Nutrient inputs to a Lagoon through submarine groundwater discharge: The case of Laoye Lagoon, Hainan, China

Submarine groundwater discharge (SGD) with inputs of nutrients in certain regions may play a significant role in controlling water quality in the coastal regions. In this paper, we have determined four naturally occurring radium isotope (²²³Ra, ²²⁴Ra, ²²⁶Ra and ²²⁸Ra) activities and nutrient concentrations in surface water, coastal groundwater and river water in the mixing zone of Laoye Lagoon to estimate the fluxes of SGD by several models. The activities of the four radium isotopes of ground water were considerably greater than those in surface water samples. Using a ²²⁴Ra/²²⁸Ra activity ratio (AR) model, we estimated the average lagoon water age to be 3.2days, which was comparable with the flushing time of 4.0days. Based on the excess radium isotopes and the water age of the lagoon, the estimated fluxes of SGD (in 10⁶m³/d) ranged from 2.64 to 5.32 with an average of 4.11. Moreover, we used Si balance to evaluate the flux of SGD (4.8×10⁶m³/d) which was close to the result calculated by radium. The SGD-derived nutrient fluxes (in mol/d) were DIN=1.7×10⁵, PO₄^3-=5.2×10², and SiO₃=5.3×10⁴. Furthermore, we applied the biogeochemical budget approach using SiO₃ as a tracer to evaluate the impact of SGD. The differences between the results estimated by radium and SiO₃ may indicate different pathways for the input of nutrients.