ACCUMULATION OF CR, PB, CU, NI, ZN AND CD IN SOIL FOLLOWING IRRIGATION WITH TREATED URBAN EFFLUENT IN AUSTRALIA

The effect of irrigation with secondary treated municipal effluent on the accumulation of heavy metals (Cd, Cr, Cu, Ni, Pb, and Zn) was inve stigated by monitoring sites that had been irrigated with effluent for 4 and 17 years. At Wodonga, seven tree species were sprinkler irrigat ed with effluent at an average application rate of 1347 mm per annum f rom 1980 to 1984. The other site at Canberra was a large grass playing field (9 ha), half of which had been effluent irrigated since 1977. T he non-effluent irrigated area served as the control area and provided reference 'background' concentrations to assess the extent of contami nation due to 17 years of effluent irrigation. Archived soil samples c ollected before the commencement of effluent irrigation were compared with those taken in 1984 at Wodonga to assess the extent of contaminat ion. The concentration of labile metals was determined by extraction w ith EDTA because this method provides a quantitative measure of bioava ilable metals. Irrigation with effluent did not increase the EDTA-extr actable metals concentration at either site. Furthermore, the EDTA-ext ractable metal values were within the natural 'background' range repor ted for Australian soils. These data suggest that it may take between 50 and 100 years for heavy metal levels (mainly Cd) in effluent-irriga ted soil to reach the currently proposed threshold values for environm ental concern. The potentially harmful effects of long-term accumulati on of heavy metals on plant growth cannot be ignored and could affect the sustainability of land-based disposal of effluent. (C) 1997 Elsevi er Science Ltd.