QUANTIFYING THE FLUX OF WATER AND LOSS OF NITRATE AS STREAM WATER FLOWS BENEATH A MEANDER OF A CENTRAL ILLINOIS STREAM

Excessive nitrate is a major pollutant of both surface water and groundwater, contributing to adverse effects on both human and ecological health. The hyporheic zone, where surface water and groundwater interact is a poorly understood but important ecotone. The aim of this study is to quantify the flux of water and loss of nitrate occurring beneath a meander of a low gradient stream, which flows across an unconfined glacial outwash aquifer. 13 monitoring wells were installed throughout the site and were screened at the water table. Physical measurements of hydraulic head, pH, specific conductance, temperature, and dissolved oxygen content were taken on a monthly basis. Water samples were also collected and processed for major ion chemistry using ion chromatography and inductively coupled plasma spectrometry. Preliminary results show a reduction of nitrate concentration as stream water passes beneath the meander, indicating denitrification or plant uptake of nitrate. Nitrate concentrations in Little Kickapoo Creek vary seasonally from 5 to 50 mg/L. Nitrate concentrations in groundwater found in the meander neck range from 10.5 mg/L on the upgradient side of the meander to non-detectable nitrate near the creek on the downgradient side of the meander. Chloride concentrations in Little Kickapoo Creek vary seasonally from 100 to 1000 mg/L, while local groundwater ranges from 10 to 15 mg/L. Chloride concentrations in the groundwater found beneath the meander exceed 100 mg/L indicating that the water is mainly stream water. Incorporating other redox species such as dissolved oxygen, nitrate, iron, and sulfate, sequential redox zones can be defined across the site. A numerical model will be constructed based on the hydraulic head measurements and conservative solute concentrations to quantify the flux of water and loss of nitrate beneath the meander.