Water Quality and Sediment Mercury Risk Analysis in an ASGM-Impacted Rawas River

Background: Illegal artisanal and small-scale gold mining (ASGM) can degrade river water quality while transferring mercury (Hg) from the water column to sediments, where contamination may persist beyond the sampling period. Objective: This study assessed water quality, pollution status, sediment Hg contamination, potential ecological risk, spatial differences, and associations between Hg and water-quality parameters in the Rawas River, Indonesia. Methods: A quantitative observational survey was conducted during a single sampling campaign at upstream, midstream, and downstream stations, with three spatial replicates per station, yielding nine water samples and nine sediment samples. Results: Total suspended solids ranged from 159.50 to 260.33 mg/L, while dissolved oxygen ranged from 0.43 to 0.57 mg/L. Pollution Index values of 2.62–3.36 classified all stations as lightly polluted. Mean water Hg concentrations remained below the Class II standard, although one downstream sample reached 0.00240 mg/L. Mean sediment Hg concentrations ranged from 0.12 to 0.20 mg/kg and exceeded the geochemical reference value of 0.08 mg/kg at every station. The CF indicated moderate contamination, Igeo indicated unpolluted to slightly polluted conditions, and the ecological risk was moderate upstream but considerable in the midstream and downstream areas. ANOVA and Pearson correlation analyses showed no statistically significant results (p > 0.05). Conclusion: The paired water–sediment evidence indicates that water-only compliance measurements may understate persistent sediment Hg risk, supporting integrated monitoring in ASGM-affected tropical rivers.