Heavy metal soil pollution is influenced by the location of green spaces within urban settings

Heavy metals are naturally present in soils but are significantly altered by anthropogenic activity which can pose both environmental and human health risks. Sixty green space reserves were chosen in New Zealand's largest city and separated into six site categories to determine the influence of site location, soil depth, underlying substrate, canopy cover, and distance from roads and central business district on soil heavy metals. Soils in native urban forests had the lowest pools (g m(-2)) of heavy metals compared with green spaces close to high-traffic, park, school, industrial and residential areas. Least variability in heavy metals was determined among forest sites, with variability increasing in the order: residential, school, industrial, park, and high-traffic sites. Using forest sites as a baseline, a pollution index' (PI) was established and deemed high for nickel, cadmium and copper and moderate for arsenic, zinc, chromium, lead and mercury. The mean integrated' PI was high at 3.3 (range 0.3-9.3), indicating elevated levels of soil pollution. The PI was considered a useful technique for interpreting data and complemented traditional ways of reporting concentrations of heavy metals through use of soil pools, which has been limited to date. This study provides important heavy metal data for use in determining where resources may be required to mitigate future risk of increased soil pollution.