Estimation Of Submarine Groundwater Discharge Into Laizhou Bay, China Using ²²²Rn

Submarine groundwater discharge(SGD) is a major component of the hydrological cycle and it represents an important process of land/ocean interactions in coastal zones. Driven by both terrestrial and marine forcing components, SGD comprises terrestrial fresh groundwater and re-circulated seawater. SGD has been widely recognized as a significant source of water and an important pathway for dissolved nutrients and chemicals transport from land to the ocean. Owing to its importance and significance in marine geochemistry and nearshore marine ecosystem,SGD has become the hot and difficult focus in the field of international hydrogeology.There is increasing evidence that the inputs of nutrients to the Bohai Sea are closely related to SGD. As one of the three bays in the Bohai Sea, China, Laizhou Bay is confronted by a variety of environmental problems. However, the estimations of SGD around Laizhou Bay are limited, especially in the eastern coast of the bay.In this study, Laizhou Bay, a semi-enclosed bay, is chosen as our study area so that the spatial and temporal variations of 222 Rn activities in groundwater and seawater are investigated. The flushing time and SGD in Laizhou Bay are estimated via the mass balance model and numerical simulations. Based on two different return flow factors obtained from a physical model and a mass balance model of 222 Rn and salinity,respectively, two different flushing times in May 2014 in Laizhou Bay were determined to be 40 d and 44.4 d. The flushing time is estimated to be 37.5 d using an apparent age of bay water model based on 222 Rn and 226 Ra isotopes. From the result of mass balance model, 222 Rn flux attributed to SGD accounted for 95.4% of the total inputs of 222 Rn to the study area. Our estimated total SGD flux into Laizhou Bay was5.92×108 m3 d-1, which was significantly higher than the Yellow Rive flux in 2014.The 222 Rn activities in groundwater were measured continuously for 24 hours in August 2014 in a typical silty-beach transect of Qingxiang Village located in the south of Laizhou Bay, China. The result indicated that 222 Rn in groundwater was spatially and temporally variable. Activity levels of 222 Rn were found to be higher inland and they generally decreased in the offshore direction. The 222 Rn activities in groundwaterdisplayed a distinct negative correlation with ORP, suggesting that reducing condition may increase the 222 Rn activities in groundwater.We deployed an in-situ 27 h continuous measurement of 222 Rn in the seawater in September 2014 in Xinzhuang Village located in the east of Laizhou Bay. A mass balance model that considered tidal effects, atmospheric loss, mixing loss and sediment diffusion and SGD was established. The estimated SGD values ranged from0 to 18.6 cm d-1, with an average of 6.93 cm d-1. Numerical simulation gave an estimation of the SGD rate to be 6.35 cm d-1, which was close to that estimated by222 Rn isotope. If the average SGD flux was applicable to the entire bay, the magnitude of SGD would be estimated to be 4.15×108 m3 d-1。The estimated nutrient fluxes via SGD into Laizhou Bay were 7.9×107 mol d-1 for DIN, 2.2×108 mol d-1 for TDN, and4.9×106 mol d-1 for TDP. These fluxes of nutrients via SGD were higher than those via Yellow River. This study confirms the importance of SGD in delivering materials to ocean, which has significant effects on the ecosystem.