Heavy metals, released by wear on tyres and brake pads, are washed off roads and can contaminate rivers, streams and harbours. NIWA has recently completed a study to measure how much copper and zinc is discharged in road runoff, and how well different stormwater devices remove these contaminants from runoff.

Between February 2008 and June 2009, we ran a field programme at four sites in the Auckland Region. We measured road runoff volumes, and collected and analysed runoff samples.

Measuring contaminants in run-off

Concentrations of copper and zinc in run-off were higher at a congested site than at two sites where traffic moved freely. However, contrary to our expectations, metal concentrations were the lowest at a moderately congested site. We think this was probably due to the recently re-surfaced road trapping sediments and metals.

Our results enabled us to produce guideline Vehicle Emissions Factors (VEFs) for copper and zinc for two categories of road: (i) congested roads and intersections,  and (ii) ‘normal’ roads. These VEFs can be used to estimate copper and zinc loads in runoff discharged from any road as long as information on traffic volumes and characteristics is available.

How well do different stormwater systems work?

We also measured how well three stormwater systems removed copper and zinc in both solid and dissolved forms, and also how well they removed suspended sediments:

(i) a stormwater pond

(ii) a grass swale (a shallow grass-lined drain running parallel to the road)

(iii) a roadside drainage channel. 

The roadside drainage channel performed best, achieving the greatest proportional reduction in loads of suspended sediments, copper and zinc, despite not being specifically designed as a treatment measure. The stormwater pond achieved the lowest reductions, partly because it was ineffective at removing dissolved metals.

Guideline load-reduction factors (LRFs) for the removal of contaminants have been developed. In combination with our guideline VEFs, these can be used to estimate the load of sediment, copper and zinc discharged in road runoff that has passed through a pond, swale or roadside drainage channel.

Contact: Jonathan Moores

This research was funded by the New Zealand Transport Agency and is published as NZTA Research Report No. 395