Left: Construction of a third runway at Seattle's busy Sea-Tac Airport—for which trucks will haul in 17 million cubic yards of dirt fill—will help alleviate the nearly $100 million additional operating costs incurred yearly due to delays. Photo: Port of Seattle. Above: Stormwater runoff systems throughout the third-runway construction area include sand-filter tanks and an 8×10-foot monitoring station. Photo: Hach Co.
Miller Creek, a natural stream in the construction area, has been enhanced with poles and wires, which hold shade cloth during warmer months to keep the creek cool until native plants can provide natural shade. Photo: Port of Seattle
Construction projects in Washington must compare the turbidity of Stormwater runoff to that of the receiving water to assess the qualitative impact. Yet, the Department of Ecology is requiring in the project 401 permit that the Port of Seattle demonstrate a daily average of no more than 5 nephelometric turbidity units (NTU) in its discharge to receiving streams during the massive embankment construction.
“The profile of the project, literally, and the runoff dynamics are changing daily, so we were looking for continuous runoff monitoring instead of batch testing,” said John Drabek, supervisor and environmental engineer for the department's Water Quality Program.
Natural Site Solutions (NSS), Redmond, Wash., is the project's Stormwater management consultant. NSS engaged Bakersfield, Calif.-based Rain for Rent to design an onsite monitoring and treatment system. The innovative solution includes a containment pond that can collect 10 million gallons or more of runoff via natural drainage after a rain event. After settling, durable online sensors from Hach Co., Loveland, Colo., continuously measure the pH and turbidity of the water and send readings to a common controller for display and recording.
Based on the influent turbidity, the operators adjust the dosing of chitosan flocculent, a 100% biodegradable biopolymer derived from crab and shrimp shells, to enhance coagulation of sediment particles and optimize filtration by sand-media filters. “If the influent is within prescribed turbidity and pH criteria for treatment, the sensor controller directs it to the sand filters. If the influent exceeds those criteria, the sensor controller diverts it back to the retention structure, to protect the filters and minimize cleaning and servicing,” said Paul Reilly, Rain for Rent division manager.
A second pair of water-quality sensors monitors the filtration effluent, which typically exhibits turbidity of 1 to 3 NTU and pH of 6.7 to 7.4, and sends measurement signals to a second common controller that diverts all runoff that doesn't meet discharge requirements back for repeat treatment, assuring that any stormwater leaving the site is in compliance. Sensor measurements recorded by this controller every five minutes are downloaded weekly for management reports prepared by NSS for the Washington Department of Ecology.
From its installation in September 2004 (the beginning of the rainy season) through April 2005, seven of these onsite systems installed throughout 180 acres of the third-runway construction area have handled about 68 million gallons of runoff prior to discharge to local steelhead streams.
“They have been very effective, consistently achieving 5 NTU or less in discharge to receiving waters,” said Drabek.