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Marsh grasses key to coastal water quality

The salt marsh is a delicate ecosystem that many forms of life call home: zooplankton and periwinkles, ribbed mussels and green crabs, fish such as the mummichog and silverside, the fox and the great blue heron, myriad marsh grasses, and on some days, a bipedal visitor armed with fluorescent dye.

The visitor is Heidi Nepf, assistant professor in the Department of Civil and Environmental Engineering. And what brings her to the marsh is Spartina alterniflora, a sea grass common to New England marsh flats.

As a buffer between land and sea, the salt marsh plays an important role in regulating the amount of nutrients and pollutants that are shunted from the land into coastal waters. With increased residential development around coastal areas, coastal water quality is increasingly affected by fertilizers, pesticides and the leaching of septic waste. The degree of this impact depends in part on the filtering capacity of the salt marsh system.

Along with tidal currents, marsh vegetation is a critical factor in determining how various substances are transported, diluted and deposited within the marsh. Professor Nepf's work focuses on understanding the hydrodynamic effects of vegetation to better predict circulation patterns in a salt marsh. Her research is funded by MIT Sea Grant through the Henry L. and Grace Doherty Professorship in Ocean Utilization.

Because vegetation is an obstruction, it enhances the diffusion of substances in the water. "If you pull something through a liquid, it will mix and stir, just like if you pull a spoon through coffee," explained Professor Nepf. "People know that, in general, vegetation can enhance mixing. I am trying to quantify the mixing effect and create a basic model that can be used to predict the mixing in different systems of marine vegetation."

The first part of Professor Nepf's study has focused on a simple model, with imitation plant stems constructed of dowels attached to plywood boards. By varying the number of dowels and their proximity to each other, Professor Nepf mimics the varying densities of marsh vegetation.

Laboratory experiments conducted with Jennifer Sullivan, a graduate student in civil and environmental engineering, will compare predictions from the model to observations in Woods Hole's Great Sipewisset Marsh on Cape Cod.

Along with an improved understanding of how nutrients, pollutants and different animal species are dispersed, Professor Nepf's findings could also have engineering applications. "There's a lot of interest in constructed wetlands," she said, because observations have shown that water flowing through natural wetlands "comes out cleaner, may have lower levels of suspended sediment and sometimes even has lower chemical constituencies." The idea "is to create a wetland and let waste water flow through it, emerging cleaner on the other side." In this manner, constructed wetlands would provide a natural way of treating waste water.

A version of this article appeared in MIT Tech Talk on July 24, 1996.

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