Quantification of the water age and submarine groundwater discharge in a typical semi-enclosed bay using stable oxygen (¹⁸O) and radioactive radium (²²⁸Ra) isotopes

Nutrient inputs from submarine groundwater discharge (SGD) play a significant role in sustaining primary productivity and nutrient cycling in the coastal area. Currently, various geochemical isotopes are used to trace the SGD processes. However, mass balance models of stable water isotopes (δ²H and δ¹⁸O) are seldom used in SGD estimating. In this study, mass balance models of ¹⁸O and radium (²²⁸Ra) isotopes were used to quantify the water age and SGD in Laizhou Bay, China based on the isotope data sampled in August 2017. The estimated age of water masses in Laizhou Bay ranged from 23.5 to 50.0 days with an average of 32.1 ± 16.3 days. The average SGD flux was (2.07 ± 1.04) × 10⁸ m³ d⁻¹, which was an order of magnitude larger than the Yellow River discharge during the sampling period. The sensitivity analysis revealed that estimated results of the water age and SGD are sensitive to the δ¹⁸O value in evaporation, as well as δ¹⁸O and ²²⁸Ra values in groundwater end-members. Based on the isotope method, the proportion of the Yellow River discharging into Laizhou Bay was estimated to be less than 27% of the total discharge. Furthermore, based on water and salt mass balance models, the estimated submarine fresh groundwater discharge (SFGD) flux ranged from (0.54–1.31) × 10⁷ m³ d⁻¹ with an average of (0.93 ± 0.46) × 10⁷ m³ d⁻¹. The SFGD accounted for 4.5% of the total SGD. Nutrient fluxes from SGD were significantly greater than those from the Yellow River, and SGD may have important effects on the marine ecosystem environments. This study reveals that stable and radium isotopes can be effectively combined to calculate the water age and SGD, which may be applied to the coastal area elsewhere.