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September 16, 2004
Vehicles contribute to air pollution. Many technological advancements try to address that issue alone. Yet vehicles also contribute to another form of pollution just as critical: roadway runoff.
Nearly all roadway runoff pollutants are related to motorized vehicles.
Oil drippings and other engine fluids deposit heavy metals, phosphorus, hydrocarbons and other toxic organic compounds on the roadway. Deteriorating tires, brake linings, thrust bearings and engine crankshafts also leave behind heavy metals.
Airborne pollutants in dust, rain and snow can contribute to the pollutant content on roadways, particularly in urban areas. Litter and organic debris also pollute the runoff.
Problems with treatment
Roadway surfaces compound the problem by preventing rainfall and roadway runoff from entering native soils where natural processes remove pollutants.
Runoff is typically collected and conveyed to central treatment facilities through storm sewers or open ditches. Treatment facilities rely on filtration, settling and chemical or mechanical treatment to remove pollutants from the runoff before releasing it back into the environment.
Such processes require a sizable financial investment in the collection system and the treatment facility. These treatment facilities are often located within sensitive areas such as wetlands or forests, and remove another crucial part of the natural environment.
Treatment without impact
Installing grass filter strips adjacent to a roadway shoulder to receive "sheet flow" directly off the road would more naturally filter out most pollutants in the runoff. Oftentimes, however, small channels form within the filter strip, creating "shortcuts" through which highway runoff bypasses the filtering qualities of the grass.
So there are two crucial questions: How can we design filtration facilities that mimic nature but do not require man-made systems to collect and convey the polluted runoff? And how can we minimize the amount of land that must be used to treat the runoff?
The Washington State Department of Transportation began testing a series of facilities on state Route 167 in King County that addressed both questions.
Initial studies consisted a small grass filter strip followed by a trench backfilled with pea gravel and other materials. The theory was that two treatment facilities would perform better than a stand-alone grass filter strip as well as reduce the overall footprint of the treatment system. This treatment system is called ecology embankment, a recently developed technique to help improve the quality of roadway runoff.
The ecology embankment's unique design takes advantage of the linear nature of most road projects and provides treatment to the runoff without collecting and conveying it in a separate system.
The pea gravel is the primary component of the bed, and it filters any remaining sediment after the runoff passes through the filter strip. The dolomite and gypsum add alkalinity and ion exchange capacity to promote the precipitation and exchange of heavy metals.
The perlite retains moisture to promote the growth of both grassy vegetation and biomass. The ecology mix has an estimated long-term infiltration rate of 25 inches per hour and a design infiltration rate of 14 inches per hour.
The SR-167 performance study has shown the ecology embankments to be extremely effective in the removal of sediment and dissolved heavy metals. Phosphorous removal is also promising.
Sediment load removal in highway runoff (referred to as "total suspended solids") was shown to be between 88 percent and 94 percent. (The state Department of Ecology Stormwater Management Manual for Western Washington requires 80 percent removal of total suspended solids.)
Removal of dissolved zinc, which is often representative of all heavy metals, was shown to be between 84 percent and 96 percent. (The DOE requires a "higher removal rate of dissolved metals than basic treatment facilities," which typically remove less than 50 percent of the dissolved metal load. )
For phosphorous removal, the DOE requires a 50 percent reduction goal for certain influents. Research is continuing to evaluate whether ecology embankments are meeting this requirement.
Ecology embankments show great promise as water quality treatment facilities. Results demonstrate that ecology embankments are extremely effective in removing most typical pollutants encountered in roadway runoff.
In the three years ecology embankments have been studied on SR-167, they've required no maintenance other than periodic mowing. The ecology mix has shown little decrease in hydraulic capacity due to sediment buildup, but a determination must be made when or if the mix must be replaced at some point.
The embankment does an excellent job of mimicking nature when runoff sheets off the roadway, gets treated on the shoulder, then flows through its natural drainage course.
Eric Dawson is a project manager at Entranco. Jay Cammermeyer is a water quality specialist with the Washington State Department of Transportation's Northwest Region Hydraulics and Water Quality Program.