University of Georgia Research Team Shows Streamside Management Zones Are Effective for Filtering Some Pollution - Streamside Management Zones Filter Nutrients, Not Herbicides

September 18, 2003

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Athens, Georgia - A new study by University of Georgia forestry researchers shows streamside management zones, a swath of trees left alongside streams to filter runoff from adjacent clearcuts, are more than 70 percent effective at filtering sediment and phosphorous -- but not herbicides. Funded by a $475,000 grant from the U.S. Environmental Protection Agency, the UGA scientists are now in the fourth of a five-year study to assess the effectiveness of current best management guidelines -- and to shed light on how they might be improved.

"If we can find a pattern to how and where breakthroughs occur, we can make corrections," said Rhett Jackson, a forest hyrdrologist in UGA's Warnell School of Forest Resources and principle investigator in the study. "Best management guidelines are always under revision as we gain more insights."

Best management practices have been in use for more than two decades, and most forest product companies follow them. But new pressures to regulate sediment levels in waterways prompted research to quantify the impacts of forestry activities on water quality.

Rather than conducting a traditional paired watershed study, researchers focused instead on the effectiveness of streamside management zones at filtering sediment, nutrients and commonly used herbicides.

Weyerhaeuser, Plum Creek Timber and International Paper made lands available for the research.

With their cooperation, and help from the Georgia Forestry Commission, researchers designed a series of experiments to demonstrate cause and effect.

They studied 30 different clearcuts on commercial timberlands across the Georgia's Piedmont, totaling more than 3,700 acres.

Where they found evidence of breakthroughs -- areas where there was obvious soil erosion and runoff moving through stream buffers after heavy rains -- they recorded the depth of the sediment in the flow area as well as instances of mud staining on leaves where the water level had risen, then receded.

An analysis showed about 5 percent of the land within these clearcuts allowed sediment to reach streams.

Erosion from clearcuts occurs in the first two years after harvest.

So, researchers say that over a typical 30-year rotation, about one-third of 1 percent of commercial timberlands contribute some sediment to streams at any given time.

"Some of our industry cooperators were genuinely surprised we found breakthroughs," said Jackson, "because they had followed the BMP guidelines closely.

On the other hand, some environmentalists were surprised that so little commercial timberland contributes to problems."

The study showed breakthroughs occurred primarily where large areas of a clearcut drain to a single point, where slopes are steep, and finally, where there is a high percentage of bare ground. Half of all breakthroughs were a result of road runoff.

"When you multiply the first three factors together, you get a very good predictor of which areas are likely to cause problems and which aren't," said Jackson.

"The data also illustrate why it's not necessarily helpful to increase buffer width [around waterways] uniformly when 95 percent of the time, current buffers are fine. The most effective thing would be to increase the buffer only where problems are likely to occur."

Jackson said it's difficult to see breakthroughs in the fall, since they're often hidden under fallen leaves. "You have to assess these areas in spring or early summer, and on a rainy day to get a true picture of what's going on," he added.

Researchers had assumed that once a breakthrough occurred, forested buffers would have little benefit, but this was not the case.

They set up silt fences in problem areas to catch and measure sediment transported in breakthroughs.

Silt fences set 40 feet inside a forested buffer collected less than 30 percent of the sediment caught by fences set at the edge of the buffer, showing that a 40-foot buffer was able to trap more than 70 percent of sediment in transport.

In another phase of the project, researchers mixed large vats of water, clay, sand and herbicides and then distributed the mixture onto experimental plots to simulate contaminated runoff moving through the forested buffers.

The plots were outfitted with samplers to collect runoff at various distances.

The buffers captured more than 70 percent of clay and phosphorous, but almost none of the soluble herbicides atrazine and picloram, which are commonly used in agriculture and forestry.

Researchers say the herbicide molecules will not attach to organic material and soil fast enough to be filtered in streamside management zones.

Experiments were conducted on plots with and without a litter layer in place. The removal of clay and phosphorous was higher where the litter layer was left intact and increased with litter depth.

"The litter layer is an important component in how well streamside management zones perform," said Jackson. "This also shows us that herbicide applications should be timed for when there is no rain in the forecast."

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