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What’s in the stormwater, and where is it going?

By: Melissa Schneider / Published: Sep 13,  2013

In order to help city managers and planners understand how various types of development impact coastal pollution, University of Southern Mississippi’s (USM) Kevin Dillon is collecting runoff directly from stormwater ditches and pipes from various locations in Harrison and Jackson counties in Mississippi. The goal is to measure exactly how much nitrogen is entering the Mississippi Sound from various types of coastal landscapes, ranging from pristine to highly developed.

Testing what’s in stormwater is important, according to Dillon.

“Just think about all the chemicals people use in and around their homes — detergents, herbicides, pesticides and fertilizers just to name a few,” he said. “It’s a pretty broad gambit of chemical inputs, and many of them contain high concentrations of nutrients, such as phosphorus and nitrogen. Additionally, wastewater and stormwater systems are not totally separated in most municipalities; there’s an overflow system that can mix these types of waters during large storms. So it’s a real mixed bag of what’s in stormwater.”

Dillon has divided his testing sites into four categories: pristine, residential, hardened and integrated. The estuary at Grand Bay, a natural area on the Mississippi-Alabama border, is serving as his pristine site. He is tapping a few purely residential sites, that is, neighborhoods of mostly single-family homes, in Ocean Springs and Biloxi. The areas he designates “hardened” are primarily commercial, with the ground largely covered with buildings, roads and parking lots. In Biloxi, he’s tagged the Edgewater Mall, the Mississippi Coast Coliseum and the downtown area as hardened. The integrated sites are stormwater pipes along the beaches, which contain a varying mix of neighborhoods and commercial properties within their drainage basins.

Specifically, Dillon and his graduate student Joshua Allen are gauging the nitrogen levels coming off these various surfaces to see how much enters the Sound and how far out it goes. Why nitrogen? Nitrogen occurs naturally in the ecosystem and, when in the correct ratio, is an important part of the chemical balance of healthy marine water. However, when too much nitrogen is introduced, it can encourage eutrophication, or excess algal growth including harmful algal blooms. This in turn can alter the oxygen levels and can lead to hypoxia, or a so-called “dead zone” in the water. Because of high nitrogen levels in the Mississippi River from fertilizer run-off in the Grain Belt, the Gulf of Mexico already houses one of the largest marine dead zones on record. 

“You always hear about the ‘dead zone’ at the mouth of the Mississippi River,” Eric Meyer, city planner for Gautier, Miss., said. “I’ll be interested in finding out if we also have excess nutrient loads, because that’s something I’ll have to deal with.” 

Meyer added that typically city planners need the kind of hard data Dillon is gathering to justify changing existing regulations to protect the environment.

Dillon’s research group is measuring different nitrogen containing compounds (ammonium, nitrate, nitrite and dissolved organic nitrogen) in water collected directly from stormwater ditch output pipes at all sites except Grand Bay, where no such pipes exist. They collect on days when the area receives enough rainfall to provide sufficient sample amounts, typically more than one inch of rain, within a certain time limit. They are also collecting surface waters in the Mississippi Sound, deploying macroalgae and oysters along transects in the Sound to test these samples for stable nitrogen isotopes and using USM’s mass spectrometer to determine if stable isotope signatures can be used to trace stormwater nitrogen into the Sound. The study will ultimately help gauge levels of nitrogen that may have been introduced by the stormwater.

*Tara Skelton is a freelance writer in Ocean Springs, Miss.*

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