ISOTOPES NOT THE END ALL – TRADITIONAL CHEMICAL SUPPORT FOR ISOTOPIC INDICATIONS OF NITROGEN CONTAMINANT SOURCES IN KANSAS, USA

As chemical methodologies improve for the use of isotopic methods to identify sources of nitrate contamination, traditional chemical ratio methods, field observations, and statistical methods are sometimes ignored. Methods such as δ¹⁸O and δ¹⁵N values from nitrate are powerful tools for identification of nitrate sources and processes, particularly denitrification, but chemical ratios of major and minor chemical parameters can offer additional identification support to isotope analyses. Chemical ratios such as bromide/chloride, sulfate/chloride, and boron/chloride are useful for identification of salinity sources and to identify septic versus animal waste nitrogen sources. Traditional methods such as piper diagrams, stiff diagrams, ion plots, and statistical analyses help to define chemical contributions to the processes affecting isotope signatures.

Much work has been done in Kansas utilizing nitrogen-15 isotopes and chemical ratios for identification of nitrogen sources. This talk is a summary of identification of impacts of evapoconcentration, location of nitrogen sources, depth to water, and bacteria on the isotopic values and the use of ion weight ratios to aide in identification of sources of nitrogen contamination. Areas with a thicker vadose zone have higher boron, bromide, and chloride concentrations due to evapoconcentration at the soil surface and subsequent migration downward. Areas with higher clay content in the vadose zone, such as in northeast Kansas, show little boron in the groundwater. However, bromide, chloride, nitrate, and nitrogen-15 analyses all complement each other to define potential nitrogen sources in higher clay areas. Parts of Kansas with thinner vadose zones composed primarily of sandy loams have rapid movement of contaminants from the surface. These areas usually do not usually show changes in the source δ¹⁵N values unless a process such as release of ammonia gas or nitrification of fertilizer occurs in the soil zone. Use of ion ratios helps to support the interpretation of the nitrate and δ¹⁵N values observed in soils and ground water. Generally the observed nitrate-N values in the soil profile are lower than found in the ground water suggesting that labile nitrate from the soil zone is often not the sole contributor to nitrate observed in ground water.