Updating NPDES Stormwater Effluent Limits for Zinc Using a Metal Translator and the Biotic Ligand Model

A receiving water study showed that zinc effluent limits for the stormwater discharged under an Illinois industrial facility’s national pollutant discharge elimination system (NPDES) permit could be increased. Data collected in 2017 (n=12) yielded a zinc translator of 0.54, which if applied, would increase the 306-μg/L total zinc limit by a factor of 1.8. Observed mean hardness (300 mg/L) has little effect on the acute zinc ambient water quality criteria (AWQC) basis of the current limit, but a biotic ligand model (BLM)-based zinc AWQC would be an average of 2.3 times greater. Together, the translator and BLM yield a potential limit fourfold greater than the current zinc limit. Increasing the zinc limit would not only improve the accuracy of environmental protection—it would also allow broader and potentially more economical source control and stormwater treatment alternatives than would otherwise be possible.