Abstract
Aquifer storage and recovery (ASR) is increasingly being used for water resource management. ASR systems fundamentally differ from purely extractive water production wells and from injection wells, in that in most systems, a specific volume of water needs to be stored and recovered. Where injected water enters a storage zone and its movement and mixing during storage are thus critical variables for system performance. Water movement in the storage zone is controlled to a large degree by heterogeneities and anisotropies in hydraulic conductivity, which have been seldom incorporated into ASR models. ASR system performance was studied by using a generalized three-dimensional model of an ASR system that was designed to simulate the effects of the degree of anisotropy and heterogeneity found in natural aquifers, and other variables, on ASR system recovery efficiencies. Simulations were performed using the SEAWAT code, a three-dimensional variable-density groundwater flow program. The most important variable among those tested in controlling recovery efficiency in systems using brackish-water storage zones is native water salinity. Low salinities allow for more mixing of stored and native waters before a target water quality standard is exceeded and buoyancy-driven fluid migration is minimized. Heterogeneity within storage zones decreases recovery efficiency in that it results in an interfingering of layers containing stored freshwater and native formation waters. Zones with high hydraulic conductivities also experience rapid migration of stored water under natural and pumping-induced hydraulic gradients.