As Pittsburgh knows too well, pollution can be relative. For instance, Pittsburgh's air remains among the dirtiest in the nation — but remember how bad it used to be?
Likewise, Pittsburgh has few environmental stories prouder than the restoration of Nine Mile Run. Decades ago, most of this stream was buried 'neath our eastern neighborhoods and suburbs. What remained in daylight was a typically trashed urban waterway flowing through Frick Park ... and past a steel-mill slag heap.
In 2006, the City of Pittsburgh and the U.S. Army Corps of Engineers completed a $7.7 million restoration of the day-lit portion of the stream (plus a tributary in Frick). Now, for the mile-and-a-half before it joins the Monongahela River — from Braddock Avenue on down — Nine Mile Run looks more like it should, with a reconstructed streambed and accompanying wetlands, native trees and wildflowers. Even in the shadow of I-376 — and that slag heap's pricey housing development — it's an oasis.
Yet this mission isn't accomplished. A new study reminds us just how far Nine Mile Run has to go.
It's about nitrogen, which can starve waterways of oxygen. Excess nitrogen helps create "dead zones" like the Gulf of Mexico's, an infamous patch of water the size of New Jersey where nothing grows.
There, the culprit is runoff from farms — especially artificial fertilizer — in the Mississippi River watershed. But for the two-thirds of its length upstream of Braddock Avenue, where it's buried in concrete, Nine Mile Run isn't fed mostly by streams and groundwater. Rather, it's supplied by storm sewers in its 6-square-mile watershed, which includes Wilkinsburg, Edgewood and parts of Pittsburgh. Sources of nitrogen include automobile engines and power plants, whose emissions coat roofs, roads and parking lots. Stormwater washes them into waterways, along with things like road salt and trash.
Another big source of nitrogen is, well, poop. Contamination of waterways by leaky sewers is a problem long known but little studied. But following a two-year investigation, University of Pittsburgh researchers recently estimated that as much as 12 percent of the sewage produced in the watershed leaks into Nine Mile Run. That's up to 20 tons of nitrogen.
Excess nitrogen can cause problems like suffocating algae blooms. "It's just not good," says Marion Divers, the Pitt doctoral candidate who was the study's lead author.
It's unclear, though, how all this nitrogen is affecting Nine Mile Run. In fact, since the restoration, the stream is returning to health in many ways: A 2010 survey, for instance, found 14 species of fish, up from just five in 2007. And the estimated total mass of those fish was up 1,000-fold.
But nitrogen pollution isn't the only problem from leaky sewers. Everybody knows that many of the region's aging sewers were designed to overflow into streams during storms, and that you shouldn't play in the river for a while afterward. In smaller urban streams, that bacteria problem is magnified. In Nine Mile Run, for instance, levels of E. coli and fecal coliform are unsafe for humans year-round, storm or no.
Even when the water looks clear, "It's truly not a good idea to walk barefoot in the stream," says Brenda Smith, executive director of the Nine Mile Run Watershed Association, the group tasked with protecting the stream. At the culvert where Nine Mile Run first sees daylight, you can smell the sewage.
The Watershed Association's new strategic plan focuses on reducing the flow of both raw sewage and stormwater runoff. It's a tall order, but the good news is that we know what works. Fixing buried pipes is costly, but you can reduce runoff by increasing surfaces for water to soak into — things like green roofs and permeable pavement. This also helps with nitrogen. "The more water you put back into groundwater, you're creating an environment that's conducive to denitrification [the process of removing or reducing the nitrogen]," says Pitt's Divers.
Moreover, bacterial contamination in Nine Mile Run is dramatically lower downstream of the culvert, simply because of the riffles, or mini-waterfalls, that oxygenate the water. Streams want to be clean. We just have to let them.