Environmental characteristics of mercury and arsenic in a eutrophic estuary under hydrodynamic influence

There is a significant lack of information regarding the interrelation between estuarine mixing, eutrophic conditions, and the presence of heavy metals dissolved in the seawater of many estuaries, particularly those affected by human activities globally. This study, based on field data amassed in summer of 2019 within the Pearl River Estuary (PRE; situated in the South China), aims to analyze the distribution patterns of mercury (Hg) and arsenic (As) within the seawater. Additionally, it identifies the extent of influence from estuarine mixing process and eutrophication on mercury and arsenic in seawater through the application of multivariate statistical methodologies. The results indicate that salinity varied within the ranges of 0.56-22.71 in surface waters and 1.56-30.59 in bottom waters. The mean level of suspended particulate matter (SPM) in surface waters was 17.15 mg/L, increasing to 23.39 mg/L in the bottom layer. The spatial variation in As presented a descending trend from the upper estuary to its mouth, with levels in the surface layer ranging from 2.8 to 5.0 μg/L (with a mean of 4.0 μg/L). Conversely, the spatial variation in Hg ranged from 0.014 to 0.029 μg/L, with a mean of 0.019 μg/L. The estuary's eutrophic status is categorized as "medium to high," indicating a significant correlation with the characteristics of these two metals. The results of principal component analysis and regression analysis have elucidated that terrestrial inputs (including nutrients, DO) and the process of fresh-seawater mixing are the predominant factors shaping the behavior of Hg and As. Furthermore, it is suggested that nitrogen and As may share a common origin, whereas Hg originates from a distinct source. These findings provide scientific backing for the integrated prevention and control of marine pollution in this estuary, extending to the broader Guangdong-Hong Kong-Macao Greater Bay Area.