EFFECTS OF MACROPORE FLOW AND SEASONAL VARIABILITY ON WATER QUALITY AT A WASTEWATER-RECHARGE SITE

The city of Dodge City, Kansas, uses irrigation of secondarily treated municipal and meat-packing wastewater as a tertiary treatment process. The treatment process started in 1986 and utilizes 2700 acres of irrigated land. The site consists of loess soil overlying the High Plains aquifer of Tertiary age. The water table varies from 22 to 45 m. Crops utilizing the waste include corn, milo, and alfalfa. Nitrate-N concentrations greater than 10 mg/L are observed in soil cores collected to 13-m depth.

The current study was undertaken in the spring of 2004 to determine the seasonal and spatial chemical variability and fate and transport of the wastewater applied at the site. The δ¹⁵N natural abundance method, chloride, nitrate, and boron concentrations were used to determine if the observed chemistry in ground water and soil cores came from the irrigation area and to document seasonal variability. On the basis of a dye-tracer study and water-quality observations, macropore flow was identified as a major contributor to the present occurrence of nitrate at the site. Macropore flow in the model employed (RZWQM) is generated only during heavy rainfall events. Macroporosity had minimal effect on soil-water content, but had appreciable effect on nitrogen distribution.

The Kendall seasonal test for trend shows increased nitrate and chloride concentrations from 1986 to 2006 in seven of the 14 monitoring wells within the study area. δ¹⁵N values of ground water for three sampling periods (2005-06) showed seasonal and spatial variation. These variations are coordinated with seasonal effects on nitrogen degradation in the wastewater-treatment lagoons. Chloride, nitrate-N, and boron concentrations in ground water generally vary spatially but not seasonally for the period of the study.

The occurrence of measurable nitrate-N (with δ¹⁵N values generally > +10‰, animal-waste source) in ground water indicates rapid flow to the underlying deep ground water. Soil-water lysimeter values of nitrate-N, δ¹⁵N, and chloride also indicate macropore flow. Nitrate-N and chloride values of soil-water extracted from the soil indicate macropore movement through the vadose zone, but depleted δ¹⁵N values indicate that mineralization and matrix-flow processes are also occurring within the vadose zone.