[A-HW27] Biodiversity, nutrients and other materials in ecosystems from headwaters to coasts

[AHW27-15] Temporal and Spatial Variations in Fulvic Acid-Bound Heavy Metal Concentrations in the Asahi and Takahashi Rivers, Okayama Prefecture

Rohdof Lactem Yengeh¹, *Hiroaki Somura¹, Toshitsugu Moroizumi¹, Yasushi Mori¹, Morihiro Maeda¹ (1.Graduate School of Environmental, Life, Natural Science and Technology, Okayama University)

Keywords:dissolved organic matter, watershed, aquatic environments, seasonal variations

Heavy metals are critical contaminants and are potentially harmful to the sustainability of coastal environments. Fulvic acid plays a crucial role in mobilizing terrigenous heavy metals by forming complex compounds that are subsequently transported to the marine ecosystem via riverine migration. Since heavy metals are non-biodegradable, their bioaccumulation in aquatic organisms eventually poses risks to human health through food chain transmission. However, the behavior of fulvic acid-bound Mn, Al, Cu, and Pb concentrations in the Asahi and Takahashi Rivers which drain into the Seto Inland Sea remains poorly understood. Therefore, the main objective of this study was to assess the temporal and spatial variations in fulvic acid-bound Mn, Al, Cu, and Pb concentrations in these rivers. Regular weekly and monthly samplings were conducted for the Asahi and Takahashi Rivers, respectively, from March 2022 to March 2024. Sampling in the Takahashi River encompassed ten sites spanning from the headwaters to downstream. Fieldwork involved on-site measurements of physical parameters (water temperature, pH, electrical conductivity, oxidation-reduction potential, turbidity, dissolved oxygen, total dissolved solids, and salinity) using a multi-parameter Water Quality Monitor (HORIBA U-50 series, Japan), alongside the collection of surface water samples for laboratory analysis. The concentrations of dissolved Mn, Al, Cu, and Pb were determined using an ICPE-9820 (SHIMADZU, Japan). Statistical analyses were performed using the R package (version 4.4.2). The results indicated that both dissolved and fulvic acid-bound Mn, Al, Cu, and Pb concentrations in the Asahi River exhibited seasonal variability across spring, summer, autumn, and winter. Moreover, in the Takahashi River, the dissolved Mn, Al, Cu, and Pb concentrations displayed distinct spatial distribution patterns. Additionally, it was observed that the concentrations of both dissolved and fulvic acid-bound Al in both rivers were higher than those of other heavy metals. Furthermore, in both rivers, the concentrations of dissolved Pb exceeded the environmental quality standards for surface water established by Japan and the World Health Organization.