Center for the Inland Bays (CIB) staff members have wrapped up study of add-on pollution filters for a stormwater management system in South Bethany – and word is they just don’t work in the field.
Town officials were hoping to boost effective pollution reduction at a Delaware Department of Transportation- (DelDOT-) installed catch basin between Route 1 and the Anchorage Canal. They were targeting hydrocarbons, in particular — fuel, oil or restaurant grease — that leave a slick on the water after a hard rain, and, specifically, in the town’s northernmost canal.
As CIB’s Chris Bason explained, the Anchorage Canal is fielding runoff from a very large area — 63 acres in all. By way of comparison, he said the other canals around South Bethany received runoff from just 2.5 acres, on average.
Anchorage receives runoff from Sea Colony and all the houses east and west of Route 1, Bason said. And between 50 and 58 percent of that area is covered by impervious surface.
Any precipitation quickly becomes highway and driveway rinse water, he pointed out, picking up pollutants as it flows toward the canal. “In addition to all that, there are already water quality problems in the canal because of poor flushing,” Bason added.
Bason said the Department of Natural Resources and Environmental Control (DNREC) gave the DelDOT catch basin, or “sediment control forebay,” an efficiency rating of 28 percent back in 2004. In other words, 72 percent of the pollution was still getting through.
The town asked the CIB to look at possible filter systems that could be suspended under the existing storm grate — but Bason’s report suggested the town will have to keep looking for solutions elsewhere.
“Lab efficiency is very high for hydrocarbons,” Bason noted of the filter-testing process. “But what they’re doing in the lab is very slowly pouring a lot of water through the filters.
“Any time you have high flows, a lot of that water’s going to bypass the filters,” Bason explained, pointing to the various overflow valves scattered atop the filter assembly.
He cited one filter study, conducted in Washington state, suggesting the filters performed quite well during 1-inch rainstorms (catching 90 percent of pollutants). But performance levels dropped significantly past the 1-inch mark, and filters caught less than 30 percent of the pollutants in 2-inch rain event.
Washington might have Delaware beat for total annual rainfall, but 2-inch downpours are hardly rare in this area.
And even if the rains fall gently, Bason said both the Washington study and another conducted by the Civil Engineers Review Foundation (CERF), suggested any filtration would require “intensive maintenance regimes” to stay ahead of clogging.
Cities could probably afford pumper truck service, but smaller towns like South Bethany would have to find some other way to keep the filters clear, Bason pointed out.
Bottom line: he didn’t recommend filters as an overall solution — although he did say they might be appropriate near “acute” sources, like gas stations or heavily-trafficked businesses like fast-food restaurants.
But since he’d been forced to advise against one option for improving water quality, Bason went ahead and offered some alternatives. They were “green technologies,” supporting low-impact development, as encouraged under Gov. Ruth Ann Minner’s Livable Delaware initiative.
“South Bethany’s pretty well built up,” Bason noted. “There’s not much room for a wet retention pond, so you need to take a decentralized approach.”
The frst rule of thumb, he said, is to increase perviousness. And for something better than the average lawn, he recommended “rain gardens” (low-lying planting beds, with good drainage) or bioretention swales (grassy ditches that slow and naturally filter runoff before it reaches waterway).
Bason suggested the town might even consider replacing asphalt pavement with porous concrete, or decorative pavers, in some areas.
Other solutions ranged from the far-out (vegetative roof covers) to the mundane (better grading), with a lot of common sense in between. “You’d be surprised by the number of simple things that can be done to increase the effectiveness of your stormwater management,” Bason asserted.
In some cases, homeowners could make a big difference simply by redirecting flow from their downspouts, he said. (This would require coordination with the neighbors in some cases, he pointed out.)
Or, they could use rain barrels at downspouts, to at least slow down the flow of water coming out of the pipe — “Imagine, if you have three of these on a house, and every house has them,” he suggested. Bason said the town might even consider some kind of cost-share program for the rain barrels.
And finally, he recommended plantings — canopy trees where possible, native plants and especially native grasses in the Route 1 median. Kentucky bluegrass might be the standard, but its root systems are typically less than 3 inches deep, he said. Switchgrass root systems, by way of comparison, could run as deep as 9 feet.
“Probably less in coastal areas, but it would still be a lot better than Kentucky bluegrass,” Bason pointed out. “And it’s attractive.”
DNREC doesn’t offer funding for low-impact development specifically, he added, but there is a possibility of some grant money from the department’s Non-Point Source (NPS) Program. (Point sources, like municipal sewer outfalls, are easy to spot, but DNREC is working to track down and regulate NPS pollution, too.)
Bason’s report is available online, at www.inlandbays.org.
University of Delaware’s College of Marine Studies in Lewes hosts the center’s STAC meetings, although the CIB will soon have a space of its own, in a remodeled U.S. Coast Guard barracks at Delaware Seashore State Park.