Impacts of Tile Drainage and Sub Irrigation On Water Quality in Southeastern North Dakota.

Installation of tile drainage to remove excess water from the root zone is increasing in popularity in the Red River Valley of North Dakota and Minnesota. The objectives of this research were to determine the impacts of tile drainage and sub irrigation on water quality.  In 2008, water samples were taken from 24 screened wells installed within 44 ha corn field that has had tile drainage (18 m spacing, and 1 – 1.2 m deep) installed in 22 ha since 2002.  In the tile drained section at four locations, four screened wells were installed 1, 4, 7, and 10 m from a tile drain, where two of these nests were in the drained-sub irrigated and two nests were in the drained only sections.  In the non drained, non sub irrigated sections, 8 screened wells were installed.  Water samples were also collected from the sump pump outlet and from the adjacent drainage ditch.  Samples were taken every two weeks and analyzed for total nutrients and major cations and anions and sampled every four weeks and analyzed for trace metals by ICP/MS.  Concentrations of nitrate + nitrate-N from the tiled section were fairly consistent over the course of the year and averaged about 10 mg L^‑1 whereas concentrations in the sump and drainage ditch were as high as 22 mg L^‑1 prior to sub irrigation.  The sub irrigation water had very minimal levels of nitrate + nitrate-N.  Although trace elements were detected in the water samples of the tiled section, drinking water standards of the outgoing tile drainage water were only exceeded by Al, Be, As, Se, and Tl.  This study indicates that outgoing tile drainage water has the potential to have elevated nutrient concentrations, soluble salts, and in some cases, trace metals, compared to the in-field water samples.