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Scientists find weirs effective against runoff

STARKVILLE — A Mississippi State University study has confirmed the success of a new technique to reduce nutrients in runoff water and protect downstream waters, including the Gulf of Mexico.

Weirs, also known as check dams, are small dams used to collect water runoff from agricultural fields. Weirs are often the size of a drainage ditch, with a two-foot channel in the center for water drainage. The weirs are concrete can be moved to various locations in a drainage ditch.

“As water from agricultural fields drains, high concentrations of fertilizer nutrients, such as nitrogen and phosphorus, can be carried downstream,” said Robbie Kröger, assistant professor of aquatic sciences in the MSU Department of Wildlife, Fisheries and Aquaculture. “These nutrients promote algal production and microbial decomposition in downstream coastal ecosystems like the Gulf of Mexico, which in turn decreases vital oxygen levels.”

Farmers throughout the Delta have been protecting water quality for years and are now using weirs as a low-cost method to immediately reduce nutrient runoff.

Most agricultural land is drained through a slotted riser that can be boarded to retain water within the drainage ditch. A pipe drains away the water.

“Several weirs can be stair-stepped throughout the drainage ditch to provide maximum nutrient retention,” Kröger said.

Wherever a weir is installed, water collects to form a miniature wetland, which may improve crop yields by adding moisture to the field.

Funded by the Mississippi-Alabama Sea Grant Consortium, the study evaluated the impact of low-grade weirs in an experimental design.

Results indicated that the weirs removed contaminants effectively. Weirs and outlets alone, however, cannot reduce nutrients. Vegetation is needed to absorb and retain nutrients.

A new project funded by the Environmental Protection Agency’s Gulf of Mexico Program Office is evaluating how weirs, slotted pipes and vegetation drainage ditches contribute to overall watershed nutrient reductions.

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