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(^(224)Ra and^(223)Ra)and other hydrological parameters in the HRE were investigated during four cruises.By solving the mass balance of^(224)Ra and^(223)Ra 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.

Submarine groundwater discharge and benthic biogeochemical zonation in the Huanghe River Estuary

Submarine groundwater discharge(SGD)has received increasing attention by studies on coastal areas;however,its effects on biogeochemical zonation have not been investigated to date.The Huanghe River Estuary(HRE)is a world class river estuary with high turbidity,and heavy human regulation.This study investigated how SGD is related to the benthic biogeochemistry of the HRE.Based on the distribution of several parameters(e.g.,salinity,temperature,dissolved oxygen(DO)levels,p H,radium isotopes,and nutrients),the HRE was subdivided into six different zones,and the SGD fluxes within each zone were quantified and compared.The highest SGD flux was found in the northwest nearshore zone,where it was more than one order of magnitude higher than in the offshore zone.High SGD resulted in low DO and p H,but high nutrient levels in the benthic boundary layer.The southeast nearshore zone was also characterized by high SGD flux,but benthic waters were more oxic because of the dominating inputs by the Huanghe River.These data suggest that such a zonation would help to understand benthic biogeochemical processes.High SGD may not only contribute to the estuarine nutrient budget,but may also contribute to the formation of hypoxia and acidification.