Soil pollution driven by duration of urbanisation and dwelling quality in urban areas: An example from Auckland, New Zealand

Abstract

Soil chemistry is driven by environmental conditions, parent material and the rate and type of anthropogenic input. Cities built on soil derived from volcanic rock make up a rare yet important subset of urban areas globally. Here we conducted an urban geochemical baseline survey over Auckland, New Zealand's most populous city, built over several volcanic fields and formations. Soils were analysed by aqua regia digestion and inductively coupled plasma mass spectrometry for 65 elements, and C and N isotopes. Multivariate analysis showed volcanic parent material was a dominant natural source of metals in Auckland soils. C4 grasses significantly influenced the isotope signature in urban soils. Metal contamination was associated with urban soils and is linked to anthropogenic sources including leaded paint, tyre wear and industrialisation, and pesticides and spray residues on land that was formerly horticultural. We compare our results against other urban areas built on volcanic rock and with either a similar duration of urbanisation (Dunedin, New Zealand: c. 180 years) or a much longer period (Naples, Italy: c. 3000 years) and one urban area built on a non-volcanic substrate (Arica, Chile: 500 year-occupation). We found Dunedin soils are elevated in Cd, Pb and Zn relative to Auckland, despite an 11-fold smaller population. We attribute these legacy elements to a high proportion of pre-1970, timber clad dwellings in Dunedin with original leaded paint, relative to Auckland. Both Naples and Arica have elevated As, Pb and Zn in soil, and Naples also has higher Cd and Cu, relative to the New Zealand urban studies. We conclude that duration of urbanisation is a dominant factor in soil pollution, but where cities have similar urbanisation periods, then factors such as the age and quality of housing can be dominant. Cities built over volcanic rock like Auckland and Naples have soils elevated in Cr, Ni, large ion lithophiles, high field strength elements and rare earth elements, relative to cities with non-volcanic substrates, such as Arica.