Publication Summary

This report compares the results of recently collected metals data in the state of Washington with models for metals partitioning that are advocated by EPA. New models were also developed using data from rivers in Washington. Technical guidance for permit managers is proposed to support water quality-based permitting for metals under the National Pollutant Discharge Elimination System (NPDES).

This report compares the results of recently collected metals data in the state of Washington with models for metals partitioning that are advocated by EPA. New models were also developed using data from rivers in Washington. Technical guidance for permit managers is proposed to support water quality-based permitting for metals under the National Pollutant Discharge Elimination System (NPDES).

The EPA screening models were found to poorly represent data from rivers in Washington. Dissolved fractions of Cd, Cu, and Zn were under-estimated, and the dissolved fraction of Pb was over-estimated by the EPA models. Under-estimation of the dissolved fraction of metals could lead to exceedence of the water quality criteria in NPDES permitting. These results indicate that the EPA models should not be used to estimate fractions of dissolved metals in Washington′s rivers. The poor performance of the EPA models for Cd, Cu, Pb, and Zn also suggests that the EPA models for other metals should not be used.

Empirical models from data in Washington were developed and proposed as an interim measure in NPDES permitting until site-specific data are available. Dissolved fractions of Cu and Zn were found to be significantly correlated with total suspended solids. Regression equations were developed to allow prediction of dissolved fractions of Cu and Zn from total suspended solids with a margin of safety to account for data variability. Dissolved fractions of Cd and Pb were found to be poorly correlated with other water quality variables. Estimated 90th and 95th percentiles of dissolved Cd and Pb fractions from available state-wide data were proposed for use in NPDES permitting in the absence of site-specific data.