A comprehensive analysis of submarine groundwater discharge and nutrient fluxes in a coast of eastern China based on radium isotope tracing

Submarine groundwater discharge (SGD) constitutes a significant land-sea interaction process in estuarine and coastal zones, profoundly influencing water and nutrient cycling. To quantify exchange fluxes and ecological effects between runoff-SGD-ocean systems in highly turbid tidal estuaries, this study investigates the densely populated, industrialized Ao River basin in eastern China. A multi-isotope radium tracing approach (²²³Ra, ²²⁴Ra, ²²⁶Ra, ²²⁸Ra) was employed. SGD fluxes and associated nutrient inputs were systematically estimated by constructing a three-endmember mixing model, a radium mass balance model, and a tidal-driven analytical analysis. Results indicate that the water flushing time in the Ao River estuary is relatively short (4.59±0.35 days). SGD flux ranged from 1.37×10⁷ to 3.65×10⁷ m³ d^-1. The fresh groundwater component (SFGD) constituted only 5.19% to 8.28%. Tidal-cycle observations revealed that tide-driven SGD fluxes ranged from 0.98×10⁷ to 2.35×10⁷ m³ d^-1, showing significant variability with tidal phase. This component accounted for 70.14% to 93.97% of the total recirculated seawater, confirming that tidal pumping is the dominant mechanism controlling SGD dynamics in this system. SGD delivered dissolved inorganic nitrogen (DIN) and phosphorus (DIP) fluxes of 4.48×10⁶ mol d^-1 and 1.79×10⁴ mol d^-1. These inputs represented 91.48% and 62.56% of the total nutrient flux to the estuary, exceeding concurrent riverine inputs by factors of 12 and 1.76, respectively, indicating that SGD is the dominant source of nutrients in this region. The DIN:DIP ratios for SGD and riverine inputs were 387 and 280, substantially exceeding both the Redfield ratio (16) and local seawater ratios (44). This reveals a pronounced phosphorus limitation risk, potentially exacerbating estuarine eutrophication and ecosystem imbalance. This study systematically elucidates the dominant role of SGD in nutrient transport within tidal estuaries, providing crucial scientific basis for assessing land-sea material fluxes and ecological management in economically developed eastern coast of China.