Expert: underground sewage is the culprit of groundwater pollution in China

Recently, the underground sewage has spread from the network to many local governments to carry out relevant investigations. Whether the evidence of underground sewage discharge by enterprises is true or not, it is an indisputable fact that China's groundwater pollution is serious. On this issue, the reporter of environment and life magazine interviewed Wang Hao, an expert in hydrology and water resources and an academician of the Chinese Academy of engineering.

Sixty percent of groundwater in China is seriously polluted

Environment and life: you are an authoritative expert on groundwater. Can you introduce the current situation of groundwater in China?

Wang Hao: more than 1.5 billion people around the world mainly rely on groundwater as drinking water. China's groundwater resources account for 1 / 3 of the country's total water resources, maintaining 70% of the country's population drinking, 40% of farmland irrigation and 38% of industrial production.

Due to the discharge of industrial wastewater and domestic sewage, the use of excessive chemical fertilizers and pesticides, the leaching of garbage dump and the leakage of underground oil tank, the groundwater is suffering from more and more serious pollution. An expert from the China Geological Survey once pointed out that 90% of China's groundwater is polluted to varying degrees, and 64% is seriously polluted.

Among the 1.97 million square kilometers of shallow groundwater in the plain area investigated and evaluated by the national water resources survey, groundwater pollution is serious in Taihu Lake, Liaohe River, Haihe River, Huaihe River and other basins.

Chemical fertilizers and pesticides cause the "three nitrogen" (i.e. ammonia nitrogen, nitrate and nitrite) in groundwater to exceed the standard. The nitrate nitrogen in groundwater exceeded the standard in all provinces and cities in the Yangtze River Delta.

There is also organic pollution in groundwater pollution. Relevant domestic departments have investigated and evaluated groundwater pollution and generally detected trace organic substances, and "three carcinogenic, teratogenic and mutagenic" substances in varying degrees. These substances are not easy to be degraded by microorganisms in water, but are easy to be biologically absorbed and return to the human table through the food chain. More than 100 pollutants have been detected in groundwater in Beijing, Tianjin, Hebei and other places, many of which are "three causing" substances.

In recent years, due to a large number of overexploitation of groundwater, a large area of groundwater funnel has been formed underground, which has exacerbated the backflow of surface sewage to groundwater. According to the official report, the groundwater in North China has exceeded 120 billion cubic meters, equivalent to 200 Baiyangdian lakes. The continuous decline of groundwater level has formed a huge groundwater drawdown funnel area in the North China Plain. Sewage discharge in these areas will not only pollute the shallow groundwater, but also easily pollute the deep groundwater with the continuous flow of shallow sewage to the deep.

In short, groundwater pollution in China has shown a trend of evolution from point to surface, expansion from east to west, urban to rural, and local to regional.

Agriculture and industry are victims

Environment and life: what harm will such groundwater pollution cause?

Wang Hao: first, it directly affects the water quality of drinking water sources. The hardness of groundwater is too high after being polluted. As a drinking water source, it is not only bitter and difficult to drink, but also causes gastrointestinal dysfunction, vomiting, diarrhea, flatulence and other symptoms; When groundwater is polluted, it often causes the change of three nitrogen content in water. If the three nitrogen content in water is too high, it will cause harm to human body (especially infants) and cause acute nitrate poisoning. Under specific conditions in the human body, trinitrogen will also be transformed into carcinogen nitrosamine, leading to cancer and even death. In addition, if the groundwater source is seriously polluted by organic pollution or even heavy metal pollution, the "three causing" substances produced in the water body will cause greater harm to human health. In recent years, the "cancer villages" in Taihu and Chaohu have been gradually appearing, which has aroused widespread concern about the relationship between water pollution and the incidence rate of cancer.

Secondly, in the field of food safety research, the impact of irrigation water pollution on the quality of agricultural products is also one of the hot topics.

Moreover, in China, especially in the north, groundwater accounts for a large proportion of industrial production water, and the pollution of groundwater will seriously affect industrial production. The increase of groundwater hardness will cause scaling in the boiler and pipe wall, which will not only waste fuel, but also directly affect the service life of the boiler and even cause explosion; In terms of textile printing and dyeing industry, washing products with high hardness water slurry will not only consume a lot of lotion, but also produce defective or waste products; In addition, high hardness groundwater will also cause harm to many industries such as chemical industry, pharmacy, brewing, power generation, papermaking and so on. This forces some industries to soften and purify hard water, which increases the production cost.

After the pollutants are discharged into rivers and lakes, in addition to causing harm to natural fish and aquatic organisms in the water body, excessive nitrate will also acidify and corrode the river and lake water, affect the normal growth of aquatic organisms, and even lead to the extinction of some aquatic organisms.

Improper treatment of sewage and waste liquid in China

Environment and life: what role does underground sewage play in the current situation of groundwater pollution in China?

Wang Hao: government departments have issued corresponding policies for enterprise sewage discharge and sewage treatment standards to minimize the negative impact of industrial development on the environment.

However, some illegal enterprises discharge industrial wastewater containing highly toxic carcinogens that are difficult to degrade (such as non degradable heavy metals such as arsenic and mercury or polycyclic aromatic hydrocarbons) into the ground. In addition to digging seepage pits and secretly draining seepage wells, in order to avoid investigation and punishment, some polluting enterprises will directly inject a large amount of sewage underground with high-pressure pumps, and some enterprises in the South even discharge sewage into underground karst caves.

For example, the cadmium pollution incident in Guangxi in February 2012 was caused by the discharge of cadmium containing pollutants by two enterprises through karst caves.

It is also common to discharge sewage through seepage pits and wells. This method provides low-cost and easy operation conditions for surface water infiltration. These sewage will often penetrate into the underground aquifer through sandstone, gravel and rock layers with good permeability in the upper layer of groundwater, resulting in groundwater pollution.

The common way of underground sewage discharge in China also adopts shallow well water layer sewage discharge, that is, the untreated sewage is directly discharged into the groundwater through a well 300 ~ 500m deep, and this location is usually where the groundwater for drinking is located. Due to the improper treatment of sewage and waste liquid and the immaturity of sewage discharge technology, underground sewage discharge has undoubtedly become the culprit of groundwater pollution in China.

An underground injection well east of Fort Worth, Texas, USA source / EPA website

90% of us hazardous waste liquid injected underground

Environment and life: it is understood that underground sewage is also used internationally. From the perspective of science and technology, what are the conditions for underground sewage discharge? Can the relevant management system be used for reference?

Wang Hao: internationally, there are three types of underground sewage discharge. The first two are seepage pit seepage well discharge and shallow well water layer discharge, which are widely used in China,

These two methods will pollute groundwater. The third method is high-pressure deep well discharge, also known as "deep well injection". This innovative technology has been adopted by many countries and proved to be a mature and leading sewage and waste liquid treatment method. The United States started early in this regard. Since the 1930s, American oil companies have used this technology to treat the waste liquid from oil and natural gas production. More than 2.84 billion cubic meters of waste liquid are injected underground every year, and the number of injection wells has exceeded 650000. About 89% of the dangerous industrial waste liquid that is difficult to treat in the United States is disposed of through deep well injection.

The advantage of deep well injection is that the injection fluid is stored in the deep geological layer, which can avoid pollutants from entering the biosphere circulation system, reduce the environmental pressure on the atmosphere, water body and shallow stratum, and the safety factor is much higher than that of other waste disposal technologies.

However, deep well grouting, as a safe underground sewage discharge method, has high requirements for geological conditions, well excavation depth and safety protection of deep wells, and is difficult to withstand the long-term test of crustal movement. First of all, in the site selection of injection wells, we should not only have simple geological structure, no complex faults and folds, but also have stable stratigraphic structure, less seismic activity or less affected by earthquake damage, but also ensure that rocks and minerals can be compatible with sewage, and there are few places that can meet these conditions at the same time. Countries like Japan at the junction of plates are not suitable for deep well injection, There are many suitable places in China.

For this new underground sewage discharge mode, China is basically feasible at the technical level, but to fully achieve safe underground sewage discharge, it is necessary to establish corresponding implementation standards and regulatory measures.