Background

Nitrogen (N) and phosphorus (P) pollution of surface waters from non-point agricultural sources is a problem nationwide, as well as in the Midwestern US, including Minnesota. Hypoxia in the Gulf of Mexico has been a serious problem since the mid 1980’s, and is largely attributed to nutrient enrichment of marine waters by N and P entering the Gulf from the Mississippi River. A federal task force has recommended a 45% reduction in N and P loads entering the Gulf of Mexico from the Mississippi River in order to reduce the long-term average area of the Gulf hypoxic zone to 1,930.5 mi²(5,000 km²) or less.

Much of the nitrogen and phosphorus that pollutes surface waters in the Upper Midwestern region flow through agricultural drainage ditches (roadside ditches). These ditches receive water from subsurface drain tiles and over land (surface) agricultural runoff. Currently, the primary function of drainage ditches is to remove excess water quickly without any treatment. Drainage ditches discharge large quantities of N and P into creeks, streams and rivers. New technologies that remove nutrients from drainage ditches offer the possibility of supplementing traditional on-farm or edge-of-field best management practices (BMPs) without taking land out of crop production. Research has recently begun to focus on using agricultural ditches as sites for treatment and removal of P and N.

This project offered the potential to remove significant quantities of N and P from agricultural drainage ditches. A bioreactor which removes N and P was installed parallel to an agricultural ditch, rather than at the outlet of subsurface tile drains as has been the case with most bioreactors. The bioreactor removes both N and P, which has not been the case with previously studied bioreactors that were designed to remove only N at the end of subsurface drains, or only P in regions where overland flow occurs. This project should lead to greater sustainability of agricultural production in Midwestern regions where corn and soybeans are produced. 

Research Objectives Completed

Objective 1: Evaluated the physical and chemical characteristics of selected P sorbing and N denitrifying media that have potential for use in a bioreactor installed in an agricultural drainage ditch.

Objective 2: Selected three P sorbing and three N denitrifying media from results of objective 1 and test the efficiency of P and N removal in laboratory flow columns under a range of temperatures and flow conditions.

Objective 3: Constructed a novel two phase bioreactor in an agricultural drainage ditch and evaluate N and P removal in agricultural runoff under field conditions. 

Materials and Methods

There were three interconnected phases for this project: 1) Initial laboratory screening of various materials for abiotic nitrate and phosphorus removal to select six optimum materials for column testing, 2) Laboratory column evaluation of the six optimum materials for their combined biotic and abiotic removals, and 3) Field installation and monitoring of the biofilter in a drainage ditch in Lamberton, Minnesota using the material selected through the laboratory column testing. The initial laboratory phases (1 & 2) were completed in years 1-3 and the field monitoring was completed in years 2-4.

Deliverables

• Identification, characterization and quantification of hydraulic residence time, and P sorbing and N denitrifying biofilter materials.
• Development of a potential best management practices for mitigating water quality impacts of agricultural production
• Water quality and quantity characterization of an in-ditch bioreactor system.