Geophysics-informed Transport and Shallow Bedrock Topography in NE and SC Wisconsin Counties

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

Nitrogen-based chemical fertilizers and manure are used throughout Wisconsin to provide crop nutrients. 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 the fate of nitrate transport 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 concentrations (present due to fertilization applications) in soil cores collected immediately after the infiltration and a year later. The results show a general trend of nitrate contamination 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.

Part B: Evaluation of Geophysical Techniques for the Determination of Bedrock Depth

 We studied different near-surface geophysical surveys as measurement techniques to comply with the requirement of Wisconsin’s N.R. 151 Maximum liquid manure application rates for Silurian bedrock as a function of depth. We evaluated ground penetrating radar, electrical resistivity, electromagnetics, and seismic refraction techniques. We assessed penetration depth, sensitivity, ease of deployment and interpretation, and implementation cost. Each method has pros and cons that might help regulators and farmers decide on selecting one type of sensor over others. We found that electrical resistivity, while it can be relatively slow to deploy, is a geophysical technique that allows reaching the depth of penetration and the needed resolution by changing electrode separation; the collected data has a high signal-to-noise ratio, a single person can deploy and operate the instrument, it is a relatively cheap instrument, and the interpretation is relatively simple. If a higher data collection rate is desired, we recommend using electromagnetic sensors. However, the instruments require careful and frequent calibrations, and users should carefully when designing and running the surveys to maintain data quality.