Levels of toxic organic compounds found in Hamilton’s roadside runoff are about the same as those found in Japan – despite the higher traffic density there.

But overall, roadside runoff is not a major issue for waterways, such as the Waikato River. That was the findings of research by a Waikato University Masters student into pollutants flowing from roads after rain funded by Environment Waikato.

In a report to the Council’s Environment Committee, Millennium Scholarship winner Angela O’Riley said vehicle traffic was a major contributor to particulates, nitrogen oxides, carbon monoxide and ozone. It was also the source of a large number of pollutants in roadside dust, nearby soil and plants.

Until 1996, lead was the most serious roadside contaminant in New Zealand, with levels at least 100 times higher than natural levels in urban roadside dust. Lead additives were banned in 1996 and current levels were now much closer to natural values.

But vehicles also emitted cadmium, chromium, copper, iron, lead, nickel and zinc, and organic compounds known as polycyclic aromatic hydrocarbons, which were toxic and could cause cancer.

She said that during rain, some contamination was washed down drains and into stormwater, and some normally benign contaminants that are relatively harmless in roadside dust could be toxic when in water. Levels could be high compared to water quality criteria.

Small amounts of copper could be extremely toxic to aquatic organisms. A significant amount of contaminants were washed down in the first few minutes of rain, so treatment to remove pollutants from runoff in the first two hours of rain should be effective.

She said that in the Waikato, raised levels of PAHs and metals dropped rapidly when diluted in water, but could accumulate in sediments near discharge points.

Separate research commissioned by Environment Waikato had shown that accumulation of contaminants in Waikato estuary sediments from stormwater was unlikely to be a problem, and levels were substantially lower than more “urbanised” estuaries, such as Auckland.

“Roadside runoff is currently not seen as being a problem for waters of the Waikato River either, due to the extremely large dilution factors involved.”

However in high-density traffic or industrial areas, contamination of sediments from roadside runoff could be a problem, and the research could be useful in finding the most effective way of minimising the effects.