PREDICTING SEDIMENT AND HEAVY METAL LOADS IN STORMWATER RUNOFF FROM INDIVIDUAL SURFACES

Impermeable urban surfaces such as roads, carparks and roofs contribute contaminants through stormwater runoff to urban waterways. Instream water quality monitoring in Christchurch has confirmed elevated sediment and heavy metals. To assist with planning of stormwater improvements to address water quality issues, an event-based contaminant load model has been developed that predicts the contributions from individual surfaces of TSS, copper and zinc based on rainfall characteristics such as rainfall intensity, antecedent dry period, storm duration, and rainfall pH. This model allows the user to spatially identify where high contaminant loads are likely to be generated. It also identifies patterns in the relative contaminant contributions from different surface types, which is helpful for guiding optimal management solutions. The model was calibrated to Christchurch’s low intensity rainfall climate by sampling contaminant loads from key impermeable surfaces and then applied to the Okeover catchment in western Christchurch as an initial case study. Results showed that road surfaces contribute the highest TSS loads (61% of catchment TSS load), followed by carparks (38%). Copper contributions are more evenly distributed between roofs, roads and carparks (27%, 45% and 28% respectively), although a small area of copper roof surfaces contributes a substantial proportion of the catchment’s overall copper load (8% of catchment copper load from 0.3% of total impervious area), primarily in dissolved form. Galvanized roofs are key contributors of zinc (81% of catchment zinc load), also primarily in dissolved form. The model was run for a full year of rain events to identify the typical event loads from individual surfaces and how these loads were spatially distributed within the catchment. The model framework accounts for the unique combination of different surfaces within a catchment, local rain event characteristics, and can support stormwater management decision-making to an individual surface or property level.