Ordering Information:
ORDER NUMBER: 90689
DATE AVAILABLE: Summer 1996

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Prepared by R. David G. Pyne, CH2M Hill and Philip C. Singer and Cass T. Miller
University of North Carolina

Aquifer storage recovery (ASR) is a water management technology in which water is stored underground in a suitable aquifer through a well during times when the water is available and recovered from the same well when needed. As of 1994, 23 ASR systems were operational in the United States, utilizing consolidated and unconsolidated, confined and unconfined aquifers containing fresh water, brackish water, and salt water. Production rates for these ASR systems range from 0.5 to 100 mgd (2 to 378 ML/d). Disinfection by-product (DBP) concentrations at several of these sites have been observed to decrease during ASR storage. This project was conducted to evaluate, under more controlled conditions, whether DBP reduction occurs and to assess the probable causes. Additional studies were conducted to evaluate well clogging relationships and to develop a hydrogeologic computer model for predicting the fate of conservative water quality constituents and also DBPs during ASR storage.

Sampling was conducted at five operational ASR sites, four in the United States and one in England. At each site, the testing program was designed to minimize the opportunity for mixing and dilution between the stored water and the surrounding aquifer water during the storage period. The degree of mixing that occurred was checked through calculations based on concentrations of conservative tracer constituents.

Results showed that trihalomethanes (THMs) and haloacetic acids (HAAs) are removed from chlorinated drinking water during ASR storage. HAA removal precedes THM removal, with the more highly brominated species in both classes being eliminated earliest. Mixing and dilution between recharge water and surrounding groundwater contribute to some of the DBP removal, more so at some sites than at others; however, mixing does not fully account for the observed DBP removal at all sites. A biological mechanism is indicated, with THM removal tending to occur after the onset of anoxic conditions in the aquifer and HAA elimination occurring before this time under aerobic conditions. Additional work will be required to establish the mechanisms responsible for removing DBPs and the conditions under which they occur, including investigations at additional sites. The results suggest that some reduction in DBP precursors may also occur, as indicated by declines in total organic carbon (TOC) concentration and ultraviolet absorbance and by reduction in HAA formation potential at most sites.

Previous studies by the Las Vegas Valley Water District indicated that THM reduction is attributable to mixing and dilution. The results of the present study suggest that other factors may also affect THM and HAA reduction at some sites.

Sampling and data collection were conducted at nine recharge and ASR wells to characterize relationships among clogging rate, hydraulic conductivity, and total suspended solids (TSS) concentration in the recharge water. Although other factors may contribute to well plugging at some sites, particulate plugging is consistently an important factor. The indicated relationships among these factors are based on limited data, but they follow intuitive reasoning and provide a preliminary basis for estimating clogging rates and redevelopment frequencies. In turn, this facilitates improved design and operation of ASR wells and associated pretreatment facilities.

Criteria are presented for assessing ASR applicability at potential new sites. Included are 22 possible applications to meet a variety of primary and secondary objectives of ASR systems. ASR is generally feasible across the United States, except in a few areas where hydrogeologic conditions are unsuitable or where insufficient variability exists in water supply, quality, or demand. The principal driving force for implementation of ASR technology is economics. Unit costs for ASR facilities generally range from about $200,000 to $600,000/mgd ($53,000 to $159,000/ML/d) of recovery capacity, with an overall average of about $400,000/mgd ($106,000/ML/d). This cost is usually less than half the cost of other water supply alternatives. Although operating costs are less well defined, available data suggest that annual operating costs are typically about $15,000/mgd ($4,000/ML/d) of recovery capacity. Environmental support for ASR solutions has been consistently strong. Several federal and state regulatory issues pertaining to ASR systems are discussed.

Suggestions for further research to advance ASR technology are presented, including the direction for additional work that would achieve improved understanding of DBP reduction mechanisms.