Chemical and isotopic constraints on evolution of groundwater salinization in the coastal plain aquifer of Laizhou Bay, China

A hydrochemical-isotopic investigation of the Laizhou Bay Quaternary aquifer in north China provides new insights into the hydrodynamic and geochemical relationships between freshwater, seawater and brine at different depths in coastal sediments. Saltwater intrusion mainly occurs due to two cones of depression caused by concentrated exploitation of fresh groundwater in the south, and brine water for salt production in the north. Groundwater is characterized by hydrochemical zonation of water types (ranging from Ca-HCO3 to Na-Cl) from south to north, controlled by migration and mixing of saline water bodies with the regional groundwater. The strong adherence of the majority of ion/Cl ratios to mixing lines between freshwater and saline water end-members (brine or seawater) indicates the importance of mixing under natural and/or anthropogenic influences. Examination of the groundwater stable isotope delta O-18 and delta H-2 values (between -9.5 parts per thousand and -3.0 parts per thousand and -75 parts per thousand and -40 parts per thousand, respectively) and chloride contents (similar to 2 to 1000 meq/L) of the groundwater indicate that the saline end-member is brine rather than seawater, and most groundwater samples plot on mixing trajectories between fresh groundwater (delta O-18 of between -6.0 parts per thousand and -9.0 parts per thousand; Cl < 5 meq/L) and sampled brines (delta O-18 of approximately -3.0 parts per thousand and Cl > 1000 meq/L). Locally elevated Na/Cl ratios likely result from ion exchange in areas of long-term freshening. The brines, with radiocarbon activities of similar to 30 to 60 pMC likely formed during the Holocene as a result of the sequence of transgression-regression and evaporation: while deep, fresh groundwater with depleted stable isotopic values (delta O-18 = -9.7 parts per thousand and delta H-2 = -71 parts per thousand) and low radiocarbon activity (