Soil type influences the leaching of microbial indicators under natural rainfall following application of dairy shed effluent

The ability of soil to function as a barrier between microbial pathogens in wastes and groundwater following application of animal wastes is dependent on soil structure. We irrigated soil lysimeters with dairy shed effluent at intervals of 3-4 months and monitored microbial indicators (somatic coliphage, faecal enterococci, Escherichia coli) in soil core leachates for 1 year. The lysimeters were maintained in a lysimeter facility under natural soil temperature and moisture regimes. Microbial indicators were rapidly transported to depth in well-structured Netherton clay loam soil. Peak concentrations of E. coli and somatic coliphage were detected immediately following dairy shed effluent application to Netherton clay loam soil, and E. coli continued to leach from the soil following rainfall. In contrast, microbial indicators were rarely detected in leachates from fine-structured Manawatu sandy loam soil. Potential for leaching was dependent on soil moisture conditions in Manawatu soil but not Netherton soil, where leaching occurred regardless. Dye studies confirmed that E. coli can be transported to depth by flow through continuous macropores in Netherton soils. However, in the main E. coli was retained in topsoil of Netherton and Manawatu soil.