干旱区生态环境知识资源中心

Groundwater is the primary source of water for irrigation and domestic purposes in the Yanchi endorheic region, an arid area in northwest China. A comprehensive understanding of the hydrogeochemical characteristics and groundwater quality is indispensable for the sustainable utilization of the groundwater sources. For this reason, twenty-seven groundwater samples were collected in three seasons. A hydrogeochemical assessment and entropy-weighted water quality index (EWQI) were used to investigate the sources of high sodium, sulfate and fluoride concentrations, to evaluate the groundwater quality and to ascertain the factors controlling the groundwater quality, the spatial distribution and the seasonal variation. The dissolution of halite and gypsum supplies most of the Na+ and SO42-, respectively, in the groundwater, and cation exchange further enhances the Na+ content. The main sources of the high F- concentrations are the dissolution of fluorite and desorption of exchangeable F-. This groundwater environment, which is Na-rich and Ca-poor, has high pH values, and the precipitation of calcite and dolomite provides favorable conditions for the enrichment of fluoride. In the three sampling seasons, 48%, 52% and 48% of the groundwater was categorized as poor- or extremely poor-quality water, which is not suitable for drinking purposes, according to the self-defined classifications of EWQI. The high EWQI values are mainly observed in the mountain front and discharge area of the groundwater runoff. The high concentrations of Na+, SO42-, F- , and TDS are the greatest contributing parameters to the low groundwater quality, and the assessed impacts decrease in the following order: Na+ > TDS > SO42- > F-. The spatial distribution of the EWQI versus four hydrochemical parameters reveals that the groundwater quality improves but the influential factors do not change with the seasonal variation.