Seasonal Monitoring of an Infiltration Experiment Using Time-Lapse Geophysics and Chemical Characterization to Understand Nitrate Transport in the Vadose Zone

Abstract

Nitrogen-based chemical fertilizers and manure are used throughout Wisconsin to provide nutrients for crops. However, nitrogen from these fertilizers may leach from the surface and root zone to the groundwater, creating health risks for communities relying on private wells for drinking water. Therefore, understanding nitrate transport’s fate and infiltration processes in the vadose zone is crucial for protecting groundwater quality and meeting drinking water standards. Infiltration of groundwater contaminants is most likely to occur during periods of intense recharge (i.e., significant precipitation events). To simulate this natural process, we conducted localized infiltration experiments on a Lower Wisconsin River Valley site with sandy soils at the farm field’s edge using a constant head infiltrometer with a bromide tracer solution. We monitored the infiltration process using electrical resistivity tomography and ground penetrating radar. We also used ion chromatography to measure bromide and nitrate (present due to fertilization applications) concentrations in soil cores collected immediately after the infiltration and a year later. The results show a general trend of nitrate contamination that is high at the surface and decreases exponentially with depth. However, the results from the infiltration experiment using a sodium bromide solution show an almost constant concentration with depth for samples collected one year after the infiltration experiment. In addition, the concentration, organic content, and grain size distribution analyses show data spikes at depths around 30 cm, 60 cm, 100 cm, and 150 cm, potentially showing a higher probability of contamination retention at subtle boundary composition changes. The difference in these results may be related to the localized injection of the sodium bromide solution vs. the uniform spreading of nitrate fertilizer in the field, the duration of the compounds’ application, and soil heterogeneities, emphasizing the complex nature of the processes.