Gill lesions and death of bluegill in an acid mine drainage mixing zone

The toxicity of an acid mine drainage (AMD) mixing zone was investigated by placing bluegill (Lepomis macrochirus) at the confluence of a stream conta minated by AMD and a stream having neutral pH. A mixing channel receiving w ater from both streams was assembled in the field, during July and October 1996, to determine the toxicity of freshly mixed and aged water (2.9-7.5 mi n). The AMD stream had elevated concentrations of Al and Fe, which precipit ated upon mixing, and of Mn, which did not precipitate in the mixing zone. Fish exposed to freshly mixed water had higher mortality than fish exposed to water after aging. Precipitating Al, but not Fe, accumulated on the gill s of bluegill, and accumulation was more rapid early during the mixing proc ess than after aging. Fish exposed for 3.5 h to freshly mixed water had hyp ertrophy and hyperplasia of gill filament and lamellar epithelial cells. Si milar lesions were observed after 6.0 h in fish exposed to water aged after mixing. Results demonstrated that Al was the predominant metal accumulatin g on the gills of fish in this AMD mixing zone, and that mixing zones can b e more toxic than AMD streams in equilibrium.