An on-site bioassay and aquatic ecological survey were conducted on the Arkansas River near Pueblo, CO. A mobile bioassay trailer was used to test the toxicity of sewage treatment plant (STP) effluent using Arkansas River water for dilution. The aquatic survey was conducted upstream and downstream of the STP's outfall. This study was conceived in accordance with the intent of the U.S. Environmental Protection Agency's (EPA) site-specific water quality criteria guidelines, which recognize that national criteria may not be applicable to species in specific bodies of water because of biological, chemical, and physical variables.

Flow-through tests were conducted using bluegill sunfish (Lepomis macrochirus), fat-head minnow (Pimephales promelas), and a daphnid (Daphnia magna). Standard field and laboratory techniques were used to evaluate physical and biological components of the stream ecosystem. The physical components included flow, substrate, and qualitative fishery habitat description, and the biological components included benthic macroinvertebrate densities, diversity calculations, and fish population surveys.

The results of the bioassay tests generally indicated the following: (1) there were distinct differences in species'; sensitivity to the effluent (that is, the sensitivity was bluegills > fathead minnows > daphnid); (2) the required dissolved oxygen (DO) coCaretncentrations in the test tanks were difficult to maintain even though the flow-through rate was increased to four times that required by protocol; (3) the toxicity of the postchlorinated/dechlorinated effluent was less than that of the prechlorinated effluent; (4) no single pollutant could be targeted as the toxic fraction, however, concentrations of chlorine, cyanide, copper, and zinc were at or near LC⁵⁰ values for the test organisms.