Submitted to: Water, Air, and Soil Pollution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/14/2011
Publication Date: 4/6/2011
Citation: Kroger, R., Moore, M.T., Farris, J.L., Gopolan, M. 2011. Evidence for the use of low-grade weirs in drainage ditches to improve nutrient reductions from agriculture. Water, Air, and Soil Pollution. 221:223-234. Interpretive Summary: Agricultural drainage ditches are now being used as best management practices to help clean up farm runoff containing pesticides and nutrients. In order for ditches to process these chemicals, water needs to be held in the systems for increased periods of time. The important balance in ditches is to allow for drainage, while still giving the system maximum opportunity for ecological processes to break down or remove potentially harmful runoff chemicals. When weirs are placed in drainage ditches, water is held at levels that are ecologically beneficial, while still allowing sufficient drainage to occur. Results from current experiments demonstrated ditches with weirs were able to clean up nutrients from the water significantly better than ditches without weirs.
Technical Abstract: The essential function of drainage ditches is to remove water from the agricultural landscape to avoid crop senescence through flooding and soil saturation. Commonly used slotted board risers provide drainage management strategies over the dormant season; however, by introducing innovative, low-grade weirs into the drainage ditch hydraulic control can occur on an annual basis with multiple sites for biogeochemical transformations. Additionally, improved hydraulic residence will affect the potential for improved N and P mitigation. This study evaluated weir vs. non-weir systems for nutrient mitigation capacity. Results showed weired systems reduced higher amounts of summed loads of dissolved inorganic phosphate (71% vs. 66%), total inorganic phosphate (60% vs. 12%), nitrate-N (96% vs. 76%) and ammonium (-47% vs. -53%) when compared to non-weired systems. Researchers need to be cognizant in differences in load reductions rather than concentrations of nutrients between weired and non-weired systems, as results showed no significant differences in nutrient concentrations. These results indicate low-grade weirs as important drainage control structures in reducing nutrient loads to downstream receiving systems. Complex relationships in nutrient mitigation and drainage occurred when a second amendment was added to the each system, post the initial storm amendment, and such future research should examine temporal relationships between nutrient mitigation, storm events and fertilizer loads.