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 3.2 days, which was comparable with the flushing time of 4.0 days. Based on the excess radium isotopes and the water age of the lagoon, the estimated fluxes of SGD (in 106 m3/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 × 106 m3/d) which was close to the result calculated by radium. The SGD-derived nutrient fluxes (in mol/d) were DIN = 1.7 × 105, PO43 − = 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.

► Determine radium isotope activities and nutrient concentrations in the mixing zone of Laoye Lagoon.
► Use a 3-end-member mixing model to calculate the fraction of each end-member and the flushing time of the lagoon.
► Employ a 224Ra/228Ra activity ratio model to estimate the average lagoon water age.
► Estimate fluxes of SGD and SGD-derived nutrient based on the excess radium isotopes and Si balance.
► Apply the biogeochemical budget approach using SiO3 as a tracer to evaluate the impact of SGD.