USE OF OUTDOOR ARTIFICIAL STREAMS TO DETERMINE THRESHOLD TOXICITY CONCENTRATIONS FOR A PETROCHEMICAL EFFLUENT

A system of six partly flow-through, partly recirculating, streams was constructed outdoors from stainless steel and divided into pool and riffle sections to closely simulate a natural stream environment. The organic fraction extracted from the effluent of a petrochemical manufacturing plant was identified as a mixture of chlorinated ethers. The effluent extract was applied to three of the streams to maintain nominal concentrations of chlorinated ethers of 0.5, 1.0, or 2.0 mg/L for a period of 28 d. A fourth stream was treated with 0.25 mg/L for 21 d; this was then increased to 4.0 mg/L for a further 7 d. Two streams were untreated and served as controls. There were no detectable effects of the treatments on macroinvertebrate population densities. There was a marked effect on the drift of macroinvertebrates at a concentration of 2.0 mg/L, but there was no effect at lower concentrations. There was an effect on the feeding rates of Gammarus pulex at concentrations of 1.0, 2.0, and 4.0 mg/L, but there were no effects at concentrations of 0.25 and 0.5 mg/L. The results of this experiment were compared with those obtained in 7-d, single-species toxicity tests. On the basis of the toxicity tests, the no-observed-effect concentration (NOEC) for the most sensitive biological response-growth of Daphnia magna - was 1.0 mg/L. On the basis of the field test, the NOEC for the most sensitive biological response-feeding rate of G. pulex-was 0.5 mg/L.