Water-quality problems in urban and suburban streams are commonly blamed on stormwater pollution -- but actual pollutant data for the receiving streams are much more limited. To understand the extent and nature of toxicity due to metals, we analyzed 18 years (1993-2010) of metals concentrations in wet-weather streamflow from 33 stream stations across King County, Washington. We assessed aquatic and human-health toxicity using state and federal regulatory water quality standards (WQSs), plus non-regulatory salmonid-specific screening values (SSVs) since concentrations below the WQSs have been shown to harm salmonids. In addition, we tested for long-term trends, and compared stormflow and baseflow concentrations (during 2001-2003 when metals were also measured in baseflow).
To begin identifying and addressing pollution sources, we used a stoichiometric signal of weathering inputs (which includes both natural and human-accelerated erosion). When weathering is the dominant input, total concentrations of many metals have been shown to correlate well with total iron concentrations, since iron is naturally present at high enough concentrations to dwarf anthropogenic non-weathering inputs.
Results/Conclusions
Metals in wet-weather streamflow caused some potential aquatic toxicity, especially for salmonids. Mercury concentrations were frequently above the chronic WQS: 70% of streams had at least one exceedance. Copper, lead, and zinc concentrations were commonly above the SSVs (30% of all samples from all streams for copper, >30% for copper, and >90% for zinc).
Compared to baseflow, 13 metals had discernably higher concentrations in wet-weather streamflow. Nearly all of these metals, plus those above WQSs/SSVs, were well-correlated with iron. This suggests that the major source of these metals of concern was increased erosion during storms, rather than other types of pollutants. The iron-correlation analysis also identified a few streams that had multiple metals from non-weathering sources and would benefit from detailed pollution tracking.
Aggregating all streams, chromium, copper, lead, nickel, and zinc concentrations decreased over time. Calcium and magnesium concentrations increased, which can reduce the adverse effects of toxic metals. However, several individual streams had increasing trends in one or more toxic metals, so water quality has not improved universally.