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Testing tile filters

Researchers take on the challenge of filtering nitrates from tile-line water.

Growers may approach field tiling differently in the future if they face rules to reduce nitrogen runoff. Tiles remove up to 90% of the water from fields, therefore serving as conduits for nitrates into waterways.

The requirement to lower nitrates in waterways may become a reality as the EPA looks at enforcing stricter water-quality rules. This means controlling nitrates in field tiles.

In an effort to find solutions for this problem, two research projects are filtering tile water for nitrates before it enters streams. Both projects use natural products or biomaterials for filtering.

Wood-chip filter. USDA Agricultural Research Service (ARS) researchers from the National Soil Tilth Lab in Ames, IA, installed the first project in Iowa last summer. The project involved digging trenches beside tile lines and filling them with wood chips to filter the water.

The original plan called for putting the wood chips above and below the tile lines, according to ARS researcher Tom Kaspar. However, the tiling contractor was unable to do this while maintaining the proper grade for drainage. So trenches were dug within 2 ft. of the tile line and on both sides. The trenches were filled with 4 ft. of chips and covered with soil. Kaspar says water is expected to flow through the chips, denitrifying it, and then exit the field in the tiles.

Ideally, the tile lines should be 6 ft. deep with the bottom 2 ft. filled with chips, he explains. The tile would be placed on top of the chips and another 2 ft. of chips dumped on top. In the future, he says, they should be able to work out the tiling problem.

Filtering results are not available from this project. Dry weather hindered measurements because water only started trickling through the tile lines in mid-May.

Another possible solution for filtering nitrates is to lower the tile lines but keep the outlets at the same depth. "Placing tiles deeper than normal will draw water from deeper in the soil," Kaspar says. "As water goes deeper, nitrate levels drop."

Corncob cell. A mixture of gravel and corncobs could be the ticket to cutting nitrates in tile-line water in another research project. Richard Cooke, University of Illinois agricultural engineer, has tested several biomaterial filters in the lab for denitrifying water. Most materials removed 90% of the nitrates in 8 hrs.

The research moves to an Illinois field this fall when Cooke will test his lab situations. He plans to install biofilters on a couple of farm sites. Each site will include five cells of biofilters on a tile line.

"Normally, one tile line leaves a field," Cooke says. "At about 50 ft. before it reaches the stream, we will branch off from the tile and run the tile water through cells."

The cells will each be 10 x 50 ft. and 3 ft. below the tile line. After testing several biomaterials for the filter, Cooke plans to use a mixture of half corncobs and half gravel. He says this waste material is a good carbon source to break down nitrates.

The biofilter should work like a wetland, he explains, except it is very concentrated and below the ground. A grower can still farm the ground above the cells. Cooke hopes the material in the cells will last at least 10 years and possibly up to 20 years.

"A lot of money is given for developing buffer strips," he adds. "But 90% of the water from fields passes through tiles."

A year from now, Cooke and his fellow researchers should know the results of their research and could start applying them to farm situations.

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