In their third year, ISU researchers are finding that wood chips may be the most useful filter in drainage systems to remove nitrates from water.

In an experiment, a team of ISU researchers made a standard drainage tile similar to those found in Iowa. They then dug two new trenches on both sides of the tile and filled them with wood chips.

“The water has to move through the wood chips to get out of the fields,” said Dan Jaynes, project leader of the Agricultural Research Service.

Jaynes, associate professor in agronomy, said that other materials were also tested in the laboratory, such as cornstalks, sawdust, cardboard and newspapers.

Colin Greenan, graduate student in soil science, tested these different materials.

“In the lab, the cornstalks helped remove the highest amount of nitrates – and really fast,” Greenan said.

Although the cornstalks worked very well, the results were found in the lab and Greenan said different results could occur when tested in the field. For example, he said cornstalks deteriorated much faster than wood chips, and this meant the material would need to be replaced more often.

Greenan said other such studies have been done with other materials in different situations, but this is the first study that studies the effects of surrounding a tile drainage system with wood chips.

The more complex issue the research is getting into is that of denitrofication and nitrogen immobilization, Greenen said.

“The whole goal was to promote denitrification,” Greenan said.

Denitrification is the process where nitrates are removed from the water, turned into nitrogen gas and released into the air.

Greenan said nitrogen immobilization hasn’t been measured in every study that is found in the literature. He said nitrogen immobilization occurs when the microorganisms that work to help remove the nitrates actually feed on the nitrates, thus keeping a percentage of the nitrates in the system.

Greenan said that in the lab, the cornstalks did a better job of removing the nitrates, leaving less immobilized nitrogen.

Jaynes said it will be a while before this system can be implemented, because there is a lot more data to collect before any conclusions as to what works best can be made.

“The big question right now is, how long would the system last?” Jaynes said.

Once they have enough data, “we can start figuring the cost benefit,” Jaynes said.

All of the materials that would be used would be from waste, wood chips, and sawdust from furniture makers, Jaynes said.

Cardboard boxes are recycled, but there is a point where they are not useable anymore. Jaynes said any inexpensive waste material containing large amounts of carbon is a prime candidate for usable material in this system.

Jaynes said not every drainage tile in Iowa would be lined – sensitive areas would be most important to implement this system, including areas where groundwater contributes high levels of nitrates and those that contribute to a drinking water source.

“These are the areas that they would start targeting first,” Jaynes said.