Associations of redox conditions and organic matter with nickel and chromium partitioning in the Bahodopi River, a tropical lateritic mining river in Central Sulawesi

Lateritic nickel mining can increase the transport of metal-bearing particles into river systems, where sedimentwater partitioning governs metal mobility and ecological exposure.Although redox conditions and organic matter are recognized as factors associated with metal partitioning, their relative importance under tropical lateritic mining conditions remains poorly understood.This study examined associations between redox-related and organic matter-related variables and nickel (Ni) and chromium (Cr) partitioning between sediment and water at nine stations along the Bahodopi River, Indonesia, sampled during the dry season, using log-transformed distribution coefficients (logKd).Oxidation-reduction potential (ORP) showed the strongest association with Ni partitioning (ρ = 0.756, p = 0.018), while sediment organic matter showed the strongest association with Cr partitioning (ρ = 0.689, p = 0.040), sediment Fe/Mn ratio and dissolved organic carbon showed weak, non-significant associations with either metal.However, because ORP and logKd both increased with distance from the upstream station, partial correlation analysis showed that the ORP-logKd association weakened substantially and lost significance once this spatial trend was accounted for, indicating that spatial confounding contributed to the observed relationship.Multiple regression models explained a large share of variance in logKd for both metals, although only the Ni model was statistically significant, and diagnostics showed that model fit was disproportionately influenced by a small number of stations.These findings suggest that Ni and Cr partitioning may be associated with different environmental variables in this system, but given the small number of stations, single sampling campaign, and evidence of spatial confounding, these associations should be regarded as preliminary and warrant confirmation through spatially and temporally replicated sampling.