Spatial and temporal trends in trace metal deposition in Canterbury, New Zealand (1989)
AuthorsStewart, Carolshow all
Spatial and temporal trends in the deposition of lead, zinc, copper, cadmium and manganese in Canterbury, New Zealand, have been studied. The two main aspects of this study were an atmospheric deposition monitoring programme set up over the study area to assess present-day spatial trends; and an assessment of the ability of kahikatea tree ring wood to record historical trends in levels of heavy metals in the Christchurch atmosphere. The main instrumental technique used was atomic absorption spectrophotometry, with both flame and graphite furnace atomisation. To study the dispersal of the metal-enriched aerosol from Christchurch, a network of deposition collectors was established over part of the Canterbury Plains. The main conclusions were that the rates of deposition of lead, zinc, copper and cadmium, but not manganese, showed an approximately exponential decay with distance away from Christchurch, and that the dispersion pattern was dependent upon the prevailing wind direction. It was also found that while rural deposition rates of zinc, copper and cadmium were lower than in other developed countries, the rates of lead deposition were similar to or higher than their overseas counterparts. Considerable seasonal variations in metal deposition rates tended to obscure an expected reduction in lead deposition fluxes following a reduction in the lead content of New Zealand premium grade petrol in July 1986. Concentrations of lead, zinc, copper, manganese and cadmium were determined in the topsoils at the deposition collection sites, to assess the influence of atmospheric deposition on topsoil composition. Substantial differences, overriding the natural variability in soil composition, were found to exist between urban and rural soils. However, in rural sites, variations in metal deposition rates did not produce effects visible above natural variations, with the possible exception of lead. To gain an historical perspective on the metal pollution history of Christchurch, the annual rings of kahikatea trees in Riccarton Bush, Christchurch, were analysed for lead, zinc, copper, manganese and cadmium. It was found that kahikatea trees appeared sensitive to environmental levels of these metals, showing accumulations of lead, zinc, copper and cadmium which appeared related to known historical emission trends. However, manganese concentrations in the ring wood appeared to be controlled by soil factors. A background stand of kahikatea trees showed no trends of increasing metal concentrations. The use of peat deposits for historical monitoring purposes was also investigated. It was concluded that the only metals which appeared relatively immobile within the peat profile, hence indicating potential for recording historical trends in deposition fluxes, were copper and cadmium. The other metals appeared to have been mobilised by the severely anaerobic conditions in the bogs, as they were depleted below the permanent water table. However, deposition rates calculated in the surface peat were similar to fluxes measured by a conventional deposition collector at a similar site.