Dry-season sources of riverine sediment from the tropical mixed urban-agricultural watershed of the Mun River Basin in northeastern Thailand

Northeastern (NE) Thailand is well recognized as the land for rice cropping and consequently, it is potentially contaminated with chemicals from soil fertilization, pest/weed control, and urban discharges. The Mun River basin in NE Thailand principally is comprised of rice paddies and sedimentation in the river has been pronounced, but the studies on sediment source identification are limited. The current study aims to identify sediment sources in the Mun River basin and discuss possible impacts on ecosystems attributed to exposure to polluted sediment. The assessment relies upon physicochemical properties and heavy metal contents (iron (Fe), manganese (Mn), copper (Cu), chromium (Cr), nickel (Ni), and Zinc (Zn)) in the fine-grained sediment (<63 μm) taken from the Mun River basin and the Chi River in the dry season of 2020–2021. The findings revealed that the heavy metals in the sediment had not accumulated to levels of concern. Most sediment was from natural sources, including Fe- and Mn-rich sediment, Cr-dominant sediment, and Ni- and Cu-rich sediment. Fe- and Zn-rich sediment was, however, found in the urban watershed of Lam Ta Klong, and it could originate from mixed urban sources. Fe-rich sediment was predominant in the Mun River basin, which and could be a significant sink and source of toxic elements from agricultural and urban discharges. It is suggested that the toxic compounds in the sediment should be consistently monitored, whereas ecological and health risk assessment should be focused on the mobile forms.