Hydrogeochemical Characterization of Groundwater in and Around a Wastewater Irrigated Forest in the Southeastern Edge of the Tengger Desert, Northwest China

Groundwater is an essential part of water resources for human survival and economic development in arid regions over the world. Human activities and environmental change have imposed significant impacts on groundwater environment. To investigate the hydrogeochemical characteristics and evolution of groundwater in and around a desert region impacted by wastewater irrigation, 84 groundwater samples were collected and analyzed for 18 indices. Statistical and graphical approaches were applied to delineate the general hydrochemical characteristics of groundwater and the major factors influencing its evolution. Stable isotopes of H-2 and O-18 were applied to identify groundwater evaporation process. Hydrogeochemical modeling was also adopted to quantify the major reactions occurring in the groundwater system. The results reveal that the abundance of cations is Na+ > Ca2+ > Mg2+ > K+ for groundwater in the entire study area, while the abundance of anions for groundwater in the desert region is HCO3 (-) > Cl- > SO4 (2-), and that for groundwater in the alluvial plain is HCO3 (-) > SO4 (2-) > Cl-. Groundwater chemistry in the study area is mainly of rock dominance, and dissolution/precipitation of minerals and cation exchange are major natural factors governing the formation of groundwater chemistry. However, stable isotopes and the occurrence of nitrate show that shallow groundwater evaporation and human activities also have some impacts on groundwater quality. Hydrochemical type transits from Ca-Cl to HCO3 center dot SO4-Ca type, and then to HCO3 center dot SO4-Ca center dot Mg type along the flow path. The transition is influenced by multiple factors with water-rock interactions the predominant one. The water-rock interactions for the upper and lower sections of the flow path, indicated by hydrogeochemcial modeling, are different due to different geologic and hydrogeologic conditions.