Evaluation Of Submarine Groundwater Discharge Into The West Of Bohai Bay,china Using ²²²Rn

Submarine groundwater discharge?SGD?is defined as all water that flows into the ocean from seabed on the continental margins.SGD includes two parts,i.e.,submarine fresh groundwater discharge?SFGD?and recirculated saline groundwater discharge?RSGD?.Due to the ion exchanges between aquifers'sediments and pore water,SGD is often with high concentration of solutes and can carry large amount of nutrients into the sea.Bohai Bay is a semi-closed bay,and the main pollution is terrestrially-originated due to human activities.The quantitative estimation of SGD is helpful for the protecting and management of the environments in Bohai Bay.In this study,samples of groundwater,seawater and river water in the west part of Bohai Bay were taken in spring(from April 28^th to May 11^th)and summer(from Aug 11^th to 22^th)of 2017 to analyze and investigate the spatial variations of ²²²Rn and its decay parent ²²⁶Ra.After using the mixing model of three endmembers?seawater,river water and groundwater?based on the data of ²²⁶Ra and salinity to discuss mixing of Bohai Bay water.Based on tidal prism model,the flushing time of water body of the west part of Bohai Bay is estimated,which is 22.03-40.48 d in spring and 10.11-15.65 d in summer.By combining ²²²Rn and ²²⁶Ra mass balance models,the flushing time of the west part of Bohai Bay is estimated to be 6.45-18.06 d in spring and 8.58-21.53 d in summer.These two results of flushing time are close to each other,and the two methods can test and verify each other.Three endmembers mixing model can be used to calculate the proportion of open sea water in the west part of Bohai Bay water bodies,and it is found that open seawater accounts for 88-93%of the west part of Bohai bay water bodies in spring and 77-85%in summer.Using the estimated flushing time and ²²²Rn mass balance model SGD in the west part of Bohai Bay is estimated to be?5.13-9.38?×10⁸m³/d in spring,and?6.24-13.1?×10⁸m³/d in summer.The mass of ²²²Rn carried by SGD into the west part of Bohai Bay in spring is about 1.6 times of that in summer.However,the mean ²²²Rn activity of Bohai Bay surface water in summer is not low bacause there is more ²²²Rn loss due to atmospheric evasion in spring.In fact,the radon atmospheric evasion in spring is 82%more than that in summer.Taking the effect of RSGD into account in radon mass balance model,together with the salinity mass balance model,the flushing time and SGD in the west part of Bohai Bay is estimated once again.Flushing time obtained from the new model is7.31-23.08 d in spring and 9.00-21.32 d in summer.The SFDG of Bohai Bay evaluated by the salinity mass balance model is?4.07-13.4?×10⁸m³/d in spring and?1.19-3.02?×10⁸m³/d in summer.In the west part of Bohai Bay,SFGD accounts for4-26%of SGD in spring and 9-49%in summer.SFGD is considered to be the main source of fresh water in Bohai Bay,which is 7.5-19.0 times of river discharge even in the wet season of summer.The results of two ²²²Rn mass balance models with and without considering RSGD-loss are very close to each other.Wit the difference ranging from 0.3-1%.