Leaching of bacterial indicators of faecal contamination through four New Zealand soils

Abstract

Land application of animal waste can result in bacterial contamination of shallow groundwater and/or waterways. Using 500-mm-diameter barrel lysimeters, we investigated the potential for bacterial indicators to leach through 4 New Zealand soils treated with dairy shed effluent (DSE). DSE was applied to soil lysimeters containing poorly drained Gley Soils (Te Kowhai and Netherton) and well-drained Allophanic (Waihou) and Pumice Soils (Atiamuri) at 50 mm/h, a typical field application rate used by farmers. Simulated rainfall was applied continuously at a rate of 5–10 mm/h, and leachate collected at a depth of 700 or 750 mm was analysed for faecal coliforms, Escherichia coli, and faecal enterococci. Bacterial indicators of faecal contamination readily moved through the Te Kowhai and Netherton soils, but not the Waihou and Atiamuri soils. Differential microbial movement was attributed to differences in soil structure. The poorly drained soils have coarse subsoil structures with macropores, favouring bypass flow. In contrast, the welldrained soils have a finer, more uniformly porous soil structure that minimised bypass flow and allowed matrix flow.