Heavy Metal Concentrations in Water and Fish Tissues of the Bagmati River, Central Nepal

Abstract

Heavy metal contamination poses a growing threat to aquatic ecosystems, yet little is known about its spatiotemporal variation and bioaccumulation in the Bagmati River, Nepal. This study assessed seasonal and site-specific patterns of Cu, Cd, Pb, and Zn in river water and examined their accumulation in two native fish species, Schistura spp. and Garra spp. Using aligned rank transform (ART) two-way ANOVA, Cu and Zn showed significant seasonal variation. Cd was influenced primarily by season on the basis of two-way ANOVA, and Pb remained largely undetectable in water. Overall metal dominance followed Zn > Cd > Cu, except at far-downstream post-monsoon sites (Zn > Cu > Cd). Bioaccumulation patterns were metal-, tissue-, and species-specific: Cu and Zn accumulated preferentially in gills, Cd levels remained low with minimal variation, and Pb displayed strong site- and season-dependent uptake. In Schistura, muscle Cu was significantly related to water Cu (R² = 0.155, p = 0.0039), whereas in Garra, gill Cu reflected water concentrations and then influenced muscle levels. Cd and Pb exhibited strong water–gill–muscle linkages, whereas Zn regulation differed between species. Zn concentrations in gills ranged from 1.048 ± 0.418 to 5.298 ± 1.777 mg/L, significantly exceeding muscle levels (p < 0.001). Cu showed strong seasonal effects in Garra gills (F = 11.53, p < 0.001) and in Schistura muscles (F = 5.68, p = 0.004). Cd accumulation remained low (0.024–0.069 mg/L) with no significant spatiotemporal variation. This study identifies downstream areas and post-monsoon periods as relatively more contaminated and provides critical baseline information for ecological risk assessment in Bagmati River system.