Genesis of salinized groundwater in Quaternary aquifer system of coastal plain, Laizhou Bay, China: Geochemical evidences, especially from bromine stable isotope

Abstract

In the south coast of Laizhou Bay, due to intensive groundwater extraction since the 1970s, salt water intrusion has occurred seriously, which increased the stress on regional fresh water resources. It is of great importance to understand origin of salinity and salinization process in coastal aquifers. To trace source and transport of dissolved bromide as a perfect representative of salinity and reveal genesis of salinized groundwater in regional aquifer system, Br stable isotopes of the sampled waters were surveyed coupled with hydrochemistry and stable H/O isotopes along the direction from inland to shore. The results show that groundwater salinity, hydrochemistry and stable isotopes exhibit regular variation from inland to shore. The TDS values of the groundwater increase gradually ranging from 0.52 to 133.74g/L, the chemical types transform from Cl–Ca and HCO3–Ca to Cl–Na, 2H and 18O are getting more and more enriched, and the δ81Br values decrease gradually, from inland to shore. Trends of major ions versus bromide and Br isotope signatures for brine indicate brine originated from seawater evaporation followed by some water–rock interaction processes. The hydrogen and oxygen isotope signatures of the brine indicate the dilution by fresh water of meteoric origin was a probable process during or after brine formation. The δ81Br values of brine samples show good anticorrelation to both δ2H and δ18O values, probably confirming the addition of fresh water. The potential contribution of evaporate dissolution as a primary process to the brine was ruled out based on their Br isotope signatures. Trends of major ions versus bromide and H/O stable isotopes for brackish water reveal brine intrusion has occurred and mixed with fresh water inland, and Br stable isotopic composition of brackish water indicates that marine aerosol input could enrich the dissolved bromide in brackish water except for brine intrusion. Other processes, such as diffusion, subaerial evaporation and anthropogenic activities may change the budget of dissolved Br−, but are not primary processes responsible for behavior of Br stable isotope in shallow brackish water. Major contribution of meteoric water in deep fresh water and more influence by brine in deep saline water were revealed based on the Br isotopic results.