ABSTRACT
Coastal aquifers serve as major sources for freshwater supply in many countries around the world, especially in arid and semiarid zones. Many coastal areas are heavily urbanized, a fact that makes the need for freshwater even more acute [Bear and Cheng, 1999]. Inappropriate management of coastal aquifers may lead to the intrusion of saltwater into freshwater wells, destroying them as sources of freshwater supply. One of the goals of coastal aquifer management is to maximize freshwater extraction without causing the invasion of saltwater into the wells. A number of management questions can be asked in such considerations. For existing wells, how should the pumping rate be apportioned and regulated so as to achieve the maximum total extraction? For new wells, where should they be located and how much can they pump? How can recharge wells and canals be used to protect pumping wells, and where should they be placed? If recycled water is used in the injection, how can we maximize the recovery percentage? These and other questions may be answered using the mathematical tool of optimization. Efforts to improve the management of groundwater systems by computer simulation and optimization techniques began in the early 1970s [Young and Bredehoe, 1972; Aguado and Remson, 1974]. Since that time, a large number of groundwater management models have been successfully applied; see for example Gorelick [1983], Willis and Yeh [1987], and many other papers published in the Journal of Water Resources Planning and Management, ASCE, and the Water Resources Research. Applications of these models to aquifer situations with the explicit threat of saltwater intrusion in mind, however, are relatively few [Cumming, 1971; Cummings and McFarland, 1974; Shamir et al., 1984; Willis and Finney, 1988; Finney et al., 1992; Hallaji and Yazicigil, 1996; Emch and Yeh, 1998; Nishikawa,
1998; Das and Datta, 1999a, 1999b; Cheng et al., 2000]. In terms of management objectives, some of these studies have addressed relatively complex settings such as mixed use of surface and subsurface water in terms of quantity and quality, water conveyance, distribution network, construction and utility costs, etc. However, saltwater intrusion into wells has been dealt with in simpler and indirect approaches, for example, by constraining drawdown or water quality at a number of control points, or by minimizing the overall intruded saltwater volume in the entire aquifer. The explicit modeling of saltwater encroachment into individual wells resulting in the removal of invaded wells from service is found only in Cheng et al. [2000]. This chapter reviews some of the earlier considerations of pumping optimization in saltwater-intruded aquifers under deterministic conditions, and furthermore, introduces the uncertainty factor into the management problem. The resultant methodology is applied to the case study of the City of Miami Beach in the northeast Spain.
