MUST estimates concentrations of dissolved copper and zinc in an urban stream based on its catchment land use and stormwater management characteristics.

Users are asked to enter details about the proportion of the catchment in each of a number of broad land use classes. One or more generic stormwater source control and treatment options can be selected. By varying inputs, users can re-run the tool to allow comparison of copper and zinc concentration estimates under different scenarios.

MUST generates estimates of the median and 95th percentile concentrations of dissolved copper and zinc and reports these relative to the respective Australian and New Zealand water quality guideline values. Please click here for more information about the methodology underpinning the MUST calculations

MUST estimates the probability that a concentration of dissolved copper or zinc meets a given threshold, rather than generating a single number. This probabilistic approach reflects uncertainty in the estimation of catchment loads and stream concentrations of copper and zinc that derives from:

• Variability in the proportion of contaminant sources (roofs, roads etc) in urban land use classes;
• Uncertainty in estimates of copper and zinc yields derived from sampling and modelling of stormwater runoff from urban contaminant sources;
• Variability in the performance of stormwater treatment devices; and
• Uncertainty in the relationship between modelled catchment loads of copper and zinc and measured in-stream concentrations of these metals.

While MUST has been developed from water quality monitoring data collected in Auckland, Christchurch and Wellington, it can be used for screening-level assessments throughout New Zealand. In particular, MUST is useful for investigating the relative difference between copper and zinc concentrations that result from different land use and/or stormwater management configurations.

However, users are cautioned that MUST relies on relationships developed from data in catchments in which the source of stream flow and contaminants is well-defined and well-matched. In strongly spring-fed streams these relationships break down, with greater dilution occurring. MUST is likely to over-estimate concentrations of dissolved copper and zinc in these situations.