04 August 2010

Roads, shopping centers and housing developments are generally seen as enhancing a community's quality of life. But they can also disrupt the delicate water cycle that provides food, recreation and hydration to communities.

When rain and melting snows flow over a city's streets, parking lots and sidewalks, that runoff picks up all kinds of pollutants such as residues of petroleum and pesticides, soaps and solvents, as well as pet and wildlife waste.

Too often, the run-off spills directly into local waterways. The U.S. Environmental Protection Agency, or EPA, lists stormwater as the primary cause of water pollution in the United States, and encourages local communities to develop management plans to treat run-off before it pollutes their rivers, lakes or streams.

Easy to use computer model

Bill Hunt, a professor of Biology and Agricultural Engineering at North Carolina State University, has developed an easy-to-use computer model to predict how landscape impacts water pollution.

"There are different ways of evaluating how well stormwater practices clean water ... things like ponds and wetlands and permeable pavement."

The program asks users - developers or state regulators, for example - to enter a description of the local land types, such as parking lots, rooftops, roads, grasses or woods, along with any water treatment areas.

"This model essentially assesses how well (stormwater practices) work in a new way. And it's a break from the way states have done it previously."

Communities across the U.S. have become more aware of the dangers of untreated stormwater. Hunt's computer program gives them a useful new tool for addressing the problem.

The model works by using a relatively new understanding of the water filtration process. Engineers have found that wetlands, grassy fields and soil or sand filters can take in stormwater with different levels of pollution and release water with a relatively constant cleanliness. One important factor affecting the water quality is the area's temperature and rainfall, and Hunt's model is set up for the mid-Atlantic region of the United States. However, Hunt says the principles can easily be applied to other temperate climates.

Nitrogen and phosphorous

To encourage widespread use among land design firms and regulators, Hunt kept his computer model simple. It uses average yearly data in its computations, and focuses on only two major pollutants - nitrogen and phosphorous.

"If you have an imbalance, or literally too much nitrogen and too much phosphorous getting into those systems, you can essentially deplete the oxygen in that water. Which, in the end, without oxygen you have fish kills, and the like."

Phosphorous from detergents often contaminates urban stormwater, while nitrogen comes primarily from fertilizer use in agricultural areas. According to Ben Urbonas, president of the Urban Watersheds Research Institute, these nutrient contaminants tend to be more of a problem in closed water systems, like lakes, where concentrations can build up.

Wastewater first, then stormwater

In developing countries, the main source of water pollution is not stormwater, but waste water. Urbonas says communities there must focus first on developing a proper wastewater treatment system, using more complex models to plan collection and treatment.

"I think the priorities are totally skewed when people start looking at stormwater when in reality they need to be looking at what is the primary biggest culprit in what their receiving waters are being degraded by."

While many developing-world communities might not be ready for Hunt's runoff management model, Urbonas believes it is important for them to be aware of the pollutants present in stormwater even as they learn to manage their community's waste water.

A small community of design firms and regulators in North Carolina is currently testing the model, and after this trial period, it will be available free to the public on Bill Hunt's university faculty webpage.