The formation of shallow fresh groundwater in the north of Yanchi county, Ningxia, China: main influencing factors and mechanism

Shallow fresh groundwater is not common in the middle of basins in arid and semiarid areas with strong evaporation where shallow groundwater is often brackish water or saline water. In this study, the shallow fresh groundwater in the north area of Yanchi county, China was investigated for the main influencing factors and mechanisms of its formation. The results showed that freshwater of HCO3 and SO4 types with mean salinity of 0.78 g/L formed in the north of the study area while brackish and saline water of SO4·Cl–Na type with mean salinity of 2.28 g/L formed in the west and south of the study area. The fresh groundwater formed owing to the special physico-chemical properties of the aeolian sand which has the good permeation of atmospheric precipitation, the weak evaporation of the groundwater, the low salt contents in the vadose zone and aquifer medium, the weak water–rock interaction and the good hydrodynamic condition. The lateral recharge of low salinity groundwater also played a mixing and diluting role in the formation of the fresh groundwater. The findings should provide a scientific basis for the exploitation and utilization of local groundwater resources and be of significance in the exploration and exploitation of fresh groundwater in arid and semiarid regions.