Speciation of Heavy Metals in Water and Channel Sediments of the Lower Mekong River

The speciation of heavy metals in water and channel sediments was investigated along a 680 km reach of the lower Mekong River in Thailand, encompassing 17 sites on the Mekong River and 6 sites on the Mun River, a major tributary. Ten heavy metals—Mn, Fe, Cr, Co, Ni, Cu, Zn, As, Cd, and Pb—were quantified in surface water and riverbed sediments using a modified 4-stage sequential extraction protocol incorporating water-soluble (F1), exchangeable (F2), reducible (F3), and residual (F4) fractions. The resulting geochemical data were evaluated against quality standards, the Risk Assessment Code (RAC), a proposed Modified Risk Assessment Code (MRAC), and the Contamination Factor (CF). Water-soluble extraction (F1) was used to evaluate the proposed MRAC. The surface water quality complies with Thailand’s National Standard at all sites. However, localized sediment contamination was identified and can be categorized into five priority groups. The group with the highest priority consists of Cr, Ni, and Cu. The concentrations of these exceeded national sediment quality standards, and surpasses the Probable Effect Concentration (PEC) at several sites, indicating a high probability of adverse ecological effects. Furthermore, MRAC identified them as having a high risk of mobilizing at ambient conditions in the dry season. The Mun River shows no substantial effect on Mekong hydrochemistry, but it may contribute Cr and Ni loading downstream.