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

Abstract

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 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 (<20 pMC) was probably recharged during a cooler period in the late Pleistocene, as is common throughout northern China. An increase in the salinity and tritium concentration in some shallow groundwater sampled in the 1990s and re-sampled here indicates that intensive brine extraction has locally resulted in rapid mixing of young, fresh groundwater and saline brine. The 6180 and delta H-2 values of brines (similar to-3.0 parts per thousand and -35 parts per thousand) are much lower than that of modern seawater, which could be explained by 1) mixing of original (5180 enriched) brine that was more saline than presently observed, with fresh groundwater recharged by precipitation and/or 2) dilution of the palaeo-seawater with continental runoff prior to and/or during brine formation. The first mechanism is supported by relatively high Br/Cl molar ratios (1.7 x 10(-3)-2.5 x 10(-3)) in brine water compared with similar to 1.5 x 10(-3) in seawater, which could indicate that the brines originally reached halite saturation and were subsequently diluted with fresher groundwater over the long-term. Decreasing C-14 activities with increasing sampling depth and increasing proximity to the coastline indicate that the south coastal aquifer in Laizhou Bay is dominated by regional lateral flow, on millennial timescales. (C) 2013 Elsevier B.V. All rights reserved.