How It Works: Prairie Line Trail Stormwater Treatment

Main page content

A rain garden on UW Tacoma's segment of the Prairie Line Trail treats stormwater from a 42-acre area above campus. Here's how it works.

Above: several "cells" of the Prairie Line Trail bioretention facility, or rain garden, on the UW Tacoma campus. Photo courtesy Jeff Caven.

View of Prairie Line Trail rain garden showing rails reinstalled along historic transcontinental train route. Photo courtesy Jeff Caven.Visitors to the UW Tacoma campus, first-timers and old-timers alike, are intrigued by a garden-like feature at the south end of the campus’s segment of the Prairie Line Trail. It’s got plants, and rusty steel stuff like brackets and troughs, and oftentimes has water more-or-less flowing through it.

Is it art? Is it a research project? Is it historic?

Depending on your point of view, it could be said to be all of the above. Actually, though, it’s a regional stormwater treatment facility.

It’s the result of a creative partnership among the City of Tacoma, UW Tacoma and the Washington State Department of Ecology. It’s a key feature in a quest to demonstrate how regions can deal with one of the most difficult problems we face in cleaning up polluted waters: the problem of non-point source pollution.

Since the passage by the U.S. Congress of the first Clean Water Act in 1972, significant progress has been made addressing pollution that originates from so-called point sources: factories, sewage treatment plants, and similar fixed, identifiable sources. The bigger challenge has been non-point sources, diffuse things like run-off from streets, parking lots, farm fields and the like.

Click to view a Google Map showing location of Prairie Line Trail biotreatment facility on UW Tacoma campus.Given that it’s the wet side of the state, it should come as no surprise that western Washington is an innovative laboratory for urban run-off pollution control. UW Tacoma, the City and other agencies have been partnering in this area for many years. The Prairie Line Trail stormwater treatment facility is just one project. The Center for Urban Waters along the Thea Foss Waterway, with its green roof, permeable pavement, water reuse and rain garden is another partnership between the City, UW Tacoma and Puget Sound Partnership.

The Prairie Line Trail bioretention facility (aka “rain garden”) treats runoff from 42 acres of existing developed urban areas upstream of the facility before the runoff is discharged into the Thea Foss Waterway and ultimately to Puget Sound. Think of all the stuff stormwater can pick up as it flows along street gutters. Not just urban trash, but sediment, gasoline, motor oil and heavy metals.

A sign alongside the facility explains, through text and a diagram, how the system works. You can see the diagram and the text from the sign below.

The plants in the rain garden are carefully selected varieties that can actually help treat the pollutants in the stormwater runoff that passes amidst them.  The filtering soil media captures and holds sediment and removes pollutants from the stormwater.

Some of the steel you see in the Prairie Line Trail rain garden is what distributes stormwater runoff evenly within the “cells”. But, running in parallel lines from one end of the rain garden to the other, suspended like little bridges over the plants and the runoff, and continuing up and down the trail into the distance in either direction, are … steel rails. Set 4 ft. 8 ½ in. apart from one another (standard track gauge in North America), the rails are in the historic location of the Northern Pacific Railway’s Prairie Line, which ran from Tacoma’s Commencement Bay south and east, creating in 1873 the first northern transcontinental line linking the Pacific and Atlantic Oceans.

Art, science and history – the Prairie Line Trail stormwater treatment facility is all those, and it’s improving the water quality of Puget Sound, too.

Schematic diagram of Prairie Line Trail bioretention facility. Illustration courtesy City of Tacoma.

A Sign Describes Where the Water Flows

A permanent sign adjacent to the Prairie Line Trail rain garden explains how stormwater flows through the system. A schematic diagram (above) is accompanied by text (below).

"When rain falls, it collects pollutants from streets, parking lots and other surfaces and washes them into local waterways. Stormwater runoff is one of the biggest sources of pollution into the Thea Foss Waterway and Puget Sound. In order to help keep our waters clean, the City of Tacoma has partnered with the University of Washington and the Washington State Department of Ecology to construct an innovative regional stormwater treatment facility along the Prairie Line Trail. The facility treats stormwater from 42 acres of previously untreated urban surfaces.

"The regional stormwater treatment system consists of two main elements: a pretreatment unit and a bioretention facility. The pretreatment unit, located in a neighboring parking lot, is an underground facility designed to remove trash and large debris from stormwater runoff. The bioretention facility, located along the Prairie Line Trail, consists of six treatment cells that are filled with filtering soil media which removes pollutants including fine sediment and metals. Plants growing in the filtering soil media aid in treatment of the stormwater runoff. Treated stormwater is collected beneath the filtering soil media and conveyed back into the City stormwater system.

"The regional stormwater treatment facility was designed to be an asset to the University of Washington Tacoma’s Prairie Line Trail that recognizes the historical significance of the railway. The rail lines from the original route of the transcontinental Northern Pacific Railroad run through the facility in their historic location."

Illustration and sign text courtesy City of Tacoma.


Written by: 
John Burkhardt / August 14, 2015
Media contact: 

John Burkhardt, UW Tacoma Communications, 253-692-4536 or