Spatio-temporal mapping of salinity in the heterogeneous coastal aquifer

Abstract

The interface between freshwater and saltwater in the coastal aquifer is a main factor that is affected by environmental threat and excessive pumping. In this study, a density-dependent numerical model was utilized to simulate the salt distribution of a coastal aquifer in the Talar region near the Caspian Sea, Mazandaran, Iran. To simulate the aquifer salinity and head contour over 2-year stress period, SEAWAT model in groundwater modeling system (GMS) package was used. Moreover, different variogram models based on 21 observation points were tested to find a best fit between observed and simulated total dissolved solids (TDS). A genetic algorithm was coupled with GMS model to identify the optimal sill ratio, range, and nugget value of each variogram. The interface between saltwater and freshwater estimated by the optimal variogram model and compared with the result of sharp interface and variable density flow assumption. The result indicates that the optimal Gaussian variogram as best estimation of spatial TDS can be used for estimation of temporal salinity with a coefficient determination about 0.91. Also, an integration of spherical and exponential variogram shows the high performance for mapping spatial TDS and neutralizes the underestimation behavior of single variogram. A comparison between variogram model and analytical solution indicates that the interface length in the eastern part of aquifer for Gaussian model is relatively small than sharp interface relation. The maximum saltwater penetration based on sharp interface and SEAWAT results is about 1600 m and 1750 m, respectively. Additionally, maximum saltwater intrusion is observed in the western part of aquifer which has a large number of active wells.