Modeling the groundwater quality (salinity) variations in Neyshabour plain using MODFLOW and MT3DMS

In this study, MODFLOW and MT3DMS codes were used to evaluate changes in groundwater quality in Neyshabour plain under different scenarios. During developing the conceptual model, it was found that the aquifer salinity increased due to the presence of fine alluvium and evaporite minerals in the margins of the plain and agricultural return flow. Groundwater models was calibrated under transient condition for 10 years from October 2001 to September 2011 and validated for a 4-year period (October 2011 to September 2015). Quantitative comparison of the head and Cl data in all the observation points indicated a reasonable match between the observed and the calculated values. The root mean squared error (RMSE) was 1.92 m and 1.65 m for the calibration and validation periods in the flow model, and the RMSE was 2 mg/l for the mass transport model in Neyshabour aquifer. The MODFLOW and MT3DMS models were used to simulate the effect of the two scenarios: (1) continue the current trend in groundwater withdrawals and (2) 40% reduction in groundwater withdrawals has been examined by the model. In the first scenario, the average groundwater level showed annual loss of 0.79 m and increasing in the groundwater salinity. Under the second scenario, which aimed to achieve equilibrium status in the groundwater balance, pumping rate for all wells were reduced by 40%. The results showed that although the average water level in the aquifer does not decrease, the groundwater salinity is still increasing, with a rate of 50% lower than in comparison to the first scenario. This reflects the fact that reduced groundwater withdrawals at the same rate for all wells may not meet water resource management goals and groundwater management requires distributed information about the aquifer