MONITORING NITRATE CONCENTRATIONS IN PORE WATER OF AGRICULTURAL SOILS USING TDR

Nitrate contamination of groundwater and surface water is one of the leading water quality problems in the Midwest. One of the main sources of nitrate contamination is fertilizer, which is often applied in excess of a crop's ability to absorb nitrates. To prevent leaching of nitrates into the groundwater, the quantity of fertilizer applied should be limited to what crops can absorb. However, determining how much nitrate is present in the soil pore water before fertilization is difficult, frequently resulting in over-application of fertilizers. This research explores the feasibility of estimating nitrate concentrations in pore water on a scale useful to agriculture using time domain reflectometry (TDR) techniques. Using TDR, both the volumetric water content and bulk electrical conductivity of the soil can be determined, and these parameters can be related to the electrical conductivity of the pore fluid. In non-saline soils, the electrical conductivity of the pore fluid can then be related to the concentration of fertilizer in the pore water using petrophysical relationships. This project develops petrophysical relationships between bulk electrical conductivity, volumetric water content, and nitrate concentrations for three agricultural soils using laboratory column experiments. In these experiments, soil columns were subjected to differing nitrate concentrations and soil moisture content conditions, and TDR probes were used to acquire bulk electrical conductivity and water content measurements. Data collected in these experiments were used to construct petrophysical relationships for each soil, and a comparsion of these relationships suggests that soil-specific calibration is required to estimate nitrate concentrations from TDR measurements.