Transport and Fate of Nitrate in a Saturated Buffer Zone as Assessed With a Chloride Tracer Test

Abstract

ABSTRACT The Upper Mississippi Basin, which includes Illinois, has highly fertile soils and experiences intensive agricultural practices. Although fertile, the soils do not drain well, resulting in the installation of tile-drainage systems. The practices of tile systems coupled with the application of nitrogen-rich fertilizers have led to the excessive export of nitrate from the agricultural fields into surface waters, contributing to eutrophication and the development of hypoxic zones in aquatic environments. Saturated riparian buffer zones (SRBs) have been proposed as a means to reduce the amount of nitrate discharged from tile-drained waters into streams. Previous works show a reduction in the nitrate as waters travel through an SRB, but in situ measurements of travel times are limited. Using the results from a 52-day tracer test, we developed a mathematical model combining end-member mixing of a tracer, chloride, and concentrations of nitrate as nitrogen (NO3-N) to determine the travel time of the tile waters in an SRB and to quantify the amount of NO3-N reduction occurring within an SRB. For the first 30 days, dissolved oxygen (DO) concentrations indicated aerobic conditions within the waters of most of the SRB, which saw a concomitant increase in NO3-N concentrations along groundwater pathways. As DO concentrations decreased below 4.5 mg/L, NO3-N concentrations began to lower along the flow pathways, resulting in NO3-N reductions ranging from 23 to 97 percent.