Evaluation Of Submarine Groundwater Discharge And Nutrient Loading Into Daya Bay,China Using ²²²Rn

Submarine groundwater discharge?SGD?is defined as any and all flow of water on continental margins from the seabed to the coastal ocean,regardless of fluid composition or driving force.It has been widely recognized as an important path way of water and nutrient from land to sea.In order to estimate SGD in Daya Bay,spatial and temporal variations of ²²² Rn activities were investigated in different seasons in this study.Continuous ²²² Rn monitoring was deployed at two locations---site A located at west bank and site B located at east bank of Daya Bay.The measurements of ²²² Rn activities were conducted in the coastal water per hour for a period of 48 h at site A in Aug 2015,for a period of 72 h and 68 h.at site A and B respectively in Dec 2015.Based on the monitoring data and an unsteady state ²²² Rn mass balance model,the rates of SGD was estimated.The result indicated that?1?the variation of rates of SGD over a tidal cycle was significant,and displayed a negative correlation with seawater depth;?2?at Site A,the average rate of SGD in summer?18.1cm/day?was twice as much as that in winter?9.5 cm/day?;?3?the average rate of SGD in east bank?17.1 cm/day?was much higher than that in west bank?9.5 cm/day?in winter;?4?²²²Rn in coastal water was contributed mainly by SGD?over 83%?.This study took 27 samples all over the bay in Aug 2015 and Dec 2015 respectively to investigate the spatial variations of ²²² Rn activities in Daya Bay.Based on the spatial data of ²²² Rn,SGD was estimated via a steady-state ²²² Rn mass balance model.As a result,?1?the total SGD flux into Daya Bay in summer?7.3×1010 L/d?was much higher than that in winter?2.6×1010 L/d?;?2?the SGD rate per unit area calculated by the steady-state model is smaller than the rate calculated by the unsteady-state model,which indicated that the SGD rate is inhomogeneous spatially;?3?the steady-state ²²² Rn budget indicted the ²²² Rn input from SGD accounted for 98%of total;?4?the flux of DIN and DIP loading though SGD was estimated to be 5.1×106 mol/d and 4.1×104 mol/d in summer,6.3×105 mol/d and 1.1×104 mol/d in winter.The total SGD and associated nutrient loadings is considerable and changed greatly with time and location.This study highlights the need to estimate SGD on large spatial and temporal scales in the future.