[Hydrochemical characteristics and formation mechanism of shallow groundwater in the Yellow River Delta].

Understanding the chemical characteristics of groundwater in the Yellow River Delta is very important. It can provide a useful reference for the development and construction of the Yellow River Delta High-efficiency Ecological Economic Zone and ecological regulation in the lower Yellow River. Based on partitioning the sediment environment and the recharge-runoff-discharge system, we studied the hydrochemical features and causes of shallow groundwater in the Yellow River Delta by mathematical statistics and geostatistics, Piper diagram, ion ratios and so on. Following results are obtained: 1) Major cations and anions such as Na+, Mg2+, Ca2+, Cl(-), SO4(2-), HCO3(-) and TDS concentrations range from 0.1-25.0 g x L(-1), 3.6-3 815.0 mg x L(-1), 5.6-3 377.0 mg x L(-1), 0.1-45.1 g x L(-1), 24.2-4 947.0 mg x L(-1), 62.6-850.0 mg x L(-1) and 0.4-80.7 g x L(-1). Average ion concentrations further indicate that Cl(-), Na+ and TDS concentrations are high while HCO3(-), CO3(2-) and K+ concentrations are very low in the study area. 2) The Cl(-) and TDS concentrations of shallow groundwater possess conspicuous directional spatial variability and gradually increase along the groundwater flow direction, showing that Cl(-) is the most critical ion of shallow groundwater. 3) From the recharge area to the discharge area, shallow groundwater changes from the Na+ -Mg2+ -Ca2+ -Cl(-) -SO4(2-) facies to the Na -Mg2 + -Ca2+ -Cl(-), Mg2+ -Na+ -Ca2+ -Cl(-) and Na+ -Mg2+ -Cl(-) facies, finally evolves into Na+ -Cl(-) facies in the coast. 4) Ion ratios indicate that the following main hydrochemical processes are inferred to control the shallow groundwater chemical composition: mixing, evaporation concentrating, mineral dissolution, cation exchange and adsorption and human activities. These findings strongly suggest that changes of the Yellow River water course and seawater intrusion are key drivers to form the chemical characteristics of shallow groundwater in the region.