides, chemical fertilizers and farmyard manure.

Natural pollution sources mainly refer to seawater, primary groundwater with high salt content and poor water quality, and ore bodies with high harmful components. When they exploit groundwater reasonably under natural conditions or nearby, it will not pose a threat to the original good groundwater quality. However, if the water sources around these pollution sources are unreasonable or overexploited, it may cause seawater intrusion, the diffusion of harmful components in ore bodies and the infiltration of primary inferior water, resulting in groundwater pollution.

Groundwater pollutants in 5.2.2.2

The so-called groundwater pollutants (or pollution components) refer to all dissolved substances or suspended substances that enter the groundwater environment due to human activities, which will cause water quality deterioration, regardless of whether its concentration worsens the water quality to the extent that it affects its use. There are many kinds of groundwater pollutants, which can be roughly divided into three categories according to their nature:

chemical pollutant

The most common inorganic pollutants in groundwater are Cl-, hardness (Ca2++Mg2+) and total dissolved solids. Trace nonmetals are mainly arsenic and fluorine. Trace metals mainly include chromium, mercury, cadmium and zinc. Many environmental concerns have little organic matter content, which is generally 10-9 or ng/g level.

Biological pollutants

Biological pollutants in groundwater can be divided into three categories, namely bacteria, viruses and parasites. There are more than 400 kinds of bacteria in human and animal feces, and more than 100 viruses have been identified. The sewage without disinfection contains a large number of bacteria and viruses, which may enter the aquifer and pollute the groundwater.

radioactive pollutant

Radionuclides in groundwater may be man-made, such as nuclear power plants, fallout from nuclear weapons tests and radioisotopes used in laboratories and hospitals. It may also be naturally formed, such as radioactive deposits.

5.2.3 Characteristics and modes of groundwater pollution

5.2.3. 1 groundwater pollution characteristics

Groundwater pollution is obviously different from surface water pollution, which has two characteristics:

disguised

Even though groundwater has been seriously polluted by some components, it is often colorless and tasteless, and it is not easy to distinguish from color, smell, fish death and so on. Even if people drink groundwater polluted by toxic or harmful components, the impact on the human body is only a chronic long-term effect, which is not easy to detect.

irreversibility

Once the groundwater is polluted, it is difficult to control and recover. Mainly because its flow velocity is extremely slow, it will take as long as ten years, decades or even hundreds of years only to rely on the natural purification of the aquifer itself after cutting off the pollution source. Another irreversible reason is that after some pollutants are adsorbed by media and organic matter, desorption-re-adsorption will occur repeatedly in the change of water environment characteristics.

Ways of groundwater pollution in 5.2.3.2

Groundwater pollution path refers to the path that pollutants enter groundwater from pollution sources. Studying the ways of groundwater pollution is helpful to formulate correct measures to prevent groundwater pollution. However, the ways of groundwater pollution are complex and diverse, and some people classify the pollution sources according to their types, such as the leakage of sewage channels and sewage pits, the leaching of solid waste piles, the overflow of chemical liquids, the pollution of agricultural activities and the pollution of mining activities, which are too complicated. This paper introduces the classification according to the hydraulic characteristics, which is simple and clear. According to this method, groundwater pollution paths can be roughly divided into four categories, as shown in Table 5. 1 and Figure 5. 1 and Figure 5.2.

Table 5. 1 Classification of Groundwater Pollution Paths

5.2.3.2.1intermittent infiltration type

It is characterized in that pollutants are leached by atmospheric precipitation or irrigation water, so that solid wastes and toxic and harmful substances in the surface soil or stratum periodically (during irrigation dry land and rainfall) penetrate into the aquifer from the pollution source through the vadose zone soil layer. This infiltration is generally in the form of rain seepage in unsaturated state or continuous seepage in saturated state for a short time. Groundwater pollution caused by this way, pollutants initially exist in solid waste or soil. Of course, it also includes irrigation of field crops with sewage, and its pollutants come from urban sewage. Therefore, when studying the pollution path, we should first analyze the chemical composition of solid waste, soil and sewage, and it is best to obtain leachate passing through the vadose zone in order to find out the source of groundwater pollution. This kind of pollution, whether in scope or concentration, may have obvious seasonal changes, and the pollution object is mainly shallow groundwater.

Figure 5. 1 Groundwater Pollution Path

Continuous osmotic type

Its characteristic is that pollutants continuously penetrate into aquifer through vadose zone with various liquid wastes. In this case, either the vadose zone is completely saturated with water in the form of continuous seepage, or the upper surface soil of the vadose zone is completely saturated with water in the form of continuous seepage, while the lower part (lower vadose zone) is in the form of unsaturated rainwater infiltrating into the aquifer. This type of pollutant is usually liquid. The most common is the leakage of sewage accumulation area (sewage pool, sewage seepage pit, sewage rapid infiltration field, sewage pipeline, etc.). ), as well as the leakage of polluted surface water bodies and sewers. Paddy fields (rice, etc.) irrigated by sewage, of course. ) will cause continuous infiltration in a large area. This kind of pollution object is mainly shallow aquifer.

The common feature of the above two methods is that pollutants enter the aquifer through the vadose zone from top to bottom. Therefore, the pollution degree of groundwater mainly depends on the geological structure, material composition, thickness and permeability of vadose zone.

Overflow type

Figure 5.2 Groundwater Pollution Paths

Its characteristic is that pollutants are transferred to other aquifers through interlayer overflow. This transfer is either through natural channels (hydrogeological skylights) or through artificial channels (unreasonable well pipes, damaged old well pipes, etc.). ), or the change of groundwater dynamic conditions caused by man-made mining [Figure 5.2 (a)], changes the overflow direction, so that pollutants can flow to other aquifers through a large area of weak aquiclude. Its pollution source may be the groundwater environment itself, or it may be foreign, which may pollute confined water or diving. The difficulty in studying this kind of pollution is that it is difficult to find out the specific location and geographical location of the overflow.

Runoff type

It is characterized in that pollutants enter the aquifer in the form of groundwater runoff, or enter other water layers through sewage treatment wells, or enter through huge karst channels with karst development, or break through the isolation layer of underground storage layer of waste liquid. Seawater intrusion is an underground runoff caused by over-exploitation of underground fresh water in coastal areas. This form of pollution, its pollutants may be man-made sources, may also be natural sources, may pollute diving or confined water. Its pollution range may not be very large, but due to the lack of natural purification, its pollution degree is often very serious.