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City presents five-year study showing permeable pavement plugs but removes key pollutants on Sharp Avenue
Summary
City wastewater staff reported results from a five-year effectiveness and durability study on permeable pavement installed on Sharp Avenue, saying the surfaces plugged over time but, after specialized cleaning, showed reductions in phosphorus and some metals; maintenance cost and durability raise questions about wide use on arterial streets.
Trey George, an environmental manager in the city wastewater management department, presented results Wednesday from a five-year study of permeable pavement installed on Sharp Avenue near Gonzaga University.
The study, which the city carried out to meet a stormwater permit requirement and with partial grant funding from the Washington State Department of Ecology, measured pavement condition, infiltration rates and water-quality treatment from 2019 through 2024. A QAPP, or quality assurance project plan, was approved before monitoring began.
City staff installed permeable asphalt in vehicle lanes and porous concrete in bike lanes and parking areas as a pilot and monitored infiltration with ASTM-standard tests, collected stormwater via an underdrain and autosampler, and analyzed pollutants. The pavement initially infiltrated at high rates but “plugged” with sediment and debris over time, reducing infiltration to near zero in monitored locations, George said. After contracting specialized high‑pressure wash-and-vacuum equipment, crews restored infiltration to the contract target (100 inches per hour) but required multiple passes and slower cleaning rates.
George told commissioners the cleaned pavement approached original infiltration performance but at notable cost: the contractor cleaned at roughly 1,000 square feet per hour and the city estimated about $0.60 per square foot for a deep clean—an expense that would be impractical to scale citywide. He also said porous concrete showed faster initial infiltration but deteriorated faster than porous asphalt in the commission’s pavement condition index (PCI) measurements.
On water quality, the city found treatment of several pollutants from the pavement and its base material—most notably reductions in total phosphorus and in metals including cadmium, lead and zinc. George said these results were “surprising” because the pavement and base contain no engineered biofiltration soils; the team expected limited treatment from aggregate alone. The presenter cautioned that background sampling locations and the origin of some analytes (for example, arsenic appearing in some samples) may complicate interpretation and will be discussed at greater length in the final effectiveness report to Ecology.
Commissioners and staff asked whether normal deicers were applied during the study; George said the city had requested minimal use to avoid affecting chemistry, but Gonzaga applied salts for pedestrian safety. Commissioners also asked about lifecycle cost, installation cost differences from conventional asphalt and whether the city would treat the study sections like adjacent asphalt now that the monitoring period has ended; George said the streets department would decide the long-term maintenance regime.
George concluded that permeable pavements can manage stormwater and remove some pollutants, but they plug and require specialized, routine maintenance; durability is challenged on higher-traffic arterials and the technology appears more viable for low-traffic areas such as parking lots and bike lanes.
The city will complete the final effectiveness study report for submittal to Ecology and said the water-quality data will be made available through Ecology’s database.
