Development of a MOORA multi-criteria decision-making framework to reduce chloride concentrations in urban aquifers using MT3DMS groundwater transport modelling

Numerical simulation of groundwater flow provides an essential basis for aquifer management because it enables hydraulic conditions to be quantified and management alternatives to be evaluated before implementation. In this study, a coupled MODFLOW-2000–MT3DMS framework was developed for the urban sector of the Tehran aquifer to simulate groundwater flow and chloride transport and to support the design and comparison of mitigation strategies. The three-dimensional groundwater-flow model was constructed from available hydrological and hydrogeological data and calibrated under both steady-state and transient conditions. During steady-state calibration, hydraulic conductivity and distributed recharge were adjusted, whereas specific yield was refined during transient calibration. The calibrated hydraulic conductivity ranged from 1 to 30 m/day, and calibrated specific yield varied from 0.01 to 0.22. Model performance, assessed using the root mean square error of hydraulic head, yielded RMSE values of 0.786 m for the steady-state simulation, 1.331 m for the transient calibration period, and 1.774 m for the 18-month validation period, indicating acceptable performance for regional-scale screening despite greater uncertainty in strongly stressed urban zones. The flow model was subsequently coupled with MT3DMS to simulate chloride transport under the assumptions of constant-density flow and conservative chloride behavior. Chloride observations from 18 sampling locations were used to define and calibrate the transport model. Both simulated and observed data showed that chloride concentrations were generally highest in the western and southern sectors of the aquifer and lower in other parts of the study area. Four management scenarios were then formulated to reduce chloride concentrations and were evaluated within a multi-criteria decision-making framework. Among the tested options, the combined scenario consisting of a 15% reduction in groundwater abstraction together with the closure of selected production wells performed best overall. Under the calibrated but not independently validated transport model, this scenario yielded the largest simulated local chloride reduction, with a maximum decrease of 70.21 mg/L at a key observation location. The results indicate that an integrated simulation–decision framework can identify groundwater-management strategies that are not only hydrogeologically effective in reducing chloride concentrations, but also more feasible when environmental, social, and economic trade-offs are considered simultaneously.