An injection well is a vertical pipe in the ground into which water, other liquids, or gases are pumped or allowed to flow.
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Injection wells are used for many purposes.
One application is waste water disposal, in which treated waste water is injected into the ground between impermeable layers of rocks to avoid polluting fresh water supplies or adversely affecting quality of receiving waters. Injection wells are usually constructed of solid walled pipe to a deep elevation in order to prevent injectate from mixing with the surrounding environment.[1]
Injection wells are widely considered to be the best method for disposal of treated waste water. Unlike outfalls or other direct disposal techniques, injection wells utilize the earth as a filter to further clean the treated wastewater before it reaches the receiving water. This method of waste water disposal also serves to spread the injectate over a wide area, further decreasing environmental impacts.
Critics of waste water injection wells cite concerns relating to the injectate polluting receiving waters. Most environmental engineering professionals, however, consider waste water treatment followed by disposal through injection wells to be the most cost effective and environmentally responsible method of waste water treatment. The only known alternatives to injection wells are direct discharge of treated waste water to receiving waters or utilization of the treated water for irrigation. Direct discharge does not disperse the water over a wide area; the environmental impact is focused on a particular segment of a river and its downstream reaches, or on a coastal waterbody. Extensive irrigation is often prohibitively expensive and requires ongoing maintenance and large electricity usage.
Another use of injection wells is in petroleum production. Steam, carbon dioxide, water, and other substances can be injected into an oil-producing unit in order to maintain reservoir pressure, heat the oil or lower its viscosity, allowing it to flow to a producing well nearby.[2] See also Enhanced oil recovery.
Yet another use for injection wells is in environmental remediation, for cleanup of either soil or groundwater contamination. Injection wells can insert clean water into an aquifer, thereby changing the direction and speed of groundwater flow, perhaps towards extraction wells downgradient, which could then more speedily and efficiently remove the contaminated groundwater. Injection wells can also be used in cleanup of soil contamination, for example by use of an ozonation system. Complex hydrocarbons and other contaminants trapped in soil and otherwise inaccessible can be broken down by ozone, a highly reactive gas, often with greater cost-effectiveness than could be had by digging out the affected area. Such systems are particularly useful in built-up urban environments where digging may be impractical due to overlying buildings.[3]
Recently the option of refilling natural aquifers with injection or percolation has become more important, particularly in the driest region of the world, the MENA region.[4]
Surface runoff can also be recharged into dry wells, or simply barren wells that have been modified to functions as cisterns.[5] These hybrid stormwater management systems called recharge wells have the advantage of aquifer recharge and instantaneous supply of potable water at the same time. They can utilize existing infrastructure and require very little effort for the modification and operation. The activation can be as simple as inserting a polymer cover (foil) into the well shaft. Vertical pipes for conduction of the overflow to the bottom can enhance performance. The area around the well acts as funnel. If this area is maintained well the water will require little purification before it enters the cistern.[6]
In the United States, injection well activity is regulated by the United States Environmental Protection Agency (EPA) and state governments under the Safe Drinking Water Act.[1] EPA has issued Underground Injection Control (UIC) regulations in order to protect drinking water sources.[7][8]