**Access the project factsheet here**

Tile drainage is required across much of the United States Midwest region for highly productive cropping systems. However, this critical infrastructure is also known to be a source of nutrient loss from fields. The purpose of this work was to evaluate some of the most common reasons why nutrient concentrations differ in tile drainage water from different fields.

This project, funded starting in 2016 by the Illinois Nutrient Research and Education Council, built on the momentum of the previously established and publicly available “Measured Annual Nutrient loads from AGricultural Environments” (MANAGE) database. Original support from the 4Rs Research Fund in 2014 led to the development of a “Drain Load” component in the database, and this new work compiled 1,564 site-years of agricultural drainage N and P concentrations (annual mean flow-weighted and annual arithmetic average) from 79 drainage studies into MANAGE’s new Drain Concentration table.

The average tile drainage nitrate-nitrogen (NO₃-N) concentration across the database was 12.88 mg NO₃-N/L (Figure 1), which was above the United States Environmental Protection Agency’s recommended limit for drinking water of 10 mg NO₃-N/L. Illinois and Iowa, the states with the most tiled-drained ground, averaged 14.09 and 13.22 mg NO₃-N/L, respectively.

Figure 1. Annual flow-weighted nitrate-nitrogen concentrations across a recently compiled database of North American drainage studies. Boxes encompass the 25^th and 75^th percentiles, stems encompass the 10^th and 90^th percentiles, the solid horizontal line is the median concentration, and the diamond is the mean or average concentration. The dashed line signifies the United States Environmental Protection Agency’s recommended limit for nitrate in drinking water of 10 mg NO₃-N/L. “n = …” is the number of site-years compiled in the database for each category.

There have been relatively fewer tile drainage dissolved phosphorus studies than nitrate studies (Figure 2). However, dissolved phosphorus can be a concern in tile drainage water depending on the location and site conditions. Very small phosphorus losses from fields are often deemed inconsequential agronomically, but such low concentrations are known to cause serious environmental impacts.

Figure 2. Annual flow-weighted dissolved reactive phosphorus (DRP) concentrations across a recently compiled database of North American drainage studies. A recommended phosphorus threshold to avoid algae growth in freshwater bodies like lakes and reservoirs in the Corn Belt and Northern Great Plains is 0.0375 mg total phosphorus/L (dashed line). The dissolved reactive phosphorus concentrations on the graph only comprise a portion of total phosphorus concentrations indicated with the dashed line, thus underestimating the potential proximity to this recommended limit.

Dr. R. Daren Harmel, USDA ARS, is a collaborator for this work.

Peer-reviewed outputs from these projects include:

1. Christianson, L., and R. D. Harmel. 2015. The MANAGE Drain Load database: Review and compilation of more than fifty years of North American drainage nutrient studies. Ag. Water Manage. 159:277-289.
2. Christianson, L., and R. D. Harmel. 2015. 4R water quality impacts: An assessment and synthesis of forty years of drainage nitrogen losses. J. Environ. Qual. 44(6):1852-1860.
3. Christianson, L., R. D. Harmel, D. Smith, M. Williams, and K. King. 2016. Assessment and synthesis of 50 years of published drainage phosphorus losses. J. Environ. Qual. doi:10.2134/jeq2015.12.0593.
4. Harmel, R.D., L.E. Christianson, M. McBroom, D.R. Smith, and K.D. Higgs. 2016 (Accepted). Expansion of the MANAGE database with forest and drainage studies. J. Am. Water Res.  Association.

The MANAGE database is published via the National Agricultural Library: DOI: 10.15482/USDA.ADC/1372907