Spatial distribution and cause of salinization of shallow groundwater in plain terrain of the Yarkant River Basin, Xinjiang

In order to understand the characteristics and causes of groundwater salinization in the plain area of Yarkant River Basin, the water quality evolution of the shallow groundwater (buried depth ≤100 m) was analyzed by means of multivariate statistics, geostatistics, remote sensing technology and geochemical methods. The groundwater pH value ranged from 6.91 to 8.07, which mainly occurred under neutral to alkaline environment in the study area. Groundwater types in single structure unconfined groundwater were mainly HCO3·SO4-Ca·Mg, SO4·HCO3-Na·Ca, and unconfined groundwater in unconfined area were SO4·Cl-Na·Ca and SO4·Cl-Na·Mg, while Cl·SO4-Na, SO4·Cl-Na·Ca and HCO3·SO4-Ca·Mgweredominant in shallow confined groundwater. The calculation results of salinization coefficient(SC) showed that the proportion of SC≤1, between >1 and 2, >2 in the water samples with single structure unconfined groundwater (17 groups of groundwater samples) were 47.06%, 29.41% and 23.53%, respectively, and the degree of salinization was high. The proportion of SC≤1 in unconfined groundwater of unconfined area (22 groups of groundwater samples) was 50.00%. The proportion of Sc between >1 and 2 was 36.36%, the proportion of SC>2 was 13.64%, and the degree of salinization was low. The SC≤1, >1-2 and > 2 of shallow confined groundwater (33 groups groundwater samples) was 42.42%, 15.15% and 42.42% respectively, which the salinization degree was the highest. Kolmogorov-Smirnov tests were carried out on Cl-, SO42-, TDS of unconfined and shallow confined groundwater, and the results obeyed normal distribution. To examine the degree of spatial correlation, the ratio of nugget to sill was advised generally. The nugget/sill≤0.25, > 0.25~0.75, and > 0.75 were the conditions in which spatial structures were supposed to be strong, moderate, and weak, respectively. In this study, the ratio of nugget to sill for all parameters of Cl-, SO42-, TDS of unconfined and shallow confined groundwater were < 0.25, suggested the strong spatial correlation for the studied regionalized variable. The trend of Cl-, SO42-, TDS in unconfined groundwater was generally low in the South and high in the North, while that of shallow confined groundwater was characterized by the distribution of high and low values. Factor analysis was carried out on 14 hydrochemical indices of 72 groups of groundwater samples. The results showed that the contribution rates of main factors F1, F2 and F3 were 49.38%, 19.53% and 13.52% respectively, and the contribution rate of cumulative variance was 82.43%.Cluster analysis showed that GW1 (controlled by hydrogeological conditions) groundwater accounted for 58.33% of the all groundwater samples, which was widely distributed and mainly affected by natural processes. The saturation indices (SI) of carbonate minerals tended to be saturated, while the SI of evaporite minerals was unsaturated. The ion concentration was mainly controlled by the dissolution of evaporite minerals. GW2 (affected by groundwater chemical environment) groundwater accounted for 15.28%, mainly distributed in grassland, and hydrochemical environment had a relatively large impact on groundwater salinization. Sodium adsorption ratio (SAR) was positively correlated with total dissolved solids (TDS) (R2=0.773), which showed that Na+ in groundwater had obvious ion exchange with Ca2+ and Mg2+ in aquifer medium. 26.39% of the all groundwater samples belong to GW3 (affected by human activities), mainly distributed in cultivated land and construction land, and groundwater salinization was significantly affected by human activities. Unconfined groundwater was greatly affected by agricultural irrigation, domestic sewage and industrial sewage, however, the pollution degree of shallow confined groundwater was relatively low.