Design of the Optimal Groundwater Quality Monitoring Network Using the Aquifer Vulnerability Map

Unplanned operation and excessive extraction of groundwater in coastal areas increases the risk of seawater intrusion; so water quality monitoring and management in these areas are of great importance. In this study, an optimal monitoring network with minimum number of wells in Talesh coastal aquifer is designed according to the aquifer vulnerability map and evaluation of the accuracy of monitoring network. In this regard, the aquifer vulnerability map was prepared using the modified DRASTIC index; and genetic algorithm was used to optimize the monitoring network. Three objectives were used in the optimization model including (1) maximizing the correlation between vulnerability index and EC values, (2) minimizing the number of monitoring wells, and (3) maximizing the Nash-Sutcliffe (NS) efficiency. NS, represented the match between the calculated EC distribution obtained from existing monitoring wells and those wells from the newly generated network. Applying the weighting factor w for economic objective, the three above-mentioned objectives are integrated in a single objective function and different weights of w were evaluated. The results showed that the selection of an optimal solution greatly depends on the weighting coefficient w and the best weight was selected according to the vulnerability index and the accuracy of monitoring network. Due to long-term quantitative and qualitative changes in groundwater, it is better to periodically evaluate and redesign groundwater quality monitoring network so that the network could effectively use in planning and applying methods to improve groundwater quality.