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Researchers try to grasp how wetland plants purify water


L
ike a clown twisting a balloon into a poodle, humans mold resources into substances that suit our needs. The countryside is dotted with fill-in-the-blank processing plants that make, among other things, our food fit to eat and our water fit to drink.

Water treatment facilities spend billions of dollars every year trying to bring contaminated water within the drinking standards set by the Environmental Protection Agency. Peter Jaffé, professor of civil and environmental engineering, is studying ways to help Mother Nature do the processing for us.

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Photo by Frank Wojciechowski

Professor Peter Jaffé researches the impact that trace metals have on wetland biology.

Studying trace metals

Professor Jaffé is studying trace metals such as chromium and arsenic in wetlands and how plantlife affects the related biogeochemical cycles.

Wetland surface waters are a combination of precipitation, road runoff, agricultural runoff, industrial waste, and other waters—all of which are served up in an unsavory cocktail of contaminants from their sources.

Vegetation in these wetlands can reduce the toxicity of surface waters, and different plants have different purification abilities.

Professor Jaffé and his group are studying a variety of flora and their effects on trace metals. This research could help scientists and planners “design” wetlands to process local waters and serve other purposes as well.

“The gist is, we’re not biologists or geologists,” Professor Jaffé said, “but we want to understand these processes in a quantitative manner sufficiently enough to design a wetland. That’s the long-term goal.”

Chromium focus

Professor Jaffé has recently focused on chromium in these wetland surface waters.

According to the Agency for Toxic Substances and Disease Registry, chromium is released into surface waters primarily by electroplating, leather tanning, and textile industries.

Chromium is generally found in three common stable valence states, chromium (O), (III), and (VI).

Chromium (VI) is water soluble and considered to be about 1,000 times more toxic than chromium (III). It may be a human carcinogen, although there is not enough evidence to definitively prove it.

In wetland sediments, chromium (VI) reacts with organic materials in the soil and is reduced to chromium (III). The aim is to pull as much chromium (VI) out of the water and into the sediments as possible. Professor Jaffé is trying to discover which plants will do that job best. He is studying two common, hardy wetland plants: cattails and phragmites reeds.

In the lab, his researchers created microcosms of various, typical wetland sediments with controls, cattails, and phragmites.

The first piece of the puzzle was to see just how much chromium (VI) was in the microcosm soils. This showed that the microcosms planted with cattails had much less chromium (VI) in the pore water of the sediments than did the phragmites.

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Photo by Peter Jaffé

Cattail was grown in a laboratory-created wetland microcosm to study its ability to purify water.

Not so simple

On this information alone, one might conclude that the cattails were reducing more chromium (VI) than were the phragmites. Yet the issue is not quite that simple.

“It’s a complex interplay, and just by looking at a single, simple measurement we can’t understand what is really happening,” Professor

Jaffé said. “So that’s why a combination of experiments and numerical modeling are done to tease out what is going on.”

The experiments also study another key element in the ability of these plants to immobilize chromium—their rate of evapotranspiration. Evapotranspiration is the loss of water from plant leaves.

“We’re looking at contaminated water moving slowly through a swamp,” Professor Jaffé said. “If a plant is losing a lot of water through evapotranspiration, it will need more water to replace it. So that plant will suck more freestanding water down into the sediments, and we want as much in the sediments as possible, where the chromium will be reduced and therefore not enter surface waters.”

After this experiment, it became clear that the phragmites reeds evapotranspired more than twice the amount of water than the cattails.

After factoring this in, Professor Jaffé believes that phragmites reeds are capable of reducing more chromium (VI) every day than cattails.

Phragmites, however, is an invasive species that can take over wetlands. Some ecologists would be wary of any wetland design strategy that pushed for the planting of this species.

One of the species that was driven out by the pushy phragmites is spartina, a native species from the Meadowlands area. Professor Jaffé and his group are now doing microcosm studies on spartina, to see how well it reduces chromium (VI).

“The question is ‘Does it make sense to replant spartina if we want to immobilize metals’?” Professor Jaffé asked.

He recently applied for a grant from the National Science Foundation to do another wetland research project to study the role of detention ponds in processing nitrogen in urban wetlands.

Research applicable to local wetlands study


S
ome of Professor Jaffé’s research has been used to educate the public and policy-makers about the integral role wetlands play in the health of watersheds. The project is run by the Stony Brook Millstone Watershed Association (SBMWA) and is supported by a grant from the Environmental Protection Agency, under their grant program Science to Achieve Results (STAR).

Professor Jaffé has assisted SBMWA in compiling a survey that tested citizens’ and public officials’ knowledge of, interest in, and opinion of wetlands. His findings about the dynamics of trace metals in wetland sediments contributed to the survey.

Survey questions examined knowledge about how to identify wetlands, wetlands’ abilities to treat contaminated water, wetlands’ ability to protect against flooding, and the protection of wetlands by state regulations.

Questions also examined opinions about developing or preserving local wetlands, the perceived benefits of wetland preservation, and the amount of trust citizens can place in various entities entrusted with the protection of the wetlands. For more information about the SBMWA, see www.thewatershed.org.

 

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