VARIATIONS IN NITROGEN WATER CHEMISTRY, SOIL CONTENT, AND PLANT UPTAKE AT A WASTEWATER IRRIGATION SITE

The municipality of Dodge City, Kansas uses irrigation of secondarily treated wastewater as a final remediation step. A study site at this location was used to resolve the fate of the nitrogen in the wastes. Soils at the study area are predominantly silty clay loams. Wastewater additions to agricultural crops resulted in elevated nitrate-nitrogen concentrations (10 to 50 mg/kg) throughout the upper soil profile. Variable concentrations of soil-nitrate at depth, observation of macropores at 3m depth (shown by dye tracing) and at 13m depth from cores support this perspective.

Enriched δ¹⁵N signatures of plants grown at two wastewater sites and a control site show variation at the various growth stages of the plants. The predominantly ¹⁵N enriched plant values show that plant uptake and utilization of the wastewater is an important removal mechanism of nitrogen from the site. The δ¹⁵N signature of total non-extracted nitrogen in soils varies from +11‰ at the near surface to +7.5 and +2.4‰ at 5m and 16m depths, respectively. The ¹⁵N signatures for inorganic nitrate (-6 to -4.5‰), ammonium (-8 to -21‰) and total inorganic nitrogen (-20.8 to -2.8‰) suggest that organic nitrogen is the primary form of N that is migrating through the vadose zone.

Nitrogen and chloride concentrations vary seasonally at the effluent irrigation site. Both constituents show an increasing trend at most of the 14 monitoring sites from 1985-2005. Two of three monitoring wells near the wastewater lagoons showed increases across the quarterly sampling done in 1991-1999.

Nitrogen-15 of nitrate and ¹³C of DOC in the waters show seasonal fluctuations, indicating differential processes and/or sources. In the fall 2005 sampling, δ¹⁵N signatures decline below +8‰, likely an indication of a Haber-process fertilizer application. In the spring 2006, more ¹⁵N enriched isotope values are observed, becoming greater than +13‰ and suggesting a source typical of the wastewater application. Differences may be related to seasonal temperature effects, volatilization, bacterial processes, and volume of input water from the city and local meatpacking plants. Lysimeter samples indicate that wastewater is moving through the unsaturated zone, increasing from +12 to +20‰. These trends suggest that several sources and processes are impacting groundwater in the area.