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

Abstract

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.