This paper presents a case study of how a weight-of-evidence approach may be used to suggest effluent limits foraluminum are not necessary to protect water quality from a pulp mill discharge to Leaf River, MS. First, theNational AWQC for aluminum (and, hence, the state WQS) is overly conservative for protection of aquatic life, andmay warrant adjustment/recalculation owing to scientific uncertainties, and lack of fish species in Leaf River whichare critical to criteria calculations. Second, whole effluent toxicity (WET) tests conducted for LRPO haveconsistently passed discharge permit requirements. This result is consistent with extensive field studies whichsuggest that Leaf River aquatic communities are not impaired by LRPO effluent discharge. Furthermore,concentrations of aluminum in Leaf River are naturally elevated, and empirical data demonstrate that LRPOdischarge has no statistically significant effect on downstream aluminum concentrations. Our evaluation ofaluminum toxicity in site waters highlighted both the lack of aluminum toxicity and the complexity of aluminumchemistry. Several regulatory mechanisms were explored as a means of avoiding a NPDES permit limit foraluminum. MDEQ was persuaded that aluminum in effluent does not present a threat to the river’s water quality butstill believed that a permit limit was necessary. The NPDES permit limit was based on a no net addition ofaluminum relative to upstream waters including accounting for statistical variation in water quality. LRPO was ableto modify the calculation used to derive the permit limit to accurately account for population variation and yielding apermit limit that was accepted to both parties.
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