Estimating Submarine Groundwater Discharge And Associated Nutrient Fluxes Into Liaodong Bay Using Radium Isotopes

Submarine groundwater discharge?SGD?is defined as all flow of water on the continental margins into the sea from the seabed,regardless of fluid composition and driving force.Driven by both terrestrial and marine forcing components,SGD comprises terrestrially-originated submarine fresh groundwater discharge?SFGD?and re-circulated seawater?RSGD?.As a major component of the hydrological cycle,SGD is one of the important sources of water,chemicals and various pollutants?nutrients and heavy metals?from the land to the sea,which has a major impact on the coastal environments.Although Liaodong Bay is one of the three bays in the Bohai Sea,the groundwater discharge and associated materials in the Bay have been seldom reported.This study applies ²²³Ra and ²²⁶Ra mass balance models to estimate the flushing time and SGD in spring 2017,and evaluate the potential environmental impacts in Liaodong Bay.The flushing time is evaluated using three different methods.With the mass balance model,the flushing time is estimated to be 57.1 to 73.3 days,with an average of 65.2 days.Apparent age model based on the ²²³Ra and ^226/228Ra yields a flushing time of 43.0-49.8d,and the tidal prism model estimates the flushing time to be 60.0-77.0 d.These results obtained by the three methods are basically consistent with each other.Based on the flushing time concluded by radium mass balance model and water-salt mass balance model,the SFGD is estimated to range from 5.79×10⁷m³/d to6.66×10⁷m³/d in Liaodong Bay,the ratio of SFGD to SGD is 20%-23%.Using the Ra-226 mass balance model,the SGD is estimated to be?2.53³.26?×10⁸m³/d during the sampling period.In addition,considering the radium loss caused by RSGD,this study also uses the improved radium mass balance model to calculate the flushing time and SGD in Liaodong Bay.In this case,the flushing time is 50.4⁶2.5d and the SGD is?1.48-5.96?×10⁸m³/d.The results show that the SGD value calculated without considering the impact of RSGD is lower than the SGD result obtained by considering the influence of RSGD by 50%,which shows that the RSGD is the major component of SGD in the study area.For the mixing process of water,the horizontal eddy diffusion coefficient in Liaodong Bay is calculated using the one-dimensional diffusion model based on the distribution of ²²³Ra and ²²⁴Ra in the bay.The results show that the horizontal eddy diffusion coefficient in the Liaodong Bay in spring is 1.99-6.10×106 cm ²/s.Based on the above results,this study also calculates the nutrient fluxes carried by SGD into Liaodong Bay.The obtained ratio of dissolved inorganic nitrate?DIN?and phosphorus?DIP?is 153 in the coastal groundwater of Liaodong Bay.Moreover,the DIN and DIP fluxes from SGD are?2.64-3.40?×10⁸ mol/d,?1.73-2.23?×10⁶ mol/d,which are 117-150 and 22-29 times the input flux of the river,respectively.We can conclude that the nutrients carried by SGD are the main sources of nutrients in Liaodong Bay,which may change the nutrient structure in seawater in Liaodong Bay.