Effects of dairying on water quality of lowland streams in Westland and Waikato

Abstract

Several New Zealand studies suggest that pastoral land use causes appreciable water quality degradation particularly in lowland streams. The water quality of eight NZ lowland streams, three near Morrinsville in the Waikato Region and five south of Hokitika in Westland, was sampled monthly for 13 months. Three of the five Westland streams are spring-fed streams on dairy-farmed alluvial plains of large rivers, and have high and steady flows and visually clear water, contrasting with two nearby reference streams in forested land which are more flow-variable and have humic-stained water. The catchments of the three Waikato streams, with the exception of a forested headwater area, are intensively grazed, mostly by dairy cows. Median conductivity, phosphorus and nitrogen concentrations, and (to a lesser extent) faecal contamination (as indicated by Escherichia coli concentrations), were all elevated in streams draining dairy pasture, often exceeding water quality guidelines. The most intensively-farmed Waikato stream (Toenepi Stream) has very high concentrations of nutrients and fairly high faecal contamination. At the other extreme, Dunlop Creek (Westland) has very low concentrations of nutrients and faecal contamination, consistent with its minimally-disturbed native forest catchment. The optical variables: humic concentration, turbidity and (inversely related) visual water clarity, show less of a pattern with land use across stream sites, apparently because of the influence of other factors, such as soil characteristics. The area-specific yields of streams in Westland dairy land were comparable with those in the Toenepi Stream, reflecting high rainfall driving runoff and leaching in Westland. However, the concentrations of contaminants mobilised by dairy farming were generally higher in Waikato than in Westland because of dilution in high water flows in the latter region. The water quality degradation of streams draining dairy land will need to be addressed by the dairy industry. Keywords: dairying, faecal contamination, nitrogen, nutrients, phosphorus, turbidity, water quality