Development of an <em>E. coli</em> runoff risk matrix

<p>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 <em>E. coli</em> 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”.</p> <p>To support these limit setting processes this project has developed an <em>E. coli</em> 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. </p> <p>Development of the risk matrix entailed several steps:</p> <ul> <li>Review of models to identify factors responsible for microbial loads and concentrations,</li> <li>Selection of the most important factors,</li> <li>Categorisation of each factor into discrete classes, and</li> <li>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.</li> </ul> <p>All of the catchment scale modelling studies that included <em>E. coli</em> 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.</p> <p>These modelling studies were investigated to identify landscape, hydrology, land use or other explanatory variables used to predict <em>E. coli</em> 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.</p> <p>The review identified 4 important factors influencing <em>E. coli</em> 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 <em>E. coli</em> risk ranking matrix would be best applied at the scale of a freshwater management unit.</p> <p>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 </p> <p><a href="https://landuseopportunities.nz/" target="_blank">https://landuseopportunities.nz/</a></p> <p><br></p>