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<p>The simulation of seawater intrusion (SWI) in coastal aquifers under complex hydrogeological conditions typically requires using "variable-density" models, which simulate the groundwater flow and the transport of salt dissolved in water. When combined with optimization algorithms, variable-density models constitute powerful tools to support the management of groundwater resources in coastal systems vulnerable to SWI, sea-level rise and unstainable groundwater abstraction. However, the application of simulation-optimization (SO) to SWI problems has so far been limited by the prohibitive computational effort required by full-scale variable-density models that simulate the aquifer response to proposed groundwater abstraction strategies. A viable solution is thus to develop “surrogate” models that emulate full-scale model responses at a fraction of their computational cost. In this study, a surrogate model of SEAWAT, a popular variable-density groundwater flow model, will be presented. This surrogate is based on the proper orthogonal decomposition (POD) method, which is a projection-based approach where the coefficient matrices and the right-hand side vectors derived through finite-difference discretization of the coupled flow and transport equations, are mapped onto a space of size significantly smaller than the model grid. Preliminary results show that the POD-based surrogate model is remarkably faster than the full-scale model, and provides results of comparable, and thus acceptable, accuracy. These features make the surrogate ideally suited for substituting the full-scale variable-density model within the SO framework adopted to support the management of coastal aquifers.</p>

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Water quality management of rivers is one of the challenges in the analysis of water resource systems. The optimal operation of the pollutant carrying capacity of these systems provides significant economic value and could reduce treatment costs. In this study, the application of the trading ratio system is investigated to control the cost of pollutants in a river and make a fair deal. In this regard, transfer coefficients between pollution sources, along with the trade coefficients, are determined, considering the system limitations and each pollutant’s contaminant impact. To provide allowable limits of river water quality concentrations, the total cost of all sources and the system is minimized, using the linear programming method. Finally, the new trading discharge permits are calculated for each source. The proposed method is successfully applied to Dez River as a case study. Results show that using a trading ratio system could maintain water quality at a standard level containing economic benefits for the participants of this program.