Artisanal and small-scale gold mining (ASGM) releases mercury into tropical freshwater systems, yet integrated assessments of Southeast Asian oxbow lakes remain scarce. We quantified total mercury (THg) in water, sediment, and fish from Lake Hanjalutung, an ASGM-affected oxbow lake in Central Kalimantan, Indonesia. Water and sediment were sampled at five river and lake stations in the dry (July 2025) and rainy (November 2025) seasons, and thirteen fish of five species were obtained from a lake net-cage unit in the rainy season. THg was measured by cold-vapor and thermal-decomposition atomic absorption spectrometry. Water THg was higher in the dry season (26.13–42.66 ng/L) than in the rainy season (8.20–24.05 ng/L). Geoaccumulation index (I_geo) values were higher in the lake interior (1.65–2.01) than in the river channels (0.53–1.46), consistent with Hg retention in the lake. Fish THg ranged from 0.16 to 1.51 mg/kg dw. Bioaccumulation factors (BAF; fish/water) ranged from 5.3 × 10³ to 39.2 × 10³ L/kg, and biota–sediment accumulation factors (BSAF; fish/sediment) exceeded unity for all species. Both indices correlated positively with body length, although this association largely reflected interspecific differences. Maximum allowable weekly intake (MAWI) estimates indicate a potential exposure risk. Intake estimated from regional data exceeded the MAWI for Wallago leeri by 9- to 12-fold for one-year-old children and approximately 8-fold for pregnant women. Preferential consumption of lower-Hg species, such as Pristolepis grootii, may increase allowable intake approximately 3.5-fold, a preliminary finding requiring confirmation.

Artisanal and small-scale gold mining (ASGM) has driven mercury contamination in tropical freshwater systems, yet integrated assessments of Southeast Asian oxbow lakes remain rare. This study quantified total mercury (THg) concentrations in water, sediment, and fish from Lake Hanjalutung, an ASGM-affected oxbow lake in Central Kalimantan, Indonesia. Water THg was higher in the dry season (26.13–42.66 ng/L) than the rainy season (8.20–24.05 ng/L), whereas lake interior sediment was more enriched during the rainy season. Geoaccumulation index (I_geo) values increased from the river channels to the lake interior (1.7–2.0, moderately to heavily contaminated), confirming that the lake acts as a mercury sink. Fish THg concentrations ranged from 0.16 to 1.51 mg/kg dw. Bioaccumulation factors (BAF) ranged from 5.3 × 10³ to 39.0 × 10³ L/kg, and biota-sediment accumulation factors (BSAF) exceeded unity for all species, with both indices increasing significantly with body length, indicating body size as the primary driver of bioaccumulation. Maximum allowable weekly intake (MAWI) analysis showed that estimated regional consumption exceeded safe thresholds for all receptor groups, by 9- to 12-fold for one-year-old children and approximately 8-fold for pregnant women consuming W. leeri. Substituting with P. grootii consistently increased allowable intake by 3.5-fold, offering a practical, species-based risk reduction strategy for communities dependent on freshwater fish for protein.