Atmospheric deposition of heavy metals in Dunedin

Abstract

Patterns of cadmium, chromium, copper, nickel, zinc, and lead deposition from the atmosphere are measured over a twelve month period in Dunedin. These patterns are related to landuse, topography, and seasonal weather. The 35 km2 field area contains a variety of landuses, a mixture of identifiable heavy metal sources, and is sampled using 108 measurement sites. Sphagnum moss is used to collect deposition over the field area. Four of the sites measure rural deposition levels. The 104 urban sampling sites cover public use areas such as recreation grounds, schools, footpaths, and backyards, as well as areas near to probable heavy metal sources. The deposition patterns show great detail as a result of the high sampling density, and are very comprehensive compared with studies reported elsewhere.

Clear deposition patterns are evident for the heavy metals measured, although they differ for each metal. Median cadmium, chromium, copper, zinc, and lead deposition is greater at urban measurement sites, though only copper, zinc, and lead differences are significant. Deposition peaks are mostly, but not always, near known heavy metal sources. Copper and zinc deposition is more widespread due to coal combustion, and lead deposition is related to distance from roads and traffic density. Seasonal trends in metal deposition are similar at rural and urban sites.

The influence of topography is very evident in confined valleys, where meteorological conditions are conducive to .air stagnation. Periods of frequent and strong inversions lead to high deposition. However, low frequency and weak inversions do not necessarily produce low metal deposition. Windspeed is not consistently related to heavy metal deposition, and wind direction reflects deposition only some of the time. Precipitation patterns relate well to temporal deposition patterns.

Median urban deposition rates are lower than for European and for other New Zealand cities. Deposition maxima are higher than air quality guidelines of Switzerland, Germany and the former Yugoslavia, and represent a risk to human health.