The toxicity of effluent from up to 10 outfalls was monitored using laboratory tests with fathead minnow larvae and/or water flea (Ceriodaphnia dubia) from 1991 through 1998. During this time period, the type of tests (chronic vs acute), frequency of tests, and compliance limits changed. In general, the permit limits required that there be no adverse affect on minnows or water fleas in full-strength effluent. This stringent limit is based on the fact that the stream flow in Big Bayou and Little Bayou creeks can potentially be very near zero. In Figure 1, the results of toxicity tests of effluent from Outfall 001 are expressed in terms of chronic toxic units. The chronic toxic unit is equal to the 100/ IC25 where the IC25 is that concentration causing a 25% reduction in fathead minnow growth or Ceriodaphnia survival compared with controls. The higher the TUc, the more toxic an effluent; based on flows, a TUc > 1.0 would have been considered a noncompliance and an indicator of potential instream toxicity.
|Toxicity test results of effluents generally indicated only isolated periods where TUc exceeded a TUc of 1 (Figure 1). Follow-up tests of those effluents that demonstrated a TUc > 1 were consistently negative, indicating that any toxicity was limited in duration.|
|In 1998, effluent from Outfall 017 which discharges intermittently to Big Bayou Creek exceeded permit limits for acute (48-h) toxicity. A toxicity identification evaluation determined that zinc from repainted storage cylinders (Photo 1) was the likely source.|
Figure 1. Chronic toxicity units for toxicity tests of Outfall 001 at PGDP using fathead minnows and Ceriodaphnia for test dates from 1991 through 1998.
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