ABSTRACT
The initial testing of ASR systems began in the United States over 65 years ago, and the implementation of the technology and other managed aquifer recharge techniques has progressively increased because of their compelling water management benefits. The historic ASR experiences in the United States provide valuable insights that can be used for more successful global implementation of the technology. The most important lesson is that ASR system performance is highly dependent upon site-specific hydrogeological conditions, which may not be locally favorable. Analyses of the hydrogeology of both successful and unsuccessful ASR systems have provided much useful information on the hydrogeologic controls of ASR system performance. Detailed aquifer characterization can identify early in a project adverse conditions that may result in poor recovery efficiencies, such as the presence of dual-porosity conditions. The current state of the art involves the use of advanced borehole geophysical logs and more conventional aquifer testing data, which are up-scaled and incorporated into solute-transport models using workflow software.
ASR systems that store freshwater in overdrafted aquifers (i.e., groundwater banking systems) require accurate evaluation of the water budgets of the groundwater basins in order to determine how much of the stored water is indeed recoverable. In systems involving multiple users, a water accounting system must be devised that balances the "credits" accumulated from recharge with the capacity of the aquifer to provide additional water during times of increased demands. More detailed impact analyses are needed to evaluate the impacts of temporal and spatial changes in groundwater recharge and withdrawals resulting from system operation.
The injection of fresh water into an aquifer containing water with distinctly different composition can result in a variety of chemical, physical, and biological processes, which may be either favorable or unfavorable. Adverse fluid-rock interactions can result in the precipitation or alteration of minerals and clay swelling and dispersion, which can result in a decrease in permeability and well and aquifer clogging. The introduction of oxic water into anoxic aquifers that contain sulfide minerals has resulted in the leaching of arsenic and metals into stored water in some systems, which exceeded applicable water quality standards. The potential of arsenic leaching can be evaluated through mineralogical evaluation of storage zone strata, water chemistry analyses, geochemical modeling, and bench top testing. Field testing is underway at several sites on pretreatment options to prevent arsenic leaching by removing dissolved oxygen from injected water. Beneficial aquifer processes include the natural attenuation of contaminant concentrations, particularly the inactivation of pathogens.
A key operational and maintenance issue remains managing well clogging, which may be an issue even in wells used to inject water of potable quality. Periodic well rehabilitation is a normal element in the operation and maintenance of ASR wells, and other injection wells. A variety of different methods are available to rehabilitate wells, but it is now clear that there is no one universal preferred method. Instead site-specific adaptive management approaches are often taken to determine the well rehabilitation program that most cost effectively maintains long-term well performance.
