Estimating Submarine Groundwater Discharge (SGD) Into Laizhou Bay Etc Using Radium Isotopes

Submarine groundwater discharges (SGD) is one of the most important and typical interaction processes between groundwater and seawater in estuary and coastal areas. Driven by both terrestrial and marine forcing components, SGD comprises terrestrially-originated submarine fresh groundwater discharge (SFGD) and re-circulated seawater (RSGD). As an essential part of global water cycle, SGD has been widely recognized to be a pathway for enriching coastal waters in nutrients, carbon and metals, and has significant impacts and implications on coastal environment and ecology. As one of the three bays in the Bohai Sea, China, and with a shoreline of ～320 km and area of ～6000 km2, Laizhou Bay is an important coastal environment. It is subject to a variety of environmental stresses, and hence provides an archetype of a semi-enclosed bay for which ecological functioning is a sensitive issue. Most existing studies on SGD in the Bohai Sea are restricted to the Yellow River delta, the largest estuary in the Bohai Sea.As important geochemical tracers, radium isotopes can provide valuable information on water body mixing and SGD. The purpose of this study is to estimate the SFGD, SGD in Laizhou Bay and others using tracer methods (radium isotopes, salinity). A physical model based on the tidal prism and radium isotopes is used to estimate the flushing time, which is estimed to be 36.6±5.3 d. Using the estimated flushing time and coupled mass-balance models of water, salt and radium isotopes based on field data of 223Ra, 226Ra and salinity, the SFGD, SGD in Laizhou Bay could be estimated. We showed that the SFGD and SGD are 0.57-0.88 times and 7.35-8.57 times the annual Yellow River flux in August 2012, respectively. The value of SFGD ranges from 4.12×107 m3/d to 6.36×107 m3/d, while SGD value ranges from 5.32×108 m3/d to 6.20×108 m3/d. The proportion of the Yellow River input into Laizhou Bay was less than 14% of the total in August 2012.Based on a 1-D mixing model and the horizontal distribution of 224Ra in the outlet section of Laizhou Bay, the horizontal eddy diffusion coefficient was estimated to be 253.1×106 m2/d and 56.5×106 m2/d for the surface and the middle layers of the outlet section, respectively. The submarine groundwater discharge (SGD) into the bay was also estimated on the basis of the 224Ra mass balance model. The estimated SGD into the bay from the 224Ra model is 4.2×108 m3/d, approximately six times the annual average flux of the Yellow River.Naturally occurring radium isotope (226Ra) was also measured in water samples collected along two transects in the northern Bohai Bay in September 2012. Based on a tidal prism model, two different flushing time of the coastal water were determined to be 9.1 days and 11.5 days, corresponding to different values of the return flow factor (b) obtained from a physical model and a mass balance model of 226Ra and salinity, respectively. Using the derived flushing time, we developed a 226Ra mass balance model including mixing, sedimentary input and SGD to estimate the SGD into the northern Bohai Bay. The 226Ra budget indicated the 226Ra input from SGD accounted for 99% of the total tracer input to the northern Bohai Bay. We arrived at the average flux from SGD of 4.83×107m3/d.With new understanding of our assessments of the SGD inputs into Laizhou Bay and the northern Bohai Bay, the management of the Bay related to fresh groundwater resources, ecology and environment in coastal and offshore areas should be reviewed.