Development of an E. coli runoff risk matrix

Water quality in New Zealand needs to improve and this will require a reduction in contaminant losses from the land. Microbial water quality impacts are particularly difficult to understand as there is a dearth of fundamental data on E. coli losses from many land uses and landscapes. Nevertheless, decisions on future land use need to be made now. The NPS-FM 2020 requires Regional Councils to set limits to manage water quality and “must not delay making decisions solely because of uncertainty about the quality or quantity of the information available”.

To support these limit setting processes this project has developed an E. coli runoff risk matrix based on expert opinion. This expert opinion was supported by a review of the literature on known mechanisms of microbial transport and a review of all modelling studies published in NZ. We thus use multiple lines of evidence to support the matrix development. 

Development of the risk matrix entailed several steps:

• Review of models to identify factors responsible for microbial loads and concentrations,
• Selection of the most important factors,
• Categorisation of each factor into discrete classes, and
• Developing a multi-dimensional risk table that included all reasonable combinations of the factor classes, assigning each combination to a risk ranking that ranged from 1 to 10, with 10 representing the greatest risk of stream contamination.

All of the catchment scale modelling studies that included E. coli were identified and summarized in an Excel database (Muirhead, 2022). Three classes of models were examined: mechanistic models, hybrid mechanistic/statistical load models, and random forest statistical models.

These modelling studies were investigated to identify landscape, hydrology, land use or other explanatory variables used to predict E. coli contamination. The development of the risk ranking matrix built on earlier development of typologies developed to explain landscape-scale variation in nitrogen and phosphorus losses to water.

The review identified 4 important factors influencing E. coli concentrations in streams: land use, soil drainage, soil wetness and elevation. The 4 factors were subdivided into various classes. Land use was subdivided into 5 classes: urban, pastoral, horticulture, arable and forestry/other. Soil drainage was subdivided into 3 classes: well drained, light soils and poorly drained. Wetness was subdivided into 3 classes: dry, irrigated and/or moist and wet. Elevation was subdivided into 2 classes: low and high. The resulting risk matrix is a ranking from 1 to 10 with 10 representing the highest risk. The risk matrix ranking can be used to indicate a direction of travel and does not represent a numerical risk factor. The E. coli risk ranking matrix would be best applied at the scale of a freshwater management unit.

The risk matrix is presented in Table i and a national scale map of this risk matrix and GIS layer is available through the Data Supermarket 

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