Application of Monensin as a Tracer for Manure Impacts on Water Quality

Understanding the path of water through landscapes is important to reconstructing the impact that land management has on water quality. For example, nitrate in groundwater is a health concern to private well owners. Anthropogenic sources of nitrate, such as human waste and agricultural practices have been successfully identified by analyzing for source specific tracers. Artificial sweeteners and other compounds unique to human use indicate nitrate is likely from nearby on-site waste systems, while the presence of herbicide metabolites signifies agricultural inputs of nitrate. Another potential source of excess nitrate can be animal manure, but adequate tracer compounds have been lacking. Monensin is an ionophore antibiotic (IPA) widely used for dairy and meat-producing animals. Because it is not well-metabolized, much of the administered dosage is excreted. We refined methods to analyze water and soil samples for monensin. We explored how monensin concentrations correlated with concentrations of nitrate and other land management-based impacts to water quality to explore its usefulness as a pathway tracer for manure impacts to water. For example, wells in Wisconsin that previously tested positive for E. coli and/or nitrate above the health standard of 10 mg N/L were evaluated for monensin. The presence of monensin in these wells would indicate animal manure as a source of nitrate contamination. The addition of this source tracer has important health and land use implications.