Subsurface water quality from a flat tile-drained watershed in Illinois, USA

Subsurface drainage is a common practice in many agricultural watersheds in the Mid-Western region of the United States. A typical drainage system in east central Illinois is not spaced in a parallel manner, but the subsurface drain lines are laid out in a random and irregular fashion. These subsurface drain lines most often discharge into numerous man-made drainage channels, which ultimately drain to the rivers and the reservoirs. The Little Vermilion River (LVR) watershed in east central Illinois, USA is an example of a watershed with altered hydrology from subsurface drainage systems. A continuous monitoring study has been conducted from 1991 to 2003 on this watershed to quantify the effects of cropping management practices and random subsurface drainage systems on fertilizer and pesticide transport in subsurface flow. This study investigates the losses and concentrations of nitrate-N and atrazine (2-chloro-4-ethylamino-6-isopropylamino-S-triazine) from the LVR watershed that is flat (1% or less slope) and has intensive crop productions. Two of the drainage areas used in this study within this watershed have corn (Zea Mays L.) and soybeans (Glycine max L.) in rotations and the other two have seed corn and soybean in rotations. Long-term data collected from the LVR suggests that surface runoff rarely occurs in this watershed, and most of the soil-water is removed by the subsurface drainage systems. The concentrations of nitrate-N in subsurface drains varied depending on fertilizer application methods; pre-planting application contributed to nitrate-N concentrations higher than 10 mg L-1 (Drinking Water Standard of the US Environmental Protection Agency for nitrate-N) in subsurface drainage water. The pre-planting N application method resulted long-term average nitrate-N concentrations of 15, 17, 19, and 20 mg L-1 at the four monitoring sites, and the long-term average losses were 33, 23, 26, and 25 kg ha-1 of nitrate-N from the respective sites. Atrazine concentrations were lower than 3 μg L-1 (Drinking Water Standard of the US Environmental Protection Agency for Atrazine) in most water samples. The long-term average annual atrazine concentrations were 0.87, 1.0, 1.22, and 0.94 μg L-1 for the four sites. The mean annual atrazine losses varied from 0 to 7.12 g ha-1 during the monitoring period, with major losses of atrazine in drainage water occurring within 3 months of atrazine application.