Subsurface Drainage and Water Quality: The Illinois Experience

Abstract

Drainage of excess water from agricultural lands is essential for crop growth. In the Midwestern U.S., Illinois, Indiana, Iowa, Ohio, Minnesota, Michigan, Missouri, and Wisconsin are some of the states that are highly drained with intensive subsurface drainage systems. Illinois alone has a total drained area of approximately 4 million ha. Many watersheds in east-central Illinois have less than 1% surface gradient and poorly drained soils, yet subsurface drains have made these lands some of the most productive farmlands in the world. Subsurface drainage enhances productivity and reduces sediment transport and phosphorous losses from fields; however, it increases NO3-N delivery to the receiving water bodies. Nitrate-N is mobile and can be lost from the soil profile by leaching and through subsurface drains (tile drains). Nitrate-N concentrations in drinking water reservoirs in many Midwestern states frequently exceed the EPA's maximum contaminant level (MCL) of 10 mg L-1. Several studies show NO3-N concentration data of more than 10 mg L-1 in discharges from subsurface drains. This article presents a history of subsurface drainage in Illinois, summarizes the results of decade-long field and watershed-scale research on subsurface drainage, and provides information on some innovative practices that are currently being evaluated to meet water quality challenges. It has been observed that infiltration is the predominant hydrologic process, and surface runoff rarely occurs from these watersheds. It has also been observed that base flow contributes significantly higher nutrient loadings into the streams than the subsurface drains from these watersheds.