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Author      Title/Abstract      

Roof Runoff As a Zinc Source: BMP Effectiveness at Seattle-Tacoma International Airport
Author: Scott Tobiason, Bob Duffner, Aaron Moldver, Sara Mueller
Date: 5/106
StormCon 2006 Conference

As part of ongoing adaptive management for reducing metals in stormwater under its NPDES program, the Port of Seattle has applied two BMP strategies to reduce zinc and associated toxicity in roof runoff at Seattle-Tacoma International Airport (Sea-Tac). One roof was retrofitted with downspout media filters in 2003, while four others were painted to reduce zinc leaching. Accounting for about 30% of the total impervious area draining to one outfall, three of these metal roofs were painted in mid 2004 to stem the source of zinc. Before and after leaching tests on control sections showed the painting would reduce zinc levels by about an order of magnitude. However, zinc concentrations in monthly samples at the “outfall” did not decrease immediately after painting as expected. Investigations found accumulated sediments in the roof gutters had substantial concentrations of total and leachable zinc that likely were contributing to the elevated zinc runoff after roof painting. After these sediments were removed, median zinc concentrations at the outfall dropped by about 37% (change in median from181 ìg/l to 114 ìg/l after BMPs), were less variable, and were less than the 117 ìg/l benchmark more often (47% after vs. 19% before). In another, much larger subbasin, a similar metal roof had runoff treated by downspout organic media filters, which have been shown to remove about 70-80% of the total and dissolved zinc. In this case however, outfall zinc concentrations have not shown an obvious decline, most likely because the particular roof is a minor portion (~1%) of the total impervious area draining to the outfall. The initial capital expense for the media treatment system was about $0.75/ft2 (area treated), which was less than ½ the cost of painting.