ISOTOPIC TRACERS (RA, B, SR) IN A PORE WATER PROFILE FROM A FRESH WATER LAKE INFLUENCED BY UPWELLING SALINE BRINES

Concentrations of Sr and B, ²²⁶Ra activity, and ⁸⁷Sr/⁸⁶Sr and ¹¹B/¹⁰B isotope ratios were measured in pore water (PW) along a 250 cm long sediment core collected at the Sea of Galilee (SG), Israel, a fresh water lake (~250 mg Cl/L) underlain by saline groundwater (up to 18,000 mg Cl/L), which appear to diffuse into the lake. This study aims to determine the processes controlling the geochemistry of the PW.

The salinity of the PW increases with depth. Sr concentrations (1.13-4.88 mg/L) gradually increase down the core, but are significantly lower as compared to the concentrations expected from simple mixing between SG and the end-member brine. The average PW ⁸⁷Sr/⁸⁶Sr (0.70762) is generally higher than the expected ratios from simple mixing between SG (0.70752) and the brine end-member (0.70824-0.70905). B content increases from 94 μg/L at the top of the core to 207 μg/L at a depth of 67.5 cm, then remains constant at ~200 μg/L. Like Sr, B content is lower than expected when compared to the mixing relationships between SG (110 μg/L) and the end-member brine (3900 μg/L). The δ¹¹B values of the PW vary from 22‰ to 37‰, and are slightly higher than the expected mixing relationships between SG (24‰) and the end-member brine (44‰). The elemental B depletion and δ¹¹B enrichment could indicate B adsorption, which increases with depth (and salinity). ²²⁶Ra measurements of PW were carried out on a Thermal Ionization Mass Spectrometer (Triton), and ranged from 0.75 dpm/L at the top to 5.43 dpm/L at the bottom of the core. Since Ra is a surface reactive element strongly influenced by adsorption processes, we modeled the expected ²²⁶Ra activity along the core, accounting for diffusion, sedimentation rate, adsorption and radioactive decay. Since Ra adsorption sharply decreases with increasing salinity, we conducted a series of adsorption experiments using SG sediments, showing that the partitioning coefficient, K_d, decreases from 640ml/g in deionized water to 400ml/g in water containing 750 mg Cl/L. In spite of accounting for adsorption, the measured ²²⁶Ra activities are lower than expected when compared to the modeled PW ²²⁶Ra activities corrected for adsorption, suggesting additional processes for Ra removal.

Overall, the underlying brine is significantly modified by extensive water-rock interactions as it diffuses through the lake sediments.