The geochemical composition of Wellington soils and other New Zealand urban soils under different land-use

Abstract

A recent soil geochemical baseline survey of the Wellington urban area analysed samples collected at multiple depths across 151 sites from a mixture of residential, commercial, industrial, pasture, native forest and exotic forest land. The parent material of Wellington soils is mostly chemically invariant, requiring data outliers to be explained by environmental or anthropogenic factors. Notable concentrations above guidance level environmental standards were found in surface (0–2 cm) and shallow (2–20 cm) soils for As, Cd and Pb. The concentration of these heavy metal contaminants in Wellington soils are likely associated with human activities, as the highest concentrations correlate to more densely populated urban areas, particularly where older pre-1960s housing stock exists. This pattern of heavy metal enrichment in New Zealand urban soils relative to regional soils is also observed in comparative urban studies in Auckland and Dunedin cities. Wellington urban soils overall have lower concentrations of Cd and higher concentrations of As, Pb, and Zn compared to Auckland and/or Dunedin. In contrast, Auckland and Dunedin urban soils have a broader enrichment in some heavy metals (including Cr, Cu, and Ni) directly associated with a strong geogenic influence from underlying volcanic geological sources, while Wellington urban soils are influenced by monolithic greywacke. This study also showed that the carbon content of soils in these three centres increases by up to 66 % with increasing latitude due to better C preservation in cooler southern climates. Moreover, urban soils are found to have lower C contents than native, pasture and exotic soils due to more rapid soil turnover rates which leads to increased microbial activity, although this is also dependant on plant type, with C4 grasses commonly found in the warmer and drier climates of Auckland being more recalcitrant in soil than C3 plants. Geochemical studies from across New Zealand show that both urbanisation and underlying geology exert a strong influence on elemental soil concentrations, with the highest heavy metal values commonly associated with cities with the longest history of European settlement. Climate and land use type are also key drivers for elemental retention in shallow soils.