Quantifying the effectiveness of a saturated buffer to reduce tile NO₃-N concentrations in eastern Iowa

Agricultural drainage tiles are primary contributors to NO₃-N export from Iowa croplands. Saturated buffers are a relatively new conservation practice that diverts tile water into a distribution tile installed in a riparian buffer parallel to a stream with the intent of enhancing NO₃-N processing within the buffer. In this study, tile NO₃-N concentration reductions were characterized through two different saturated buffers at a working farm site in eastern Iowa. Study objectives were to (1) evaluate the hydrogeology and water quality patterns in the saturated buffer and (2) quantify the reduction in tile NO₃-N concentration from the saturated buffer installation. Results showed that the two saturated buffers are reducing NO₃-N concentrations in tile drainage water from input concentrations of approximately 15 mg/l to levels < 1.5 mg/l at the streamside well locations. The reduction occurs rapidly in the fine-textured and organic-rich alluvial soils with most of the reduction occurring within 1.5 m of the distribution line. Denitrification is hypothesized as being primarily responsible for the concentration reductions based on soil and water chemistry conditions, completion of a geophysical survey (quantifying low potential for N loss to deeper aquifers), and comparisons to other similar Iowa sites. The study provides more assurance to new adopters that this practice can be installed in many areas throughout the Midwestern Cornbelt region.