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Underground Injection Control (UIC)

Aquifer Recharge and Aquifer Storage and Recovery

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Aquifer recharge (AR) and aquifer storage and recovery (ASR) are manmade processes or natural processes enhanced by humans that convey water underground. The processes replenish ground water stored in aquifers for beneficial purposes. Although AR and ASR are often used interchangeably, they are separate processes with distinct objectives. AR is used solely to replenish water in aquifers. ASR is used to store water, which is later recovered for use.


Background

Projects for AR and ASR are increasing in number nationwide, especially in areas with potential for water shortages. AR and ASR projects are frequently found in areas of the United States that have high population density, proximity to intensive agriculture, dependence and increasing demand on ground water for drinking water and agriculture, and limited ground or surface water availability. Northeastern and midwestern states with relatively abundant water supplies may not have used AR and ASR widely. However, in many southeast, southwest, and western states, AR and ASR are popular options to provide a reliable water supply.

The objective of AR is to replenish water in an aquifer. Injecting water into AR wells can prevent salt water intrusion into freshwater aquifers and control land subsidence. In contrast, ASR wells are used to store water in the ground and recover the stored water for drinking water supplies, irrigation, industrial needs, or ecosystem restoration projects. The stored water may be recovered from the same well used for injection or from nearby injection or recovery wells.

Several methods of introducing water into an aquifer exist including:

  • surface spreading
  • infiltrations pits and basins
  • injection wells

Injection wells are used for AR and ASR in areas where surface infiltration is impractical. EPA’s UIC program regulates AR and ASR injection wells. The UIC program does not regulate the recovery of the stored water.

Construction of injection wells for AR and ASR varies depending upon site-specific conditions and project objectives. Wells may be either deep pits draining into porous layers above a USDW, or use multiple layers of casing and tubing to inject water directly into a USDW.

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UIC regulations for AR and ASR wells

The regulating agency will either authorize the AR or ASR well by rule or by permit. The well is typically authorized by rule if both the owner or operator submits the well information and the well injection does not endanger a USDW. The regulating agency may require an individual permit if additional operating requirements are needed to ensure USDW protection.

Additional regulations adopted by primacy states for AR and ASR wells vary. State-specific AR and ASR regulations do not supersede federal regulations that prohibit USDW endangerment. 

As of 2007, nine states require water used for AR and ASR injection be potable or treated to national or state standards. Potable water is defined differently in each state. Generally, "potable" refers to water of high quality posing no health risk when consumed. 

Primacy states may adopt additional regulations for AR and ASR wells. However, state-specific AR and ASR regulations do not supersede federal regulations that prohibit USDW endangerment. Federal regulations state:

“no owner or operator shall construct, operate, maintain, convert, plug, abandon, or conduct any other injection activity in a manner that allows the movement of fluid containing any contaminant into underground sources of drinking water, if the presence of that contaminant may cause a violation of any primary drinking water regulation under 40 CFR part 142 or may otherwise adversely affect the health of persons.” (40 CFR 144.12L)

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Impact on underground sources of drinking water

Water injected into AR and ASR wells includes:

  • Drinking water from a public water treatment system
  • Untreated ground water and surface water
  • Treated effluent
  • Reclaimed or recycled water

Some states allow additional types of water to be injected for AR and ASR. The water sources are subject to state regulations or state water criteria. 

The type and quality of injected fluid, called "injectate," and the geology affect the potential for endangering a USDW. The following examples illustrate potential concerns.

  • Pathogens may enter aquifers if water is not disinfected prior to injection. Some states allow injection of raw water and treated effluent. In these states, the fate of microbes and viruses in an aquifer is relevant.
  • Disinfection byproducts can form in the aquifer if water is disinfected prior to injection. Soluble organic carbon should be removed from the injectate before disinfection. If not, chlorinated disinfectants may react with the carbon to form contaminating compounds. Contaminants include trihalomethanes and haloacetic acids.
  • Metals and radionuclides may be mobilized from the rock depending on the chemistries of the injected water and the aquifer. Differences in pH and reduction-oxidation potential between the injected water and aquifer may cause arsenic, iron, manganese, or radionuclides that are present in the rock to dissolve into the USDW.
  • Carbonate precipitation in carbonate aquifers can clog wells when the injectate is not sufficiently acidic.

EPA is aware of some ASR operations that have exceeded the National Primary Drinking Water Regulations for arsenic and the National Secondary Drinking Water Regulations for iron and manganese. Additionally, the presence of disinfection byproducts has occurred in USDWs due to ASR activities.

AR and ASR injection can have positive impacts on USDWs.  Recharge into aquifers of poor quality water has, in some cases, improved ambient water quality.


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