Study On The Characteristics And Formation Mechanism Of Groundwater Chemical Composition In The Yelllow River Delta

The Yellow River delta, located in the transitional zone of land and sea, is asediment deposit which forms from the interaction of river dynamics and seadynamics. Salt water and brine is widely distributed in the delta, but fresh water andbrackish water is scarce. Marine transgression and regression has happened threetimes in the area since Late Pleistocene, so the groundwater storage and distribution iscomplex, and the water origin and water quality are different. Marine transgressionand regression is a complex process with multicomponent reactions, including mixingerosion of saltwater-freshwater, evaporation-concentration process, cation exchangeand mineral dissolution/precipitation process. They are the most important processesof the groundwater chemical formation in the delta. The study on chemicalcompositions and formation of groundwater will provide scientific evidence for theidentification of environmental carrying capacity and reasonable use of groundwater.Based on the fully studying of geology condition and hydrogeology condition ofthe Yellow River delta, groundwater here is divided into three systems: the phreaticand confined groundwater in the south diluvial fan, phreatic water and shallowconfined groundwater in the Yellow River delta. The chemical characteristics ofgroundwater in each system are analysed. The relationship between groundwater andYellow River, rainwater and seawater is studied by analyzingδ¹⁸O-δD andδ¹⁸O-Cl- ofgroundwater in aquifers. The age and origin of groundwater are estimated accordingto the result of 14C and the relationship between ¹⁴C and Cl^-. And then the formationmechanism of groundwater chemical composition is revealed through experimentanalysis and PREEQC soft.The study shows that: the phreatic and confined groundwater in the southdiluvial fan is fresh water, and the hydrochemical types are mainly HCO₃-Ca·Mg, HCO₃·Cl-Ca·Mg and HCO₃-Na. Phreatic water in the Yellow River delta is brackishwater, salt water and a small amount of lenticular freshwater. Cl·HCO₃-Na·Mg waterand HCO₃·Cl-Na·Mg water are the main hydrochemical types. Shallow confinedgroundwater is salt water, even brine, and hydrochemical types are Cl·SO₄-Na·Mg andCl-Na.Formation period of deep groundwater (depth of confined aquifer is more than100m) in both sides of Xiaoqing River is much earlier than shallow groundwater. Itprobably originated from paleo-rainwater recharge. Phreatic water in the Yellow Riverdelta is relatively young and originated from modern rainwater and the Yellow River.High-salinity shallow confined groundwater, aged between 4000 and 7500, originatedfrom paleo-seawater which remained in the area and became concentrated.In the study area, evaporation-concentration process, cation exchange andmineral dissolution/precipitation are the most important effects related to theformation of groundwater chemical composition. Cation exchange occurred on thesurface of aquifer medium decreases with increasing distance to the Yellow River. Cl,K and Na of the groundwater exist in the form of ions, while Ca, Mg, S, and C haveother complex compounds except ion forms. The saturation index of gypsum andhalite is below 0, so groundwater is erosive for gypsum and halite. And the erosiveintensity becomes weak along the direction of groundwater flow. Calcite, dolomiteand aragonite are in a supersaturation state and will be precipitated.