Site-Specific Water Quality Standards
Frequently, effluent limitations are derived using multiple factors set at worst-case levels to derive numeric water quality standards (e.g., ammonia-nitrogen toxicity is pH/ temperature dependent; toxicity for several metals is hardness-dependent). H&A assesses the multiple contributing factors to determine which have the most significant impact and prepares evaluations to assess the appropriate levels that should be used for limit derivation. These evaluations assess ambient concentration data and correlate these concentrations with the other factors used in the limit derivation procedure.

In certain circumstances, the toxicity of a pollutant is significantly altered following the treatment of municipal effluents (e.g., copper). This is due to complexing between the pollutant and dissolved organic carbon in the treated wastewater. Significant relief can be obtained if this effect can be documented. H&A has instituted several studies to determine the water effect ratio (WER) that results under these circumstances. Once documented, the WER acts as a multiplier to increase the effluent limitation from the unadjusted condition.