Using hydrochemical and stable isotopic (δ2H, δ18O, δ11B, and δ37Cl) data to understand groundwater evolution in an unconsolidated aquifer system in the southern coastal area of Laizhou Bay, China

Abstract This study illustrates the origin of Quaternary brines and groundwater salinization processes at Laizhou Bay using hydrochemical and δ 2 H, δ 18 O, δ 11 B and δ 37 Cl data. The characteristics of the major ionic components of these brines demonstrated that brines originated from evaporated seawater. The δ 2 H and δ 18 O values of brines showed that seawater underwent strong evaporation and that these brines underwent mixing with paleo-meteoric water or surface water. The fact that the Cl/Br mass ratios of the Laizhou Bay brines (180–291) are lower than that of halite dissolution brine (>1000) contradicts the hypothesis that these brines were derived from halite dissolution, and the fact that the δ 37 Cl values (0.14‰–0.21‰) of these brines are more enriched than those derived from seawater evaporation represents positive evidence that evaporated seawater in the lagoon infiltrated through clay layer, during which ion filtration resulted in the positive Cl isotopic compositions of these brines. The higher B isotopic compositions (49.88‰–64.42‰) and lower B/Cl molar ratios (8.5 × 10–5∼1.6 × 10–4) of these brines are consistent with the brine origin determined based on the analysis of Cl isotopes. As the evaporated water infiltrated the clay layer, the preferential adsorption of 10 B onto clay dominated the B isotopic composition of the brine. Five different mixing processes were calculated using B contents and B isotopic compositions. These calculations illustrated that one shallow saline groundwater (No. 05) was derived from the mixing of fresh groundwater and deep brine, thus indicating that brine intrusion occurred in the northern area of Changyi, which was caused by the over-exploitation of fresh groundwater. While one shallow saline groundwater (No. 10) and one deep saline groundwater (No. 12) in the northern of Xiaying were derived from the mixing of brine and seawater. The δ 11 B values of the two shallow brackish groundwater samples were close to that produced by the desorption of B from marine clay, and their B/Cl molar ratios were relatively higher, which demonstrated that B-desorption dominated the characteristic B hydrochemical compositions of brackish groundwater.