Tools For Water Quality Prediction And NPS–FM Implementation In Urban Settings

Implementation of the National Policy Statement for Freshwater Management (NPS-FM), in urban catchments is likely to require quantitative models to enable limit-setting of contaminants, including copper and zinc. Some models currently available require considerable resources in terms of the time to set-up and implement, and many have substantial data requirements for calibration (e.g., continuous flow monitoring data). More simplistic models, such as spreadsheet or GIS-based catchment load models, do not provide information on in-stream concentrations required for assessing attribute states. We are exploring new modelling methods that are suitable for catchments with minimal data (e.g., no water quality data, no continuous flow monitoring) and provide the required information for comparison to in-stream concentration-based attributes and source-based load limits. These methods can be applied as screening tools, to indicate where more resource is required (either additional monitoring or more sophisticated modelling).

Our work is focusing, firstly, on methods to incorporate uncertainty into the loads predicted by contaminant load models for suspended solids, copper and zinc. Current yield-based models traditionally provide single estimates of loads, ignoring potentially significant uncertainties introduced, for example, by the values adopted for the source yields which are often derived from limited data. We are developing a method to quantify this uncertainty using a Monte Carlo approach, implemented within a simple spreadsheet interface. The incorporation of uncertainty will improve our ability to discriminate between major contaminant sources and inform comparisons of differing scenarios.

The second part of the work is a way to convert these predicted catchment loads into in-stream concentrations. The catchment loads, calculated from a contaminant load model, are related to in-stream concentrations through an empirical relationship developed from State of the Environment monitoring data. This enables estimation of the likely median or 95th percentile in-stream concentration for a given catchment load, and uncertainty ranges around these concentration estimates. We have developed useable relationships for Auckland and undertaken initial tests on data from the Greater Wellington region and Christchurch City.

In combination, further development of these two methods aims to provide tools to undertake screening level assessments of contaminant loads and in-stream concentrations, for baseline and future scenarios, with uncertainty ranges for these estimates. This will enable more informed decision-making for catchment management under the NPS-FM.