Factors that influence the chemical composition and evolution of shallow groundwater in an arid region: a case study from the middle reaches of the Heihe River, China

To investigate the chemical characteristics, spatial distribution and evolution of groundwater in the middle reaches of the Heihe River, 100 shallow groundwater samples were collected and tested. The results of this assessment indicate that the shallow groundwater in the study area is weakly alkaline and brackish. Concentrations of K+, Cl− and NO3− are highly variable in groundwater and are the highest in areas with intensive human land use activities. The TDS content of groundwater gradually increases from the recharge area to the discharge area, and the chemical composition gradually changes from SO42−–HCO3−–Na+·Mg2+ to SO42−–Na+–Mg2+. Overall, the chemical composition and groundwater types in the study area are primarily affected by water–rock reactions, and certain areas are more affected by human activities. Isotopic evidence indicates that the main source of shallow groundwater in the study area is snowmelt.