Radium isotopes distribution and submarine groundwater discharge in the Bohai Sea

Abstract

In order to characterize the radium distribution and submarine groundwater discharge (SGD) in the Bohai Sea area, the three isotopes 223Ra, 224Ra and 228Ra in different water bodies were measured in a 4-year period with the radium-delayed coincidence counting (RaDeCC) system. The results indicated that radium activities take an order of groundwater > coastal seawater > river water > central seawater. The radium activities of groundwater were controlled by the lithology, human activities and salinity. Groundwater in aquifers with higher content of Th and U like igneous and metamorphic has higher radium activities, high salinity and oil exploitation which leads to groundwater contamination can also cause the higher radium activities. The radium isotopes of offshore waters were controlled by their respective onshore groundwater and SGD flux rates. The relative level of radium Ra isotope activity of coastal groundwater and nearshore seawater is determined by the interaction between groundwater and seawater. It is concluded that smaller radium mass transport from bottom sediments and the majority radium were supplied from landward groundwater from horizontal and vertical distribution of Ra isotope. The seasonal investigation showed that the seawater radium activities of 223Ra, 224Ra and 228Ra in autumn were higher than that in spring. Based on the seasonal variation of radium activities and the mass balance of radium, SGD of the Bohai Sea during rainy season were calculated to be 1.43 × 108 m3/d (3.73 × 10−3 m3/m2/d) in northern region and 2.84 × 107 (1.76 × 10−3 m3/m2/d) in western region by 228Ra.