Preferential flow and water quality in two New Zealand soils previously irrigated with wastewater

Preferential flow of bromide tracer through two New Zealand soils, Horotiu (Typic Hapludand) and Te Kowhai (Typic Ochraqualf) soils, was explained. These two soils differed in physical, chemical, and drainage characteristics. These soils had been irrigated with meat-processing wastewater for 38 mo that ceased 3 mo before this study. Impact of heavy rainfall on residual water quality from such wastewater applied soils was examined by measuring the concentrations of anions (nitrate and phosphate), and cations (Na, K, Ca, Mg, and ammonium) in leachates collected. Heavy rainfall did not appear to have any impact on leachate quality, as indicated by low nitrate-N and phosphate concentrations compared with those measured during wastewater irrigation. While no preferential flow- occurred in the Horotiu soil when it was wetted to field capacity, preferential flow occurred to depth in the Te Kowhai soil under similar conditions. Other water quality problems (e.g., pathogens), which were not investigated in this study may arise in Te Kowhai soils under heavy rainfall.