The Performance of Several Current Interpolation Methods for Variability of cations in Groundwater in Esfarayen Plain, Iran: A Case Study

Acquiring information about groundwater quality is essential in developing management strategies. In this article, spatio-temporal variations of cations in groundwater in Esfarayen plain were investigated using data monitored in ۱۳۴ groundwater wells, active in ۱۹۸۸, and ۴۷ wells, active in ۲۰۱۹. To evaluate groundwater quality, interpolation methods have been used to interpolate existing limited spatial data. The performance of ۸ current interpolation methods on the data for the two selected years (۱۹۸۸ and ۲۰۱۹) was compared. Finding the optimum interpolation method for the considered groundwater quality parameters is essential. Cross-validation and three indexes of R۲, mean absolute error (MAE), and root mean square error (RMSE) were used to compare the performance of the methods. By identifying universal kriging (UK) and global polynomial interpolation (GPI) methods as the optimum methods and using those for the selected years (۱۹۸۸ and ۲۰۱۹), spatial variation of the concentration of cations in groundwater across the plain has been presented. In ۱۹۸۸, the maximum concentration of the cations occurred in the southwest of the plain (about ۸۰ mg/L), and the minimum concentration of the cations was observed in the northwest of the plain (approximately ۸ mg/L). Similarly, in ۲۰۱۹, the highest concentration of the cations was found in the southwest of the plain (almost ۶۴ mg/L), and its lowest concentration was observed in the northeast of the plain (roughly ۱۳ mg/L). Moreover, temporal variations of the concentration of cations in groundwater from ۱۹۸۸ to ۲۰۱۹ have also been presented. The concentration of the cations increased by approximately ۲۳ mg/L in the northwest and decreased to about ۳۷ mg/L in the southwest of the study area from ۱۹۸۸ through ۲۰۱۹. According to the results, changes in the quality of groundwater are a complex problem and it is necessary to adopt proper strategies to reduce its adverse effects.