ASR, Useful Storage, and the Myth of Residual Pressure

Abstract

Aquifer storage and recovery (ASR) is the underground storage of water using wells during periods of excess supply for later use during periods when demand exceeds supply. The advantages of ASR as a water resource management tool are compelling because it can provide very large volumes of storage at a small fraction of the cost of other storage options, such as tanks and surface reservoirs. However, ASR is not a panacea for water resources management. Not all ASR systems are created equal in their water resources benefits. For an ASR system to be of hydrologic value, it must create a water resource that would not otherwise be available. In other words, the injection of fresh water must result in the useful storage of fresh water. Injection of fresh water into an aquifer does not necessarily create a hydrologic benefit. In ASR systems that store fresh water in unutilized aquifers that contain poor quality (e.g., brackish) water, the injection of fresh water results in useful storage by displacing the native water and emplacing a new fresh water resource that can be exploited in the future. The main design concerns are achieving acceptable recovery efficiencies and avoiding fluid-rock interactions, such as metals leaching, which would render the water stored unusable or require expensive post-treatment. The hydrologic benefits of ASR systems that inject fresh water into fresh water aquifers are less clear-cut. Injection of water into an aquifer locally increases heads, in a manner analogous to the decrease in heads that occur during the pumping of a well. Once injection is terminated, the pressure buildup dissipates, in the same manner and approximate rate as water levels recover when pumping is terminated. There is no residual pressure, and thus, no long-term increase in storage, unless the aquifer is unconfined and sufficiently bounded so that water levels actually rise. If additional physical storage of water has occurred after injection, then aquifer heads must be greater after injection than before. Whether or not aquifer heads have increased at an ASR site over time thus provides a simple criterion for evaluating whether the system is achieving useful storage. Once aquifer pressures return to static (preinjection) levels, any future withdrawals from the aquifer, whether they be from an ASR well or from a purely extractive production well, will result in drawdowns that may contribute to local adverse impacts such as dehydration of wetlands, reduction in spring flow, and saline-water intrusion. The local benefits of injection (as far as increased aquifer heads) are lost because of the absence of a residual pressure increase to counter the drawdowns during future recovery. ASR systems that inject fresh water into regional fresh water aquifers may thus have little, if any, local hydrologic benefit. However, the ASR systems may still have a regional benefit if the storage zone aquifer is in overdraft, such that the long-term withdrawal rate exceeds the rate of replenishment and aquifer water levels are declining. Injection of water into an ASR system would mitigate the effects of ground water withdrawals on the aquifer overdraft. Although an ASR system may not provide significant local hydrologic benefits, it may still be worthwhile to the owner and operator if it confers the right to withdraw additional water when needed (i.e., it results in “regulatory storage”). In an aquifer that is fully allocated, injection of fresh water into an ASR system may be the only means by which a utility would be allowed to withdraw additional water during peak demand periods, under the concept that the operation of the ASR system is neutral within the context of the long-term aquifer water budget. The ASR system may also allow the owner to capture unused surface water rights that would otherwise be lost. Often regulatory issues play a greater role in the implementation of ASR projects than hydrogeologic issues. ASR has been shown to be technically possible in many hydrogeologic settings, but it does not work everywhere and the benefits vary. Emplacing fresh water underground is not enough. The challenge is to design ASR systems that result in actual water resource benefits commensurate with the cost to construct and operate the systems.