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Finding room for 100,000+ bioreactors, saturated buffers

By Keegan Kult, ISA environmental scientist

The INRS lays out a scenario in which 120,000 – 130,000 edge-of-field practices such as bioreactors and saturated buffers are needed to reach the 41 percent nitrogen reduction goal. Scaling up the installation of bioreactors and saturated buffers to meet the goals of the Iowa Nutrient Reduction Strategy (INRS) is a daunting task. 

As the first in a series examining the factors that will lead to wide-scale adoption of these edge-of-field practices, this article will discuss sites appropriate for installation.

Currently, 40-50 bioreactors and saturated buffers are installed on private Iowa farms. Before enough of these edge-of-field practices are installed to significantly impact the amount of nitrogen leaving the outlets of even small watersheds, many factors need to be addressed.

  • Farmers, drainage contractors and watershed coordinators need to understand where practices will work.
  • The design process needs to be simplified in order to decrease the time from when a farmer signs up to when the practice is installed.
  • Innovations to the practice need to continue in order to increase operating efficiency and convenience.
  • A steady funding source must be secured to help with installation expenses.

Bioreactors and saturated buffers typically are installed on smaller scale tile systems that drain one or two fields, about 30-100 acres. Historically, larger tile systems require a practice, such as a nitrate removal wetland, however, research in underway to treat larger systems with bioreactors and saturated buffers.

Even though bioreactors and saturated buffers treat similar drainage areas, the siting requirements differ. Bioreactors act as more of a generalist as to where they fit in the landscape, whereas a saturated buffer has more specific requirements.

A bioreactor redirects a portion of tile flow through an underground pit of wood chips to remove nitrate-N. Bioreactors can be accommodated in many landscape positions, but require an area approximately 100 feet long by 20 feet wide. The area devoted to the bioreactor should not be driven over, which is why they typically are located at the field edge, or ideally in a filter strip. When a filter strip is not available, bioreactors have been located near field borders where traffic impacts are kept to a minimum.

Historically tile systems with 6 to 10 inch field tiles with consistent flow are treated with bioreactors. Tiles known to flow sporadically or only in response to storm events should be avoided.  The longer the bioreactor is saturated throughout the year, the longer the life expectancy of the practice, approximately 10-15 years. Sites with high water tables should be avoided as the bioreactor chamber will be inundated with groundwater and the tile water that intended to be treated will bypass the system.

A saturated buffer redistributes tile water into the soil profile of an existing streamside buffer or filter strip to remove nitrogen before reaching the stream. It requires a minimum filter strip width of 30 feet and should outlet to a stream with banks less than eight feet in height. Saturated buffers are not recommended for fields with minimal elevation change from the crop field to the stream because they work by raising the redistributed tile water to within a couple feet of the soil surface. Elevating the redistributed water provides the opportunity to interact with the portion of the soil profile that has high amounts of carbon or organic material. The organic rich soil material spurs denitrification.

By positioning the saturated buffer in a filter strip where the crop is situated at a higher elevation, the risk of backing water into the field is minimized. The gradient also helps drive the redistributed tile water towards the stream. It also is crucial to have tighter soils for the length of the distribution line and the width of the saturated buffer area. If sandy subsoils or gravel lenses are located within the saturated buffer area, subsurface preferential flow paths are likely to develop, which allows water to bypass treatment. Additionally, lower order streams — streams in the upper portion of the watershed — likely have more suitable sites than the larger stream systems in the lower portion of a watershed. Larger streams tend to have a wider migration in their floodplain, leaving behind soil layers with high hydraulic conductivity.

In the next part of this series, Advance will examine some of the innovative approaches are being taken to treat larger tile systems of 200 to 700 acres.

This article is the first in a series of how ISA plans to help reach the goals laid out in the INRS. To continue to follow the series, subscribe to the ISA Research Advance, a bi-weekly newsletter exclusively dedicated to the research activities of ISA and our partners.

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