Evaluating the effectiveness of stacked practices: Utilizing modified blind inlets at terrace sites for N and P load reductions

Issue

Currently, blind inlets are listed in the Iowa Nutrient Reduction Strategy (INRS) as an erosion control practice that reduces phosphorus (P) loss. Blind inlets have been shown to reduce sediment and phosphorus losses by 80 to 98%. However, gravel filter blind inlets are not effective for trapping dissolved nutrients like nitrate. Denitrification woodchip bioreactors are an INRS listed edge-of-field practice in which a large pit is dug at the edge of an agricultural field and then backfilled with woodchips. Tile water is then directed through the bio-rich medium before being exported from the field thereby reducing N losses via denitrification. This practice has been shown to reduce N losses by 43% on average. However, edge-of-field bioreactors generally have a high initial investment cost and do not capture sediment and sediment-bound P.

Stacked conservation practices are often necessary to provide adequate nutrient reduction benefits for a given agricultural field. While stacked practices are often thought of as process of multiple different practices (e.g., cover crops followed by wetland), PIs Streeter and Schilling have been investigating stacking nutrient reduction benefits within the same basic practice. In a pilot project recently completed for the Iowa NRCS, they looked at the effectiveness of incorporating a woodchip bioreactor into a traditional in-field blind inlet placed behind contour terraces and WASCOBs. The single economical conservation practice was stacked with both traditional blind inlets and edge-of-field bioreactors. 

A primary concern of the new system was that, unlike a bioreactor, the blind inlet system may not maintain an anaerobic environment for long periods of time, but rather, will be most effective during storm events and until ponding and runoff cease. However, the team identified significant reductions in N and P concentrations at two sites in southeast Iowa during even small rain events that showed the denitrification process reduced N concentrations almost immediately after saturation of the woodchip medium occurred. 

Objective

Researchers will build on a previous pilot study to investigate in-field load reductions at blind inlet locations and compare traditional standing and modified blind inlets alone and in combination with other edge-of-field practices. They envision development of a new combined practice that incorporates the benefits of both traditional blind inlets and edge-of-field bioreactors to reduce both N and P export from agricultural fields through the drainage network.

The study area includes the Southern Iowa Drift Plain Region of southeast Iowa, which has extensive contour terracing and WASCOB installations. This region has large areas of highly erodible soil and may be considered one of the most underserved regions in Iowa in terms of agricultural conservation research and development. Farmers, NRCS staff and local Soil and Water Conservation Districts have expressed willingness to work with researchers to develop and test new conservation practices.

The two-year study will quantify nutrient load reductions at modified blind inlet sites in southeast Iowa and model watershed-scale load reduction benefits based on field monitoring results. Specific objectives are to:

1. identify locations for two new blind inlet installations based on soil mapping, Lidar, and existing maps of Best Management Practices (BMPs) in southeast Iowa in proximity to an existing traditional tile intake;
2. install new blind inlets based on sites identified in objective one;
3. collect soils data and install a shallow groundwater monitoring well at each inlet to document subsurface soil and water quality conditions;
4. instrument the new modified blind inlets and a traditional standing tile inlet with precipitation gauges, flow meters and runoff activated water samplers to collect pre- and post-BMP as well as control water during rain events;
5. analyze the effectiveness of the modified blind inlets for N and P load reduction by comparing pre and post-BMP nutrient loads with control loads;
6. make predictions about the best locations for new modified blind inlet installations within the study area watersheds using output from the ACPF toolbox; and
7. model watershed-scale nutrient load reductions based on new monitoring data to provide evidence to support a new INRS practice.

Approach

Methods for this project are organized around each objective.

1. Work with the NRCS and local conservation groups and utilize results from the Iowa BMP mapping project, statewide Lidar and existing soil map data (SSURGO) to identify the most appropriate locations for two new modified blind inlet installations.
2.  Install modified blind inlets based on specifications determined from information gathered in objective 1, with modifications to intercept tile water downstream of each inlet to allow for post-practice water sampling. The modified blind inlet will be installed in a paired design in proximity to a traditional standing inlet to allow for a comparison between the two inlet types.
3. Analyze soil samples for a variety of relevant parameters at each site, adjacent to the modified blind inlet and install a monitoring well to characterize the depth to the water table and collect groundwater samples for biogeochemical analysis to ascertain groundwater nutrient contributions.
4. At each study site, install monitoring equipment supplied by the Iowa Flood Center to track rain events and estimate overall runoff and nutrient load exports, as well as other parameters. 
5.  Analyze results of the water quality analyses from each study site to determine reductions in N and P loads. 
6. Run the ACPF toolbox for the HUC 12 watershed that encompasses each study site. The ACPF toolbox will provide predictions for the best locations of future terrace and WASCOB installations that could benefit from modified blind inlets and estimate the potential effect of large-scale adoption of modified blind inlets. 
7. Utilize Iowa DNR ambient monitoring data where available to model potential N and P load reductions based on the incorporation of this new practice in multiple watersheds and at multiple watershed scales.