How to repair contaminated groundwater

Groundwater is water that is buried underground. The water that we bring up from the well or the spring water that comes out of the stone in the mountains is groundwater.

Where does the groundwater come from? It does not originate inside the earth, but is buried underground by rainwater or river water. The water in the well seems to be only a small one, but it can never be used. Why is this? Because the soil under our feet has a layer of sand and a layer of clay , layer by layer, like a stack of layers. book of. The sand layer has a large pore, just like a sponge. When it rains, the rainwater penetrates into the ground and is stored in the sand layer. This layer of sand is called an aquifer. When the well is hit in the aquifer, water is taken from the well and the water in the surrounding sand layer flows into the well. The water in the well can be recovered quickly within the aquifer's tolerance. However, if we hit a lot of wells and then take water on a large scale, the groundwater surface will gradually decline. In the North China Plain, due to large-scale mining, the groundwater level has changed from a few meters deep to a few tens of meters or even hundreds of meters deep.

How is groundwater contaminated?

The soil layer has a filtering function. When rainwater and river water infiltrate into the ground, the litter, animal carcasses, feces and those greeny plankton in the water will be filtered and left on the ground or shallow soil. However, there are also harmful substances in the water that we can't see. For example, the nitrate in the fertilizer has a strong water solubility. If the fertilizer is too much, the crops cannot be fully utilized, and some of the fertilizer will infiltrate into the ground with the irrigation water. This is also the shallow groundwater nitrate in many places in China. The main reason for drinking.

In addition to nitrate in fertilizers, groundwater has many sources of pollution. Since the reform and opening up, our cities have expanded rapidly and the amount of garbage generated has increased. In order to accommodate these garbage, many landfills have been built in our country. Although the landfill is covered with anti-seepage layer and drain pipe at the bottom to collect the landfill leachate for centralized treatment, the large-scale landfill anti-seepage layer will age and break, and the garbage with high concentration of pollutants will seep. The filtrate penetrates into the ground from these breakages, causing serious groundwater pollution.

Another source of pollution is a large chemical plant, a mechanical processing plant, and the like. The factory uses solvents such as chloroform and carbon tetrachloride which are highly toxic, as well as chemical raw materials such as benzene and chlorobenzene. These organic liquids will inevitably penetrate into the ground by running a drip during use. They are mostly insoluble in water, and some are relatively light. They infiltrate into the ground and float on the surface of the water. They are called "non-aqueous phase liquids (LNAPL) that are lighter than water." Some are heavier than water and sink below the surface of the water. They are called "non-aqueous phase liquids (DNAPL) that are heavier than water." There is a layer of clay underneath the aquifer, and it is difficult for oil and water to penetrate again. It is called "water barrier". LNAPL and DNAPL are highly toxic, and even a small amount of dissolved into groundwater can cause serious pollution, resulting in excessive groundwater.

Metal smelter is one of the sources of pollution. Metal smelters use a variety of chemical raw materials to dissolve the metal ore, and then extract the dissolved metal, which will discharge waste water and waste residue containing high concentrations of heavy metals. The water oozing from these waste water and waste slag will also infiltrate the contaminated groundwater. Heavy metals such as chromium and arsenic are not easily adsorbed by the soil layer and will infiltrate into groundwater. Both of these heavy metals are very toxic, for example, arsenic is the main component of arsenic.

Another is oil and gas extraction. Oil and gas wells are deep and isolated from groundwater. In the later stage of oil exploration, it is necessary to fill the oil field to drive the oil out. Natural gas extraction, especially shale gas mining, requires hydraulic fracturing, that is, high-pressure water injection to fracture the formation so that natural gas can emerge. These high-pressure injected waters often contain some chemicals, and if the fracturing is not good, they may rise upwards and pollute the shallow groundwater for human consumption.

The use of oil can also cause groundwater pollution. Normally, fuel, oil, hazardous chemicals and solvents, and chemical waste are stored in underground storage tanks. These oil storages may leak into the groundwater due to corrosion of internal or external metals [k1].

All wastes generated by human activities, such as waste liquids, waste gas, and domestic garbage generated by people, may slowly percolate into the ground along with rainfall, thereby polluting the aquifer and threatening people's water safety.

How to repair contaminated groundwater?

There are many kinds of groundwater pollution remediation technologies. The commonly used ones can be divided into physical treatment technology, chemical treatment technology and biological treatment technology. To repair contaminated groundwater, first understand the nature and distribution of underground pollutants, and then select the appropriate measures.

If the subsurface contaminants are large, concentrated, water-insoluble liquids (LNAPL and DNAPL), they can be welld or trenched to extract or extract them, and then concentrated on the surface to remove contaminated bodies. However, this simple physical treatment can only remove about 50% of the pollutants, and other residual pollutants become long-term sources of pollution.

There is a physical treatment technology for soluble groundwater pollutants, that is, the polluted groundwater is pumped out and treated in the sewage treatment plant, and the uncontaminated water in the underground will flow over and replenish; however, the pollutants adsorbed on the aquifer cannot All are pumped away with groundwater, continuing to release harmful substances and contaminating new sources of water. This treatment method is costly and inefficient at the later stage. Because the concentration of pollutants in the soil is reduced, a large amount of water needs to be extracted, and the pollutants that can be extracted are less and less. If the pumping is stopped, the concentration of pollutants will rebound.

Soil gas phase extraction (in situ soil ventilation) is a physical repair method for volatile and semi-volatile organic pollutants. Volatile and semi-volatile organic compounds volatilize into gas in the ground and diffuse in the soil. Soil gas phase extraction is the use of a vacuum pump to extract these "dirty" gases. During the extraction process, the organic pollutants adsorbed in the soil will continue to volatilize, and the soil will be slowly cleaned after continuous pumping. Sometimes in order to enhance the effect, some gas injection wells are added, and when the air is pumped, the air is blown to the ground, so that the dirty things can be blown out more quickly to achieve the purpose of purifying the soil. The technical equipment is simple and easy to operate; the damage to the on-site environment is small, and the surface construction is not affected; the repair time is short, the cost is low; and it is easy to be combined with other repair technologies. However, it is difficult to reduce the concentration of pollutants by more than 90%; the effect on soils with low permeability (low permeability soil) and uneven soil is uncertain; the extracted gas needs subsequent treatment; it is only suitable for soil in unsaturated areas.

Chemical treatment techniques include chemical oxidation and reduction, permeable reaction walls, and the like. Chemical oxidation and reduction are substances that convert pollutants into non-toxic and harmless substances through redox reactions. This treatment requires the addition of an oxidizing agent or a reducing agent to the soil to ensure that the added compound is non-toxic and harmless, and is environmentally friendly during the reaction.

The permeable reaction wall is a man-made “wall” that is permeable to water on the road of underground pollutants. The reaction medium filled with zero-valent iron , zeolite and carbon source for enhancing microbial activity. When groundwater flows through the reaction wall, the pollutants may biologically or chemically react, or be trapped in the wall, or be converted into non-toxic and harmless substances. This technique requires a good understanding of the direction of the groundwater flow and the thickness of the aquifer in order to select the correct location and create the right size of the wall. In addition, it is important to understand the nature of the contaminants and fill the walls with appropriate reactants. The following figure is a schematic diagram of the design of two permeable reactive walls. Figure a is a linear infiltration reaction wall that allows the pollution plume (contaminated groundwater) to flow through; Figure b is designed with impervious retaining walls on both sides to Gather together to ensure that contaminated groundwater flows through the intermediate permeable reaction wall.

Biological treatment technologies include biological aeration, monitoring natural degradation, and the like.

In addition to soil, water and various compounds, microorganisms are present in the soil. Some microbes can “eat” organic pollutants, but they require certain living conditions. The biological aeration method is to inject air (or oxygen) into the ground to greatly increase the activity of the microorganisms, thereby eliminating organic pollutants. This treatment technique requires an understanding of the "food intake" of the microorganisms, and the flow rate of the injected air is determined according to the requirements of microbial degradation. This method is most effective in treating petroleum product leakage and is not suitable for biological pollutants that are difficult to degrade.

Another way is to monitor natural degradation. Under favorable conditions, monitoring natural degradation is the most economical method with minimal disturbance to the site. As mentioned above, some microorganisms can react with pollutants to produce non-toxic and harmless substances, and the conditions required for the microorganisms to survive are some inorganic substances in nature, such as dissolved oxygen, nitrates, sulfates, and the like. What we need to do is to monitor the changes in inorganic matter in the contaminated site. When the conditions are not suitable for microbial survival, we should put some inorganic substances in time to improve the living conditions of the microorganisms and ensure that they can “eat” the pollutants continuously.

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