Investigation of Temporal and Spatial Variations of Groundwater Quality in Sefiddasht Plain and Suitability Evaluation for Agricultural Use

Groundwater resources are valuable resources to supply water for various uses in all areas, especially arid and semi-arid areas. Due to the limited resources and increasing population and the growth of agriculture and industry, the optimal use of groundwater needs more attention. In this research, the temporal and spatial variation of groundwater quality of Sefiddasht plain located in Chaharmahal and Bakhtiari province and its capability for agricultural purposes have been investigated. For this purpose, the time series of qualitative variables collected in 9 sampling stations (including 6 wells, 2 qanats and a spring) in the Plain during a 29-year period (1991-2018) was used. The trend of changes in groundwater quality variables of this plain was investigated using the modified Mann-Kendall test (after complete elimination of the autocorrelation effect). The trend line slope was also calculated for each time series using the Sen slope estimator. The results showed that groundwater quality has decreased in recent years in some places, especially in the southern parts of Sefiddasht. In order to draw zoning maps of qualitative variables at the beginning and end of the studied period, the accuracy of various geostatistical methods, including inverse distance weighting (IDW), Kriging, Co-Kriging, etc. for Sefiddasht plain was evaluated and the IDW method was selected for this plain. Comparison of zoning maps of qualitative variables at the beginning and end of the studied period showed that the groundwater quality of this plain has decreased during the considered 29 years. Finally, the usability of groundwater in Sefiddasht plain for agricultural use was evaluated using the USSL diagram for the beginning and end of the studied period. The results indicated a decrease in the quality of groundwater for agricultural use in this plain during the studied period.