Executive Summary
Environment Waikato has collected E. coli data from 73 stream sites across the Waikato region that encompass a diverse range of faecal contaminant sources. Examination of this microbial dataset has been conducted in conjunction with a range of environmental factors including the physical characteristics of each catchment, the land management practices within it, and dynamic processes such as hydrological and meteorological conditions. This approach has highlighted the key processes determining faecal contamination of waterways, and identified some mitigating practices.
With the exception of a few sites, the discharge of point sources direct to waterways appears not to influence median E. coli concentrations. This is attributed to the relatively low number of consented discharges that cause faecal contamination, and to improvements in the treatment of waste water.
Median E. coli concentrations across the region range from 1 to 1300 cfu/100mL and, at 53 of the 73 sites sampled, they exceed the guideline for freshwater recreation (a median value of 126 cfu/100mL). The pattern of contamination across the Waikato is dominated by the presence of grazing livestock and the highest median E. coli concentrations are associated with the most intensive dairy farming in the centre of the region. Conversely, the lowest median values are found in forested catchments, although E. coli concentrations are always measurable, indicating contamination by wild animals.
Strategies to reduce faecal contamination of streams and rivers in the Waikato region should focus upon grazing livestock. Cattle access to streams and near-channel areas is likely to dominate faecal contamination, and mitigation measures may, therefore, be best directed at riparian zones. Permanent fencing to exclude livestock from stream channels and a proportion of riparian land is likely to be the most effective means of reducing faecal contamination by grazing cattle. There are also a number of riparian management alternatives to permanent fencing that may not be as effective but should still reduce faecal contamination.
The percentage of poorly drained soil within a watershed is a relatively strong predictor of median E. coli. This is probably attributable to the generation of appreciable overland flow that can rapidly transport faecal material to waterways. It is also likely that artificial drainage in poorly drained soils accelerates the transport of faecal microbes to streams. The bacterial water quality of streams draining such soils is likely to be particularly sensitive to livestock grazing and the application of effluent to land. Appropriate mitigation measures on land underlain by poorly drained soils may include the adoption of less intensive farming practices, optimising the timing of effluent application to land to avoid periods when the soil is saturated, wetland treatment of wastes, surface runoff and sub-surface drainage, and retirement of riparian areas from grazing.
A weak inverse relationship was found between the presence of a wetland within a catchment and median E. coli. A tentative inference is drawn therefore that wetlands may act to attenuate faecal contamination.
A statistical model explains almost 70% of the observed variance in median E. coli across the region. There are 3 explanatory variables: the percentage of poorly drained soil, density of cattle, and median turbidity at the catchment outlet. The model potentially provides a means of predicting faecal contamination through the use of basic environmental data.
Some future directions for research are identified. A major challenge for researchers and resource managers alike will be to develop a method of characterising riparian zones with respect to livestock access to channels and the function of vegetation in trapping overland flow of faecal contamination.
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