Characteristics, hazards, causes and prevention measures of groundwater quality deterioration
The deterioration of groundwater quality is an important research topic of global environmental pollution. The deterioration of groundwater quality mentioned in this section mainly refers to the increase of some chemical and microbial components in groundwater due to environmental pollution, changes in hydrodynamic and hydrochemical formation conditions and poor exploration during groundwater development, which leads to the deterioration of water quality and exceeds the prescribed use standards. Its main features are as follows:
1) Many organic compounds (such as synthetic dyes, detergents, detergents, solvents, oils and organic pesticides, etc. ) what does not exist in natural groundwater appears in groundwater;
2) Toxic metal elements (mercury, chromium, cadmium, arsenic, lead) and some radioactive elements in natural groundwater enter groundwater in large quantities;
3) All kinds of bacteria and viruses multiply in underground water, far exceeding the drinking water quality standard;
4) The hardness, salinity, acidity and the content of some single conventional ions of groundwater are rising continuously, which has exceeded the prescribed use standards.
(B) the harm of groundwater quality deterioration
The deterioration of groundwater quality environment has seriously damaged the use value of groundwater resources, brought various adverse consequences to human society, damaged human health, and led to disability and death; Damage to the quality of industrial products; Reduce crop yield and salinize the land; Reduce the recoverable resources of groundwater, so that the whole water source is abandoned; In order to treat water quality, the unit cost of water is increased.
The problem of groundwater pollution in China cannot be ignored. Among the major cities in China,12 cities use groundwater as water supply source, and13 people in China drink groundwater. According to the investigation of 100 cities in China, more than 80% of the groundwater is polluted to varying degrees, among which Beijing, Shenyang, Tai, An, Bao and Chang are seriously polluted. Some indicators of 78% well water in Shenyang do not meet the drinking water standard. The hardness of groundwater in many cities in northern China is increasing year by year, and the problem of seawater intrusion in some coastal cities is also quite serious.
(C) the reasons for the deterioration of groundwater quality
There are many reasons for the deterioration of groundwater quality, which can be summarized as the following three aspects.
1. There are some pollution sources that cause the deterioration of groundwater quality.
These pollutants can exist underground and on the ground. According to the genetic types of pollutants, they can be divided into two categories. The first category is natural pollution sources, that is, all kinds of inferior water bodies existing in nature, such as seawater, underground high salinity water or other inferior water bodies. In addition, some water-bearing media in aquifers or vadose zones contain some minerals (especially various soluble salts), which may also be the source of groundwater pollution. The second category is man-made pollution sources, which refer to pollution sources formed by human activities, such as various wastewater, sewage, garbage, fertilizers, pesticides and so on. According to relevant data, the average annual sewage discharge in China is 500× 108t, of which 75% is industrial sewage and 25% is domestic sewage. More than 80% of the sewage is directly discharged into the waters, causing water pollution. Man-made pollution sources can be divided into direct pollution sources and indirect pollution sources All kinds of sewage, wastewater, fertilizers and pesticides, whose pollutants directly enter the aquifer through the vadose zone, are direct pollution sources. Pollutants first enter the atmosphere or surface water body, and then enter the aquifer, which is called indirect pollution source. For example, "acid rain" containing sulfuric acid and nitric acid formed near industrial cities is an indirect pollution source. The infiltration of acid rain, on the one hand, directly acidifies groundwater; On the other hand, acidified water will enhance solubility, greatly increase the content of metal elements in groundwater and pollute groundwater. Moreover, acid rain pollution is a large area, often more harmful than local point pollution sources.
There are many examples that industrial wastewater and domestic sewage are discharged into surface water without treatment, thus causing groundwater pollution. Especially in those riverside water sources, piedmont alluvial fans and karst underground river water sources, the groundwater pollution caused by river water pollution is more serious. For example, an underground water source in Anshan City mainly depends on the leakage of Taizi River during the dry season. Because the wastewater containing nitro compounds discharged from the upstream chemical plant enters the aquifer through the river, more than 40 wells in the water source area are polluted to varying degrees, with a polluted area of 200km2.
In addition, in some cases, the corrosion of well pipes or water pipes, metal pipes and concrete water pipes will also cause water pollution.
2. Pollutants enter through channels.
When the groundwater quality deteriorates, there must be not only pollution sources, but also ways for pollutants to enter the aquifer. Pollutants usually enter aquifers in three ways:
1) Within the mining funnel of the aquifer, pollutants directly penetrate into the aquifer through the permeable rock above the aquifer. Because of the short access route, groundwater is often seriously polluted soon. Under the same pollution source, the degree of groundwater pollution mainly depends on the permeability of rock between the surface and aquifer, the adsorption and purification ability of geotechnical particles to pollutants and the buried depth of aquifer. Therefore, generally confined water has better pollution prevention conditions than diving. If there is cohesive soil layer in the vadose zone of phreatic aquifer, it will also have better protection ability.
2) Pollutants enter the mining area from other parts of the aquifer. Various natural inferior water bodies (such as seawater and mainland high salinity water) and polluted surface water bodies penetrate (flow) into the aquifer through the direct contact zone with the aquifer (especially the recharge zone), and then move to the mining area. When the pollution source is located in the upstream of the water source, it poses a great threat to the water quality pollution of the water source.
3) Pollutants enter the aquifer through some natural or artificial centralized channels. Natural centralized channels mainly refer to all kinds of water-conducting fault channels, fissure channels and karst channels (including "skylight") that communicate with pollution sources. This kind of channel is generally distributed in points or lines, which can pollute deep confined water. Artificial centralized channel mainly refers to the direct channel for inferior water to enter the aquifer due to the damage of anti-fouling effect of waterproof top plate (or bottom plate) of the aquifer in various underground projects and well construction processes. It can often be seen that due to the design and construction defects of the well (failure to stop water or unsatisfactory water stop), the upper sewage flows into the exploitation aquifer along the gap between the well pipe and the hole wall; Sometimes, because abandoned wells are untreated or backfilled, they become the invasion channels of surface sewage; In some wells that have been in disrepair for a long time, due to well pipe corrosion or earthquake disaster, the well pipe breaks, and the upper sewage can also invade the exploitation aquifer.
3. The factors causing the deterioration of groundwater quality are hydrodynamic and hydrochemical factors.
If the existence of pollution sources and pollution channels is a necessary condition for the possible deterioration of groundwater quality, then the hydrodynamic and hydrochemical effects under mining conditions are the direct reasons for the deterioration of groundwater quality.
When sewage invades the exploitation aquifer, certain hydrodynamic conditions are needed. First of all, there must be some direct or indirect hydraulic connection between the exploitation of aquifer (or part) and sewage; Secondly, due to pumping, a negative pressure area relative to the polluted water body is formed in the mining aquifer (or section), which urges the sewage to flow directly or indirectly (through the weak permeable layer) and pollutes the mining aquifer (or section).
A typical example is that the change of hydrodynamic conditions of offshore water sources leads to seawater intrusion into continental aquifers. Under natural conditions, fresh water in continental aquifers is discharged into the ocean, and the equilibrium interface between salty and fresh water bodies is maintained by the head pressure of fresh water in aquifers higher than the sea surface. Under the condition of exploitation, if the exploitation amount of water source exceeds the recharge amount, it will inevitably lead to the continuous decline of fresh water level in aquifer. When the water level drop funnel extends to the coastline, it will lead to seawater intrusion and groundwater salinization. In some cases, although the exploitation amount does not exceed the fresh water supply, when the head pressure of the fresh water body has been reduced to the point where it is difficult to maintain the original balance of the salty and fresh water body, the salty and fresh water interface will also move to the mainland. If the interface is pushed into the falling funnel of the pumping well, it will also cause salt water to invade the mining area and worsen the water quality.
Large-scale exploitation of groundwater will also change the hydrogeochemical conditions of aquifers. The appearance of some new hydrogeochemical processes is also one of the important reasons for the deterioration of groundwater quality in some areas. The phenomena of salinity, hardness, iron and manganese ion content increasing and pH value decreasing in many underground water sources in China are mainly caused by dehydration and oxidation of aquifers. Because in the process of groundwater exploitation, with the decline of groundwater level, oxygen enters the drainage area with air, which promotes the oxidation of sulfur, iron, manganese and nitrogen compounds in rock strata, especially the role of sulfur-oxidizing bacteria, which further intensifies the oxidation process of metal sulfides. For example, pyrite (FeS2), which is widely distributed, is very stable in reducing environment and almost insoluble in water, but it is easily dissolved in oxidizing environment to form acidic water. The compounds of calcium, magnesium, iron and manganese, which often exist in the soil layer, are also easily dissolved, which greatly increases the content of iron, manganese, calcium, magnesium and sulfate ions in groundwater, and the salinity and hardness of groundwater also increase.
In recent years, the fluorine ion content in the submerged area of reservoirs (or channels) in semi-arid areas of northern China has increased, which is also caused by the change of hydrogeochemical environment. After the establishment of the reservoir (canal), the groundwater level in the flooded area rose, and the evaporation and concentration of groundwater intensified, further increasing the fluoride ion content in the groundwater with high background value under semi-arid alkaline conditions; Due to the construction of the reservoir, the surrounding environment has become relatively humid, which has promoted the further development of vegetation. CO2 secreted by plant roots will accelerate the decomposition of carbonate in soil layer and the accumulation of bicarbonate in groundwater, thus alkalizing the environment. This weak alkaline hydrogeochemical condition further creates conditions for the enrichment of fluoride ions.
Due to the change of hydrogeochemical environment caused by groundwater exploitation, there may still be many environmental hydrogeologic problems that lead to the deterioration of groundwater quality. It should also be noted that these changes are not all unfavorable. For example, under certain conditions, due to strong pumping, the alternating circulation of groundwater is intensified, and leaching is strengthened, thus accelerating the dissolution and removal of soluble salts in aquifers. With the decline of water level in aquifer, the original closed reducing environment of groundwater has become an open oxidizing environment, which leads to the precipitation of some compounds in water, and can also reduce the content of some harmful ions in water or dilute the water quality. Therefore, in the hydrogeological investigation of water supply, a thorough and comprehensive analysis should be made according to the local stratigraphic lithologic structure, the types and contents of soluble salts in vadose zone and aquifer, the types and chemical properties of recharge water sources, and the expected mining intensity and depth reduction of water sources, so as to make a correct prediction on the possible changes of groundwater quality after mining.
(4) Measures to prevent the deterioration of groundwater quality
Groundwater is an inseparable and important part of the hydrosphere and even the whole earth environment. Therefore, the prevention and control of groundwater quality deterioration must be combined with the prevention and control of environmental deterioration for comprehensive treatment, which requires both technical measures and management measures. The deterioration of groundwater quality is often slow and hidden, and it is not easy to be found in time. Once it deteriorates, it is difficult to control and recover. Therefore, the policy of combining prevention with treatment must be adopted to ensure the quality of water supply.
1. Preventive technical measures
The most important preventive technical measure is a comprehensive and reasonable planning and layout of urban development and water source construction. When making urban development planning, especially industrial layout, we must consider minimizing urban environmental pollution and groundwater pollution. Those factories that are easy to cause groundwater pollution should be arranged downstream far from the water source, or use pipelines to discharge pollutants. When building new water sources, we must also consider the environmental conditions of groundwater pollution. We should choose the water source or groundwater recharge area in the upper reaches of the city, or choose a place with better conditions from the lithologic structure of the stratum to prevent pollution. In short, in order to protect groundwater resources, the overall planning of urban construction must consider the requirements of environmental protection; We should have the viewpoint of preventing pollution and maintaining ecology; Environmental protection should be planned and implemented synchronously with economic development, so as to realize the coordinated development of economy, society and environment.
In addition, when there are inferior water layers or water bodies above, below or near the water intake layer (especially in coastal water sources), the exploitation amount and depth of the water source should be strictly controlled to prevent inferior water from invading the aquifer. In the design of wells, it is best to take water in layers. When there is salt water in the deep, the well depth should be controlled to keep a certain distance between the bottom of the well and the interface between light and salt water. Ensure the quality of water stopping and backfilling in water well construction. For wells that have been in disrepair for a long time, the well tube should be replaced in time; Abandoned wells should be backfilled and sealed. It should also be noted that when excavating underground buildings, the waterproof protective layer above, below or around the mining aquifer shall not be destroyed.
Although it is impossible to completely eliminate the pollution of sanitary protection zone, it can be controlled in a certain period of time and under certain hydrogeological conditions. For shallow groundwater and water sources with thin surface coverage, the building health protection effect is obvious. Therefore, China's Environmental Protection Bureau, Ministry of Health, Ministry of Construction, Ministry of Land and Resources and other departments jointly issued the Regulations on Pollution Prevention and Control of Drinking Water Source Protection Areas. It stipulates that underground water sources must set up first-class protected areas and second-class protected areas; When necessary, quasi-protected areas should be set up in the periphery, and corresponding protective measures should be stipulated for each area. The size of each area should depend on the specific hydrogeological conditions and mining intensity. If the health protection zone 1 is a no-activity zone, its scope should not be too large. According to the national documents of the former Soviet Union, the radius of Area I is not less than 30m, and the diving is not less than 50m.
2. Governance measures
The control measures of polluted water sources should be formulated according to the main causes of groundwater pollution, pollution routes and current national economic conditions. The main measures are as follows.
(1) Control pollution sources
Pollution sources include point sources and non-point sources. Point source refers to the pollution source composed of industrial "three wastes" and urban domestic sewage and garbage. They are the main sources of water pollution in centralized water sources at present. Among them, industrial wastewater is the most harmful. Therefore, the key to control and treat groundwater pollution should be the comprehensive treatment of industrial wastewater. In addition to taking legal measures such as controlling sewage discharge and discharge standards, it is mainly to reform backward production technology and do a good job in closed-circuit circulation of industrial water. Only in this way can we minimize the discharge of industrial wastewater and eliminate industrial wastewater in the production process; At the same time, it also saves water resources and improves the economic benefits of enterprises.
For wastewater (including industrial and domestic sewage) that must be discharged, it is necessary to prevent them from leaking into the aquifer on the way of discharge and in the sewage treatment plant. In order to reduce leakage, it is best to discharge in a place with stable water-resisting layer distribution, or take anti-seepage lining measures, and arrange the discharge point as far away from the downstream of the water source as possible. When using underground caves or deep mined-out rocks to discharge pollutants, it is necessary to find out the local hydrogeological conditions, which are proved to be harmless to the environment by experiments.
In the treatment of point pollution sources, the possible pollution impact of urban garbage, especially some industrial waste residue, on groundwater can not be ignored. For example, the landfill in Lanzhou polluted the water and groundwater of the Yellow River. In order to prevent the groundwater from being polluted by garbage and industrial waste residue leachate, comprehensive treatment measures such as garbage recycling, incineration, power generation, biochemical treatment and composting should be taken. Generally speaking, it is best to choose a dump or landfill for garbage or waste residue where the weak permeable soil layer on the surface is widely distributed, thick, low-lying, closed and with thick vadose zone; At the same time, they must stay away from water sources or recharge areas where aquifers are exploited.
Non-point sources mainly refer to the pollution caused by agricultural sewage irrigation, fertilization, pesticides, acid rain and urban storm runoff. According to American statistics, the pollution load caused by non-point sources accounts for more than 50% of the total pollution load, which is a factor that cannot be ignored in groundwater pollution. The following measures can be taken to control non-point pollution sources:
1) seriously carry out sewage irrigation. The most important thing is to strictly control the water quality standard of sewage irrigation, control the irrigation quota, and rationally plan the location of sewage irrigation area according to the environmental hydrogeological conditions. For example, it is not suitable for sewage irrigation near diving areas, irrigation areas and water sources with thin surface soil or high permeability.
2) Use fertilizers and pesticides that are easily absorbed by plants or decomposed by soil, strictly control the use amount of fertilizers and pesticides, and minimize their residual concentration in the soil layer and the amount flowing into the aquifer.
3) Pretreatment of irrigation sewage.
(2) Building supporting environmental projects and vigorously carrying out sewage treatment and utilization.
Building supporting environmental projects and vigorously carrying out sewage treatment and utilization are the fundamental measures to control the deterioration of groundwater quality. A large amount of untreated sewage is the main pollution source that causes the current environment, especially water pollution. The treated sewage can be used for different purposes of water supply according to its water quality, so as to improve the reuse rate of wastewater and increase the total amount of water resources.
(3) Take hydraulic measures to prevent inferior water (or sewage) from invading the exploitation aquifer.
When seawater or other inferior water intrudes into the aquifer from the side, so-called "hydraulic" measures can be taken to prevent the intrusion of inferior water bodies. Now take the prevention of seawater intrusion abroad as an example to introduce it.
1) "Recharge water mound" or "fresh water barrier" method. That is to say, a fresh water injection well is arranged between the coast and the inland mining area, and a "recharge water mound" higher than the natural groundwater level is formed by water injection (figure 12- 1), so as to control the inland movement of the salt water surface. It is reported that this method has been used in a coastal area of California and the coast of Israel, successfully preventing seawater intrusion.
Fig. 12- 1 schematic diagram of recharge water mound (fresh water barrier)
2) "water pumping tank" method. A pumping line is arranged between coastal and inland mining areas to form a "pumping trough valley" to prevent salt water from moving inland. If the extracted salty and light mixed water cannot be used, it will be discharged into the sea. The advantage of this method is that it does not need to replenish water. This method of preventing seawater intrusion has been widely used in freshwater sand dunes along the coast of the Netherlands (Figure 12-2).
Figure 12-2 Schematic diagram of water pump box (the legend is the same as Figure 12- 1).
3) The method of "combining water injection with water pumping". Generally, pumping tanks are arranged near the coast, and water injection wells are arranged near the exploitation water source.
4) Construction of "underground water retaining wall". This method is mainly used in areas where salt water invades along narrow seepage channels. For example, in the northwest of Nagasaki Prefecture, Japan, a grouting curtain was built in the gravel layer containing clay near the entrance of the passage to prevent seawater intrusion. An artificial underground reservoir has been formed offshore, but the effect of this measure is limited. Even if the dynamic water level remains above sea level, it is difficult to prevent the "rising cone" formed by deep brine from moving into the well.
In addition to the above situation, there is also a situation that a large area of land is secondary salinized due to unreasonable irrigation, which worsens the ecological environment. For example, in Kashgar, the area of land salinization has reached 64.7% of cultivated land, which is amazing. The main prevention and control measures should be reasonable irrigation.