Excess nitrogen and phosphorus are a great concern for Minnesota water bodies and beyond. The Crow River is one of the highest nutrient loading watersheds in the Upper Mississippi River Basin. Water quality monitoring conducted by the MFCRWD and the Kandiyohi County Drainage Inspector confirm a variety of water quality issues in drainage ditches. The most critically important issues revealed by the data are high nutrient loading (phosphorus and nitrogen) and high amounts of suspended solids flowing downstream to receiving waters. Woodchip bioreactors and rock inlets (see photo) are two practices that improve water quality while keeping impacts to farmed acreage at a minimum. Research at the University of Minnesota has shown an average 50% reduction of nitrate-N and a 54% reduction of phosphorus from installing woodchip bioreactors (Ranaivosan et al., 2010). Also, rock inlets have been shown to significantly reduce the delivery of sediment and phosphorus into subsurface tile drainage systems (Gieseke, 2000).
In March of 2012, the MFCRWD was awarded BWSR funding for the installation of four woodchip bioreactors and five rock inlets.
A rock inlet is an excavated area approximately 2 to 3 feet wide and 12 feet long perpendicular to the existing tile line. The depth depends upon the depth to existing tile line. A muck pipe with a muck sock and end cap is then covered with clean pea rock approximately 1/4″ to 7/8″ in diameter. The pea rock is then mounded 6″ to 12″ above the grade. Landowners can work over the rock inlet by easily lifting equipment instead of driving around the open tile intake.
Rock Inlet Advantages:
- Excess surface water is removed as effectively as open inlets
- Less sediment is delivered into the subsurface tile system
- Producers can plant through the rock inlet and will not harm their equipment
- Subsurface drainage is increased in the area around the inlet
- Freeze up time during cold winter months is shorter allowing drainage to occur in late fall, early winter and early spring
- Suction voids do not form around rock inlets
- Allows for venting of the subsurface drainage system
- Porosity can be up to 10 times that of a standard 6-inch open inlet around the open tile intake.
Woodchip bioreactors have been shown to be effective for removing nitrates from subsurface drainage water. Nitrates are removed from the system as the carbon from the wood chips is used by bacteria that break down the nitrate through the process of denitrification. Advantages of woodchip bioreactors, also known as denitrification beds, are that they have a relatively high rate of nitrate removal, small footprint, minimal to zero maintenance during the design life, and low installation cost.
Information obtained from Carver County SWCD