Abstract: When assessing the origin of salinity and the mechanisms of salinization in coastal aquifers, hydrogeologists may consider the combined use of certain geochemical tools to assess critical aspects of the hydrogeological setting of the system. These tools are based in the integrated use of chemical (major ions, trace elements and ionic ratios) and isotope parameters (oxygen, hydrogen, sulphur, carbon, strontium and boron). The problem of groundwater salinization in coastal aquifers, besides active seawater intrusion, may be affected by several human activities that accelerate the progressive deterioration of water quality, such as concentrated pumping, intensive agricultural practices including return flows or reuse of waste waters from urban or industrial origin. The characterisation of the perating processes and mechanisms of salinization is a requisite for a proper management of groundwater resources and for adopting remediation strategies. In this contribution the potential role of several isotopic tools in these studies is briefly described.

Abstract: In the southern coastal plain of Laizhou Bay, due to intensive exploitation of groundwater since the early 1970s, the shallow aquifer has been severely influenced by saltwater intrusion, which causes the extraction to shift from shallow to deeper aquifer changing the hydrogeological condition greatly. This study was conducted to investigate the groundwater salinization using hydrochemistry and H, O and Li isotope data. Dissolved Li shows a linear correlation with Cl and Br in seawater, brine and saline groundwater indicating the marine Li source, whereas the enrichment of Li in surface water, brackish and fresh groundwater is impacted by dissolution of silicate minerals. The analyses of hydrochemistry and isotopes (H, O and Li) indicate that brine originated from seawater evaporation, followed by mixing processes and some water-rock interactions; shallow saline groundwater originated from brine diluted with seawater and fresh groundwater; deep saline groundwater originated from seawater intrusion. The negative correlation of δ(7)Li and Li/Na in surface water, brackish and fresh groundwater is contrary to the general conclusion, indicating the slow weathering of silicate minerals and hydraulic interaction between surface water and shallow groundwater in this area. The analyses of hydrochemistry and isotopes (Li, H and O) can well identify the salinity sources and isotope fractionation in groundwater flow and mixing, especially groundwater with high TDS. As both mixing with saltwater and isotope fractionation can explain the combination of high δ(7)Li and low TDS in brackish groundwater, isotope fractionation may limit their use in recognizing salinity sources of groundwater with low TDS.