Optimization of storage and recovery of seasonal surplus desalinated water in a coastal aquifer

An increase in groundwater storage in aquifers in arid areas improves water security. Most desalination water production around the globe involves the private sector in the form of “build, operate, and transfer” or “build, operate, and own” agreements. Take-or-pay contracts are the most dominant contracts in the desalination industry. The water utility buys a fixed volume of water from the desalination company over a fixed period of 20 to 25 years. The contract between the two parties is established prior to building the plant to help ensure a profitable investment for all stakeholders. This regularly implies a surplus supply of desalinated water during low water demand periods. Given the absolute water scarcity in arid regions, maximizing the banking of surplus water in an aquifer is considered in this paper. For this purpose, a numerical groundwater flow simulation model, called MODFLOW, and a heuristic multiobjective optimizer, namely, NSGA-II, are coupled to optimize the injection and recovery of seasonal excess desalinated seawater in an alluvium coastal aquifer in Oman. Dual wells are considered for injection and abstraction of the water. The optimal daily abstraction and injection rates are determined by defining a multiobjective optimization framework. The four objective functions considered in this study are maximizing the total volume of desalinated water recharged into the aquifer; minimizing the groundwater losses to the sea; minimizing seawater intrusion by minimizing the maximum seasonal mean drawdown; and maximizing the total benefit from the recharge and recovery of the desalinated water. Analysis of the results revealed that we would be able to use 84% of the excess produced desalinated water (i.e., 8.4 of the 10 Mm3/year) that is currently returned to the sea. The net benefit from storage and recovery ranged between $14.77 million/year and $17.80 million/year. The increasing number of desalination plants at the global level calls for an integrated approach to bank the excess desalinated water and to improve the water security of coastal cities in arid and semiarid regions.