Characterization of Groundwater Nitrates in Area 29

Technicians with the Lower Loup Natural Resources District collected 22 samples from 20 irrigation wells and 2 domestic wells in northeast Wheeler County for this project during summer 2022. Samples were chemically and isotopically characterized to help determine the potential source of nitrogen.    

This type of analysis is described as “fingerprinting,” a method for tracing sources of contaminants such as nitrate. By understanding the isotopic composition, the lab can determine if the nitrate source originated from inorganic sources like commercial fertilizer, or from organic sources such as animal waste. Ammonia, total organic carbon, and chloride concentrations were also measured as potential indicators of organic nitrogen (e.g. livestock manure) sources to groundwater. 

Dan Snow, director of the University of Nebraska’s Water Sciences Lab, provided the included plot image comparing the isotope composition of nitrate with different potential sources. The isotope composition suggests a mix of multiple nitrogen sources, both organic and inorganic. The majority of the wells are impacted strictly by commercial fertilizer sources. Three wells had all organic sources of nitrogen. 

Proper nitrogen fertilizer management is key, no matter if commercial fertilizers are used or if manure is applied. Next steps are to investigate the results and compare well depth and hydrogeology. The LLNRD will also collect samples within the vadose zone (soil profile below the root zone) to evaluate potential nitrogen losses, which ultimately end up in the water table. 

For more information about the isotope study, please contact Jason Moudry, Lower Loup NRD, (308) 728-3221.