Using radium isotopes to quantify submarine groundwater discharge at different scales in the Huanghe River Estuary, China

As an important land-ocean interaction process, submarine groundwater discharge (SGD) is composed of multiple dynamical processes at different scales and plays an important role in the study of coastal ocean geochemical budgets. However, most of the existing studies focus on the quantification of the total groundwater discharge, few studies are about the differentiation and quantification of groundwater discharge processes at different scales (i.e., short-scale SGD and long-scale SGD). As a world-class river, the Huanghe River is highly turbid and heavily regulated by humans. These natural and anthropogenic factors have a significant impact on groundwater discharge processes in the Huanghe River Estuary (HRE). In this study, the distribution patterns of the natural geochemical tracer radium isotopes (224Ra and 223Ra) and other hydrological parameters in the HRE were investigated during four cruises. By solving the mass balance of 224Ra and 223Ra in the HRE, the long-scale SGD flux was quantified as 0.01-0.19 m/d, and the short-scale SGD flux was 0.03-0.04 m/d. The rate of short-scale SGD remained essentially constant among seasons, while the rate of long-scale SGD varied considerably at different periods and showed a synchronous trend with the variation of river discharge. The results of this study are significant for understanding the SGD dynamics in the HRE and the contribution of SGD to the ocean geochemical budgets.