Highway stormwater media filter drains: modified design evaluation

Maxwell Freimund

Stormwater runoff from highways is known to contain zinc and copper which may impact aquatic life. The media filter drain (MFD) is an approved Best Management Practice (BMP) for treating stormwater in Washington State. The original MFD design (old mix) is based on an aggregate gradation that is no longer consistent with the Washington Standard Gravel Specification. An alternate design (new mix) has been proposed that is consistent with the current specification which has fewer aggregate fines passing the U.S. No. 4 sieve (10-30% for new mix versus 30-56% for old mix). Experimentation was carried out using 30 6-inch diameter by 12-inch deep columns. Of these, 22 were filled with the new mix, and eight were filled with the old mix. 16 of these new columns and all eight old columns were subjected to five events of simulated stormwater loading representative of large storms with contributing runon from 10 times the MFD area, at typical (10 and 100 ppb for copper and zinc, respectively) or high (100 and 500 ppb for copper and zinc, respectively) concentrations at loading rates of 30 or 50in/hr. These tests showed that the new design had slightly better removal rates for copper than the old design (medians of 82% and 63%, and means of 78% and 65%, respectively) and the new design had slightly lower removal rates for zinc than the old design (medians of 88% and 96%, and means of 89% and 95%, respectively). Both designs had higher median removal rates for both zinc and copper than a variety of typical stormwater control BMPs, which may qualify them as enhanced treatment methods. The six remaining new columns were subjected to low volumes of very highly concentrated metals solution to simulate years of metals loading in nine months. Periodically, two of the new columns were tested for performance using the aforementioned high concentration metals solution. These tests indicated no loss in performance for the new design even after many years of simulated metals loading (more than 14 years based on 40 inches annual rainfall and runon from 10 times the MFD area).

Highway stormwater media filter drains: modified design evaluation

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