success are described in Chapter 4.

Factors that can preclude MUS development include low available aquifer storage; low hydraulic conductivity; high probability of clogging during recharge; anticipated loss of recharge water; anticipated degradation of water quality due to physical, chemical, or biological processes, and anticipated changes in patterns of potentiometric gradients that would adversely affect existing water supplies.

The significance of these factors must be considered on a case-by-case basis. Depending on the operational goals of the MUS system, some of these negative factors may be acceptable provided regulatory requirements are met. Addressed briefly in Chapter 6 and not covered here are operational issues that affect MUS viability.

This chapter reviews the status of knowledge on the hydrogeology of recharge, storage and recovery processes as they relate to MUS. The chapter includes discussion of the hydrological properties of the geological formation to be used for storage, the aquifer boundary conditions, recharge and recovery methods to be used, and potential impacts of the MUS system on the groundwater flow and aquifer integrity. In addition, knowledge gaps and research needs related to the hydrogeology of MUS systems are identified.

A requirement for the success of an MUS system is a comprehensive understanding of the hydrogeological properties of the aquifer to be used for storage. An aquifer is a layer, formation, or group of formations of permeable rock or sediment saturated with water and with a degree of permeability that allows water to be withdrawn or injected (Fetter, 2001; Marsily, 1986; Lohman et al., 1972). Sand and gravel layers, sandstone, and carbonate rocks usually form aquifers. This section describes hydraulic and hydrogeologic properties of aquifers, including flow and storage characteristics, and discusses aquifer classification with emphasis on considerations that are important to MUS.

Aquifer classification is generally based on composition, degree of confinement, and geometry at local and regional scales. Each of these is described below.