Tuesday, April 17, 2012

Bioretention to Meet Port Water Quality Goals

By Kim Slack, PE

The Port of Vancouver, USA operates more than 800 acres of industrial and marine transportation property with about 50 tenants. Primarily used for exports, the Port’s five marine terminals are used for the trade of bulk, break bulk, automobiles, and a variety of special project cargoes with Japan, Southeast Asia, China, South America, Europe, and Egypt. About 16 percent of US wheat exports are shipped through the port, which also provides one of Subaru’s two points of entry on the West Coast.

Providing permanent treatment Best Management Practices (BMPs) for stormwater runoff was required when the port developed its 50-acre Terminal 2 facility for the handling and staging of marine cargo in 1999. The port constructed a parallel bioswale to treat stormwater runoff from Terminal 2 in accordance with the City of Vancouver’s Phase II Municipal Stormwater Permit requirements. In Washington, many industries including ports are required to monitor their stormwater discharges and compare monitoring results to benchmark levels included in the State of Washington Department of Ecology Industrial Stormwater General Permit (ISGP). Sampling results showed over time that the bioswale was not performing up to ISGP benchmark levels so the port took corrective action.

Kennedy/Jenks Consultants worked with the port to analyze several supplementary treatment alternatives, and recommended installation of a high flow bypass and conversion of the bioswale system to a bioretention system. The port constructed the bioretention system in late 2009. Effluent water quality data collected in 2010, 2011, and the first quarter of 2012 has shown vastly improved removal of total and dissolved copper, zinc, and turbidity when compared to the previous bioswale system.

Terminal 2 Activities and Water Quality Challenges
Terminal 2 contains four berths and approximately 50 acres of paved area used for the handling and staging of marine cargo, including bulk, breakbulk, wind turbine components, equipment, and mobile harbor cranes. Within Terminal 2 is a handling area for the export of recycled scrap metal. While stormwater runoff from this area is treated separately and discharges to the municipal sewer system, truck dragout and fugitive emissions from the process frequently reach surrounding areas at Terminal 2.

With the exception of some tenants’ individual stormwater permit coverage areas, stormwater discharges from Terminal 2 are covered under the ISGP. The ISGP requires quarterly monitoring of turbidity, pH, oil sheen, total copper, and total zinc in stormwater discharge against established benchmark values. Water quality data collected from the bioswale effluent from 2003 to 2009 showed good removal of solids and oil/grease, but poor removal of metals and turbidity.

Bioswale-Bioretention Conversion
In response to having exceeded these benchmarks and in preparation for increasing cargo volumes and new commodities to be handled at Terminal 2, the port initiated a Level 3 Corrective Action to re-evaluate the treatment of stormwater runoff from Terminal 2. Kennedy/Jenks Consultants conducted an analysis of Terminal 2’s drainage and treatment system and provided four alternative corrective actions. These alternatives included rehabilitation of the existing bioswale, conversion of the bioswale to a bioretention system with a high flow bypass, installation of a stormwater filtration system downstream of the bioswale, and installation of an active stormwater treatment system downstream of the bioswale.

The port elected to convert the bioswale to a bioretention system, which would offer hydrologic and water quality benefits through the retention of runoff, evapotranspiration, and plant uptake of the metals that have proved to be so problematic and prevalent in industrial stormwater runoff. Further water quality testing of the facility’s influent and effluent flow, including both total and dissolved fractions of metals, was also recommended to more fully assess the site’s treatment needs.

The Terminal 2 bioswale was converted to a bioretention system in December 2009. The bioretention system was designed to treat up to and including the water quality flow rate defined as the flow rate at or below which 91 percent of the runoff volume, as estimated by an approved continuous runoff model, will be treated as required by Ecology and City of Vancouver standards. A high flow bypass structure was installed upstream so that flows exceeding the water quality flow rate are diverted directly to the system’s effluent piping.

Treatment flows first enter the upgraded system through CleanWay MetalZorb® media inserts installed in the system’s two inlet pipes for preliminary metals removal through ion absorption and filtration. An inlet bay with an oil boom installed along its perimeter then collects runoff and provides preliminary sedimentation and oil reduction. Flow is then dispersed evenly across the facility’s 12,500 square feet of surface area and infiltrated through 30 inches of engineered soil media, which was specified in final design as gravelly sand with 40 percent compost by volume to be mixed until a minimum infiltration rate of 20 inches per hour is achieved.

The surface of the bioretention system was planted with common and spreading rushes, Columbia sedges, and New Zealand orange sedges, which are particularly suited for metals uptake in the Pacific Northwest environment. The system design allows for ponding of the treatment flows to a depth of one foot in the facility, and 3 catch basins installed along the outlet end provide internal bypass drainage for depths exceeding one foot. All three internal bypass catch basins are equipped with CleanWay MetalZorb® inserts to provide treatment for bypassed flows.

As silty soils exist beneath the facility severely limiting infiltration, 8-inch perforated PVC underdrain piping installed beneath the engineered treatment media collects and discharges treated runoff through the existing outfall to the Columbia River.

Initial performance of the bioretention system saw “bathtub” conditions following irregular heavy rains that occurred during the 2009-2010 winter season, indicating that infiltration capacity of the media blend that was initially installed was insufficient to handle high flows. This caused ponding to occur up to the internal bypass catch basins, so most runoff passed through the system in bypass mode. During this time, an infiltration rate of approximately one-sixteenth of an inch per hour was estimated. After the heavy rains had passed and the ponding had subsided, plant plugs were found uprooted and deposited around the internal bypass catch basins. Post-construction soil testing indicated that the engineered soil blend was nearly 90% compost by volume, rather than 40% as specified. These conditions were rectified in August 2010 by the removal and re-installation of the plants and infiltration media, ensuring the originally specified compost ratio.

Although much of the preliminary influent and effluent water quality testing was conducted during bypass conditions, the data shows good removal efficiency for turbidity and metals. Additional influent and effluent testing for dissolved metals will be conducted as the site activities occurring in Terminal 2 continue to evolve to more fully assess the system’s treatment efficiency. 2011 and early 2012 quarterly monitoring under the port’s ISGP requirements shows effluent concentrations of turbidity, copper, and zinc below benchmark values.

The construction project to convert the existing bioswale to a bioretention system took 62 days and required a city grading permit and stormwater plan review. The project was completed according to the State Environmental Policy Act (SEPA) process. Final design and permitting costs were approximately $43,700 and the final construction cost was approximately $246,700.

Largest of its Kind
The Port of Vancouver should be commended for its vision in development of this ground breaking retrofit project. The Terminal 2 bioretention system is the largest facility of its kind known to exist and represents one of the first port facilities implementing Low Impact Development technology to treat runoff from a large marine terminal. The early success of the system in reducing concentrations of difficult to control metals in stormwater runoff has piqued the interest of the Department of Ecology and is the model for other large bioretention facilities being designed for other northwest Port industrial properties at the Ports of Tacoma and Seattle.

This article was originally published in the February 15 issue of The Water Report (www.thewaterreport.com).

Kim Slack is a civil/environmental engineer with the Portland office of Kennedy/Jenks Consultants, Inc., an engineering and environmental consulting firm. She is experienced in the design of stormwater treatment, detention, and retention systems. She received a Bachelor of Science in civil engineering from Montana State University and is a registered engineer in the state of Oregon.