Impact of land use on the faecal microbial quality of hill-country streams

Abstract

Faecal contamination of rural streams is of increasing concern in New Zealand. This study assessed hill-country streams in the Whatawhata district that were impacted by pastoral farming, indigenous forest, or Pinus radiata forest; by measuring Escherichia coli bacteria at 14 sampling sites fortnightly for 2 years. E. coli concentrations were highest in streams flowing through grazed pasture. In both years there was a noticeable seasonal pattern in all streams irrespective of land use, with highest bacterial concentrations in summer and autumn and lowest in winter and early spring. There was no obvious correlation between E. coli concentration and rainfall or stream flow. In those streams impacted by a change in land use from pastoral to pines during the study, E. coli concentration fell rapidly and remained at levels lower than those in streams impacted by either indigenous or 7-year pine forests. As E. coli was detected in all but two samples, the water in these streams is not suitable for human consumption. The pastoral streams consistently failed to meet stock drinking-water guidelines (median concentration not greater than 100 E. coli 100 ml-1) and the forest streams failed to do so in summer. Twenty-eight percent of pastoral samples, 25% of indigenous forest samples, 14% of 7-year pine forest samples, and 5% in New Pines stream samples (after planting) had E. coli concentrations associated with a high level of risk for contact recreation (>500 E. coli 100 ml-1) and the high concentrations usually occurred in summer.