Aquifer Recharge (AR) and Aquifer Storage & Recovery (ASR)

Water: Underground Injection Control

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Aquifer Recharge (AR) and Aquifer Storage & Recovery (ASR)

Artificial aquifer recharge (AR) is the enhancement of natural ground water supplies using man-made conveyances such as infiltration basins or injection wells. Aquifer storage and recovery (ASR) is a specific type of AR practiced with the purpose of both augmenting ground water resources and recovering the water in the future for various uses.

This pages explains general use of AR/ASR and the how the EPA UIC program regulates these injection activities to protect underground sources of drinking water (USDWs).

This page explains general use of AR/ASR and the how the EPA UIC program regulates these injection activities to protect underground sources of drinking water (USDWs).

Background
UIC Regulations for AR and ASR wells
Inventory and Geographic Distribution
Impacts on USDWs
Stakeholder workshop
Links

Background

States, communities and water suppliers are seeking ways to augment natural water sources as climate change and development pressures increase demand on these finite supplies. One approach to augmentation is storage of excess water in an aquifer using AR or a specific type of AR well called an ASR well. Several methods of introducing water into an aquifer exist, and injection via wells is regulated by the EPA Underground Injection Control (UIC) Program.

Many water suppliers and states are eager to employ ASR well technology to meet current and future water demands by storing water during wet periods or periods of low demand, and recovering it during dry periods or times of high demand. ASR projects are increasing in number throughout the US, especially in areas with potential water supply shortages.

AR and ASR wells are found in areas of the U.S. that have high population density and proximity to intensive agriculture; dependence and increasing demand on ground water for drinking water and agriculture; and/or limited ground or surface water availability. AR wells, for example, have been utilized to deter salt water intrusion into freshwater aquifers and to control land subsidence. While an AR well is used only to replenish the water in an aquifer, ASR wells are used to achieve two objectives: (1) storing water in the ground; and (2) recovering the stored water either using the same well or by pairing injection wells with recovery wells located on the same wellfield. ASR wells have been used to store and recover water for drinking water supplies, irrigation, and more recently, ecosystem restoration projects such as the Comprehensive Everglades Restoration Project. Although the process of ASR includes production of the injected water, the UIC program does not regulate recovery activities.

Conventional methods of AR include surface spreading, infiltration pits and basins in addition to injection wells. Injection wells are the selected method of artificial recharge in areas where the existence of impermeable strata between the surface and the aquifer makes recharge by surface infiltration impractical or in areas where land for surface spreading is limited.

Well construction ranges widely and is directly related to the intent of operations. Some AR wells may be little more than a deep pit into the unsaturated (vadose) zone above the USDW that allows the water to percolate through the soil before reaching the USDW. Other wells frequently inject water under pressure directly into or close to the top of the USDW and are constructed of multiple layers of casing and tubing to withstand pressurized injection and prevent leakages of injection fluid into zones above the USDW.

UIC Regulations for AR and ASR wells

ASR wells are regulated as Class V injection wells. As such, ASR well owners and operators are required to submit basic inventory information to the primacy enforcement agency. EPA may directly implement a program, or a state may have primary enforcement authority, or "primacy". If the owner or operator submits the inventory information and operates the well in a manner that does not endanger a USDW, the well is typically authorized by rule. However, a primacy state or EPA, in the case of a state without primacy, may require a permit for a Class V well.

Additional regulations adopted by primacy states for ASR wells vary. As of 2007, nine states require that water used for ASR injection be potable water or drinking water treated to national or state Drinking Water Standards or state ground water standards. Potable water is defined differently in each state but generally refers to water that is high quality and poses no immediate or long term health risk when consumed. Some primacy states allow additional types of water to be used in ASR, including treated effluent, untreated surface and ground water, reclaimed water subject to state recycled water criteria, or “any” injectate. However, state-specifc ASR regulations do not supersede the prohibition of movement of fluid into a USDW. Specifically, EPA regulations provide that “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 theat contaminant may cause a violation of any primary drinking water regulation under 40 CFR part 142 or may other wise adversely affect the health of persons.” (40 CFR 144.12).

Inventory and Geographic Distribution

The use of ASR in the United States varies by region at this time. Many Northeastern and Midwestern states have not used ASR widely, most likely due to the relative abundance of available drinking water. However, in other regions such as the Southeast, Southwest and Western states, ASR has become a more prevalent tool for providing a reliable supply of water throughout the year or treatment cycle.

Approximately 1200 AR and ASR wells are operating or capable of operation (operable but not in operation for some reason) in the United States as of February 2009 (Table 1; Figure 1). Only EPA Region 1 does not have ASR wells. Most of the wells are located in coastal areas in EPA Regions 4, 9, and 10, with the majority of AR wells located in EPA Region 9. The number of ASR wells has more than quadrupled since the Class V Underground Injection Control Study was published in 1999.

Figure 1. Geographic distribution of aquifer recharge and aquifer storage & recovery wells in the US by EPA Region (last updated February 2009).

Table 1. Operating status of aquifer recharge and aquifer storage & recovery wells in the US as of February 2009.

**Well Operating Status**
Well Type	Capable of Operation	Non-functional	Plugged & Abandoned	Project Sites Containing Wells
ASR	542	14	65	307
Other AR	661	0	375	441
Total	1203	14	440	748

Impacts on USDWs

Water injected into AR and ASR wells ranges from potable drinking water treated at a public water system to untreated ground water and surface water and even recycled water. Water injected into ASR wells typically meets or is treated to meet primary drinking water standards as required by several state regulatory agencies to prevent degradation of the ambient ground water quality.

Depending on the type and quality of injectate and/or the geology, the potential for endangering a USDW may increase.

Pathogens may be introduced into an aquifer if injectate is not disinfected. In states which allow injection of raw water and treated effluent under state regulations, the fate of microbes and viruses in the aquifer after injection becomes particularly relevant. The growth of microorganisms within the aquifer could cause decreased water recovery efficiency by clogging the wellscreen or risks to public health from contamination of the aquifer.
If water is disinfected prior to injection, the possibility of disinfection by-products (DBPs) forming in situ increases. If soluble organic carbon is not removed from the injectate before disinfection, a chlorinated disinfectant may react with the carbon to form compounds such as trihalomethanes and haloacetic acids.
Chemical differences between the injectate and receiving aquifer may be different enough to create problems within the recharged aquifer. If the reduction-oxidation (redox) potential of the injectate varies enough from the receiving aquifer, leaching of arsenic and radionuclides may occur if they are present in the geologic matrix, increasing public health risk. Carbonate precipitation within the aquifer may occur if the pH of the injectate is not sufficiently acidic and may cause clogging of the wellscreen.
Injected water has been known to cause the dissolution of metals such as arsenic, manganese, and iron from the surrounding geologic formation. At the time the Class V Underground Injection Control Study was published in 1999, there were no reported cases of contamination of underground sources of drinking water by ASR wells although changes in water quality of the aquifer after recharge have been noted. Recharge into brackish aquifers or aquifers with poor quality water has, in some cases, improved the ambient water quality of the aquifer. Since the 1999 report was published, however, EPA revised the drinking water standard for arsenic (lowering the threshold for an exceedance to 0.01 mg or 10ug arsenic/L) and some ASR test wells and operations have had concentrations of arsenic exceeding the standard. In addition, some ASR test wells and operations had manganese and iron concentrations exceeding National Secondary Drinking Water Regulations in recovered water. While the presence of disinfection by-products has occurred in USDWs due to ASR activities, EPA is not aware of exceedances of applicable primary drinking water standards as of 2007.