Using Radium Isotopes To Assess Submarine Groundwater Discharge And Its Nutrient Input In Bohai Bay

Submarine groundwater discharge?SGD?refers to the flow of all water from the sea floor to the nearshore waters on the edge of the continental shelf,regardless of the composition and driving force of the water.SGD is an important part of the global water cycle,and it is also an important path for the transport of water and various chemicals from the land to the ocean.Numerous studies have shown that near-shore groundwater is rich in nutrients,various metal ions,organic matter,and inorganic matter.It is an important source of various pollutants in coastal waters and has a very important impact on the ecological environment of coastal waters.Bohai Bay is surrounded by land on three sides and has poor exchange capacity with the open sea.Therefore,it is very important for protecting the ecological environment to carry out research on the discharge of submarine groundwater to Bohai Bay.There areonly a few local areas in Bohai Bay where SGD studies of have conducted.In this paper,²²³Ra with short half-life and ²²⁸Ra with long half-life are used to establish a radium isotope mass balance model of Bohai Bay water.The flushing time of Bohai Bay water body in spring is estimated to be 45.23 days,and the flux of submarine groundwater discharge flux is estimated to be 2.58×10⁹m³/d.By establishing the mass balance model of water and salt in Bohai Bay,it is estimated that the submarine fresh groundwater discharge flux in the Bohai Bay is 1.16×10⁸m³/d.The tidal prism model and apparent age model of the water body are also used to estimate the flushing time of Bohai Bay and the results are 47.55 days and43.07 days,respectively,which are similar to the results of the radium isotope mass balance model.The sample data show that,in spring,the isotope activity of radium in seawater is greater in the southern part than in the north.Therefore,this paper divides Bohai Bay into two regions:north and south,and sets up a radium mass balance model respectively.The flushing time of water bodies in the northern and southern parts of Bohai Bay is estimated to be 51.32 days and 48.99 days,respectively.SGD fluxes of the south and north were 1.15×10⁹ m³/d and 1.28×10⁹ m³/d,respectively.In the southern Bohai Bay,the SGD flux is close to the north,but the volume of seawater in the northern part of Bohai Bay is 1.7 times that of the Southern District.In this paper,we measured the contents of heavy metal ions and nutrient salts in samples.Seawater,groundwater,and river water in Bohai Bay reached the grade?standard for water quality with respect to the heavy metas chromium,copper,zinc,cadmium,and lead.The average concentrations of the Inorganic nitrogen and active phosphate in seawater are 26.27?mol/L and 2.90?mol/L,respectively.The average concentrations of inorganic nitrogen and active phosphate in groundwater were 37.84?mol/L and 4.74?mol/L,respectively.The SGD associated flux of inorganic nitrogen and active phosphate into the sea was 4.78×10⁸ mol/d,and 1.22×10⁷ mol/d,respectively.