Gold mining impairs the biological water quality of a culturally important river and UNESCO World Heritage Site in Nigeria.

The UNESCO World Heritage Sites have special conservation regulations because they are recognized as having significant cultural and/or natural value. One of Nigeria's two UNESCO-designated World Heritage Sites is the Osun River. It has immense biodiversity values, which are currently being threatened due to gold mining activities within its catchment area. Thus, this study seeks to ascertain the current biological water quality of this culturally and internationally important river using ecological and ecotoxicological approaches. Samples were collected from three environmental compartments (i.e., water, benthic sediment, and macroinvertebrates) in both dry and wet seasons from the section of the river that flows through the UNESCO site. Findings revealed that the river's physiochemical water quality has immensely degraded on account of extremely high total suspended solids, extremely low water transparency, and low dissolved oxygen. Three heavy metals exceeded their permissible limits in drinking water; i.e., Cd (364-367 times higher), Pb (75-104 times higher), and As (35-67 times higher). The contamination factor indicated that the benthic sediments were contaminated with Cd and Au, while the geo-accumulation (Igeo) index only indicates contamination on account of Cd, though Au ranks second in order of Igeo. The composition of the river's macroinvertebrate fauna suggests that it has been severely impacted because there was almost no stress-sensitive Ephemeroptera-Plecoptera-Trichoptera taxon present (one taxon and one individual only). Almost all of the chosen heavy metals had bioaccumulated in the three functional feeding guilds of macroinvertebrates. The current levels of heavy metals in the three environmental compartments of the river pose a great risk to both freshwater biodiversity and human health. Thus, a re-channeling of the upstream effluents from the river or any other natural water is advised. Also, a periodic environmental audit of the river should be conducted until it recovers from the pollution stress.