Abstract

Disposal of cull onions (Allium cepa L.) in unlined landfills in the Treasure Valley region of eastern Oregon and southwestern Idaho is a historic management practice that has been identified as potentially causing NO₃-N contamination of regional groundwater. To investigate NO₃-N generation and transport during a 6-yr period, monitoring wells were placed in the cull onion waste inside two landfills. The soil below each landfill was instrumented with soil-solution samplers and matric-potential sensors. Four to 6 Mg of organic N were loaded into each landfill in the form of onion waste. Little leaching of NO₃-N was observed during the study. With the exception of a NO₃-N pulse during loading, solution NO₃-N concentrations within and beneath both landfills remained <1 mg L⁻¹. Concurrently, NH₄-N increased to 500 mg L⁻¹ at 0.5 m and to 7 mg L⁻¹ at 3 m below the landfill bases. Build-up of NH₄-N and lack of NO₃-N in the landfills are hypothesized to result from nitrification inhibition in an anaerobic environment. Hydraulic heads showed that saturated waste resided over unsaturated soil. Formation of a hydraulically restrictive layer at the interface between the landfill contents and the underlying soil probably caused these conditions. Saturated cull-onion landfills do not appear to be a source of NO₃-N contamination. Potential groundwater contamination from nitrification upon drying of landfill contents can be mitigated by removing and land-applying residual waste after sufficient degradation.

Copyright © . .