Water Quality and Tile Drainage
This section will discuss several studies related to tile drainage, nitrogen, and phosphorus discharge.
Despite the obvious benefits of tile drainage, subsurface drains increase nitrate transport by flushing water out of the soil matrix. The major source of nutrient loading to the Mississippi River is the North Central Region of the U.S. Alexander, et al. Differences in Phosphorus and Nitrogen Delivery to the Gulf of Mexico from the Mississippi River Basin U.S. Geological Survey Department of the Interior/USGS. Accessed from: http://water.usgs.gov/nawqa/sparrow/gulf_findings/index.html.
Sources of Nutrients delivered to the Gulf of Mexico
- Corn and soybean crops
- Other crops
- Pasture and range
- Urban and population-related sources
- Atmospheric deposition
- Natural land
Source: Alexander, et al. Differences in Phosphorus and Nitrogen Delivery to the Gulf of Mexico from the Mississippi River Basin U.S. Geological Survey Department of the Interior/USGS. Accessed from: http://water.usgs.gov/nawqa/sparrow/gulf_findings/index.html
Nutrient contributions to the Gulf, by State
It has been estimated that 15% of Mississippi River nitrogen loading and 10% of Phosphorus loading comes from Illinois. Source: P.K. Kalita, R.A.C. Cooke, et. al. Subsurface Drainage and Water Quality: The Illinois Experience. Transactions of the ASABE. Vo. 50(5): 1651-1656.
- < 1 Percent Share
- 1 – 5 Percent Share
- 5 – 10 Percent Share
- 10 – 17 Percent Share
Hypoxia is a term for low oxygen levels in water, occurring when oxygen falls below 2 parts per million.
When excess algal growth, caused by heavy nutrient loading from the Mississippi River, dies, it consumes oxygen from Gulf waters in the process.
Hypoxic Zone Gulf of Mexico, 2010. Courtesy U.S.E.P.A.
Area of Mid-Summer Bottom Water Hypoxia
(Dissolved Oxygen < 2.0 mg/L)
Tile Drained Soils and Nitrates in Surface Waters
Little Vermilion Watershed
An example of nutrient loading is given in a study conducted on the Little Vermilion Watershed in Illinois.
Almost 90% of this watershed is in row crop production, with 80% being very flat and poorly drained soils, with extensive tile drainage in place.
In this watershed, it was shown that nitrate levels in groundwater exceeded Maximum Contaminant Level when early pre plant nitrogen application methods were used. From: Mitchell, J.K, et. al. Transactions of the ASABE. VOL. 43(2): 337-342 . @2000.