Long-term effects of agricultural chemicals and management practices on water quality in a subsurface drained watershed.

摘要

The Little Vermilion River (LVR) watershed in east central Illinois, United States, is an example of a watershed with altered hydrology from subsurface drainage systems. A continuous monitoring study was conducted in the LVR to quantify the effects of cropping management practices and random subsurface drainage systems on nutrient and pesticide transport in subsurface flow and surface runoff. This study investigated the effect of nitrate-N, ortho-phosphate, atrazine, and alachlor on water quality from the LVR watershed, which is flat (slopes of 1% or less) and has intensive crop production. Four sites that included both subsurface and surface runoff monitoring programs and two river stations were selected for this study. Three of the sites had corn (Zea mays L.) and soybeans (Glycine max L.) planted in rotation, and the other site had seed corn and soybeans planted in rotation. Statistical analysis was performed to test the effects of several variables such as site, crop, discharge, and the interaction among them, on nutrient and herbicide concentrations in subsurface flow, surface runoff, and river flow.; Data analysis revealed that nutrient and herbicide concentrations and losses varied from year to year, from season to season, and from station to station. Generally, nitrate-N, atrazine, and alachlor concentrations and losses were found to be greater in subsurface flow than in surface runoff. Ortho-phosphate concentrations were higher in surface runoff than in subsurface flow, whereas ortho-phosphate losses were greater in subsurface effluents than in surface runoff. Nutrient and herbicide transport increased from late winter to early spring and declined following crop harvest.; The factors that showed some effect on nutrient and herbicide transport one way or another were climatic conditions, N application rate and timing, tillage system, and cropping practices. Statistical analysis showed that the effects of site, crop, discharge, and the interaction among them may be significant in nutrient and herbicide transport.; Double mass curve analysis indicated that the increase of nutrient and herbicide loading began when high amounts of rainfall occurred following chemical application. The primary factor that was found to affect nutrient and herbicide loading was subsurface flow, especially during the growing seasons.