Mapping the potential risk of Escherichia coli leaching through soils of the Waikato River catchment, New Zealand

A regional-scale map showing the potential risk of Escherichia coli leaching through soil on flat to rolling land was developed for the soils within the Waikato River catchment. Microbial breakthrough curves were developed for representative soils of the catchment that are commonly under dairy land use. These microbial breakthrough curves were generated under laboratory conditions using large, intact soil lysimeters irrigated with farm dairy effluent followed by continuous clean water irrigation. Some of the lysimeters were later used to monitor E. coli leaching under field conditions. The soils were rated as having a high, medium or low potential for leaching E. coli based on the microbial breakthrough curves or concentrations of E. coli in leachates from the field investigation. The potentials were correlated with the New Zealand Soil Classification to allow regionalisation to all soils within the Waikato River catchment. The catchment-scale map revealed that similar to 60% of the flat to rolling soils in Waikato are rated as having a low potential for leaching E. coli. These soils are typically Pumice or Allophanic Soils. The map can be used to raise the awareness of management of farm effluent irrigation to minimise microbial pathogen contamination of water bodies.