Evaluation Of Submarine Groundwater Discharge And Associated Nutrient/Heavy Metal Fluxes Into Daya Bay

Submarine groundwater discharge?SGD?,asone of the research prioritiesin groundwater–seawater interactions,comprises terrestrial fresh groundwater discharge?SFGD?and re-circulated seawater?RSGD?.As an important part of the water cycle,SGD is amain channel of chemicalinput into the sea from land,and has significant impacts and implications on coastal environment and ecology.In recent years,there have been many reports that SGD driven material cannot be neglected.In this paper,Daya Baywhere SGD has not been studied is chosen as the study areausing radium isotopes.Analysis of the radium isotope activity measurement resultsshow a decreased trendfrom northeast to southwest.We coupled all the four radium isotopes to assess the flushing time,whichranges from 13.41 d to 18.85 d.The result of tidal prism modelgives a value of 18.83 d for the flushing time.Meanwhile,the water apparent age model was applied to calculate the water apparent age,which rangesfrom 9.35to26.72 d.The three results are consistent with each other.Based on the distribution of ²²⁴Ra in Daya Bay and the 1-D mixing model,the range of horizontal eddy diffusion coefficient is estimated to be?7.80¹1.65?×10⁶cm2/s.Based on the horizontal distribution of ²²⁴Ra in a station within Daya Bay,the range of horizontal eddy diffusion coefficient was estimated to be?1.12⁴.44?cm2/s.Using the coupled mass-balance models of radium isotopes,the SGD is estimated to be?2.68³.34?× 10⁷ m³/d.The SFGD is estimated to be?3.98⁷.12?×10⁶ m³?d in July 2015 based on coupled mass balance models of water and salt.Based on drainage basin water mass balance model,the averaged SFGD is?1.23².41?×10⁶m³/dor?0.22⁰.43?cm/d.The results of SFGD of the two different models are very close,and they are mutually validated.The flux of heavy metal assocated with SGD was significantly greater than that of the river,and the fluxes of nutrientsassocated withSGD were comparable to those of the river.The SGD-driven nutrient loadings have an important influence on the nutrient budgets in the coastal zone.This study may provide useful information for coastal ecological evaluation in Daya Bay.