MONITORING NITRATE TRANSPORT AND STORAGE IN THE VADOSE ZONE OF AN AGRICULTURAL SUB-WATERSHED AT LAKE BLOOMINGTON, IL

Lake Bloomington, a drinking water source for the city of Bloomington, IL, has nitrate levels that occasionally exceed the drinking water standard. The main source of nitrate is from fertilizers throughout the watershed. The city's goal is to understand nitrate transport in the sub-watershed and encourage agricultural practices that reduce nitrate in surface waters. A nitrogen mass balance will be established along with a conceptual model of subsurface flow and storage within the sub-watershed.

Ongoing research with the City of Bloomington is being conducted at a farm adjacent to Lake Bloomington. The city has installed tile drains, piezometers, soil moisture and temperature data loggers. Tile discharge is measured and tile water chemistry is analyzed to determine off-site nitrate flux through the tiles. Tensiometers and suction lysimeters have been installed for vadose zone study. Objectives of this research include the characterization of physical and chemical properties of the shallow subsurface, nitrate mass balance, and the importance of preferential flow paths.

Current samples show nitrate (as NO₃) concentrations in the vadose zone ranging from 1 to 260 mg/L during a crop season in which no fertilizer was applied, indicating an extended period of nitrate storage within the zone. The lowest nitrate level in the vadose zone is in a grassy area adjacent to the field within the shallow root zone in silty loam. The highest nitrate level is at the highest elevation in the middle of the field within a mottled clay loam. This trend relates to available vegetation for N uptake and also indicates storage potential related to soil properties.