DOCUMENTING SEASONAL VARIATION AT A WASTEWATER IRRIGATION SITE THROUGH STABLE ISOTOPES

The use of treated wastewaters in the fertilization for agriculture is increasing, and with it are potential impacts on ground-water quality. In the area of Dodge City, Kansas, which consists of silty clay loam soils, treatments have resulted in elevated nitrate-nitrogen concentrations (10 – 50 mg/kg) throughout the upper soil profile. Variable concentrations of soil-nitrate suggest that preferential flow processes have occurred at the area. Observation of macropores at depths up to 15-m below the surface and indicated by dye tracing done at two of the sites supports this perspective. Cores taken during previous studies in the area show nitrate-N values of 30 mg/L, converted from soil mg/kg values, at depths of up to 15-m.

Nitrogen and chloride concentrations appear to vary seasonally at the effluent irrigation site. Nitrogen-15 and carbon-13 on the DIN and DOC show seasonal fluctuation indicating differential processes or sources. In the fall, 2005 sampling, δ¹⁵15N signatures are lower (less than +8 ‰) and more indicative of fertilizer application. The spring, 2006 δ¹⁵N enriched isotope values (greater than +13 ‰) suggest a source more typical of wastewater application. Lysimeter samples indicate that wastewater is moving downward through the unsaturated zone (+12 to +20 ‰). These differences suggest that several sources and processes are impacting groundwater in the area.