Chemical Characteristics and Sources of Groundwater Sulfate in the Kashgar Delta, Xinjiang

Abstract

Poor water quality is one of the main reasons for a shortage of water resources in the Kashi Prefecture of Xinjiang. The area of the Kashgar Delta is located in the western Kashi Prefecture and covers an area of 13329 km2. Sulfate concentrations exceed the groundwater quality class Ⅲ standard in 73.2% of the unconfined groundwater area and 53.2% of the confined groundwater area. In addition, the TDS content and the hardness of the water are high. The chemical characteristics and sources of groundwater sulfate were analyzed using the isotope method of δD, δ18O-H2O, and δ34S-SO42-. The results showed that:① The dissolution of salt minerals such as calcareous siltstone, calcareous sandstone, and gypsum in denuded mountainous areas controlled the hydrochemical compositions in the basin, and formed surface water and groundwater dominated by SO4. The evolution trend of groundwater chemical types was HCO3·SO4→SO4→SO4·Cl. Alluvial-proluvial gravel plains on the piedmont slopes were the main dissolution-runoff zones and the fluvial plain was main runoff accumulation zone. The chemical composition of the groundwater tended towards salinization, from upstream to downstream; ② The groundwater was mainly derived from atmospheric precipitation, and was affected by evaporation. The distribution of δD and δ18O in the groundwaters of different hydrogeological units was clear; from upstream to downstream, isotope values varied from low to increasingly higher, and evaporation varied from weak to strong, respectively. The distribution of isotopes in the confined groundwater in the river alluvium plain was relatively discrete, which was affected by the mixing effect of unconfined groundwater; ③ The sources of sulfate in the unconfined groundwater in the southern and northern alluvial-proluvial gravel plains on the piedmont slopes were the dissolution of marine-terrestrial and terrestrial deposit evaporates, respectively. The sources of sulfate in the unconfined groundwater in the river alluvium plain were the dissolution of terrestrial deposit evaporates and pollution from fertilizer leaching. The confined groundwater was affected by the dissolution of evaporates and the mixing effect of the unconfined groundwater and bacterial sulfate reduction.