Long-term effects of sewage sludge application on heavy metal distribution in a loam soil and indigenous vegetables.

Abstract

Heavy metal accumulation in soils could have serious negative effects on soil life, plants, ground water and human health. Tons of sewage sludge, a mixture of activated and anaerobically digested sludges, with high concentration of heavy metals, are produced at Darvill Waste Water Works (DWWW) and irrigated on land as a suspension. The aim of this study was to investigate the spatial and vertical distribution of different heavy metals and their availability in soils and tissue composition of indigenous vegetables after long-term disposal of sewage sludge (more than 50 years) at a dedicated site in Pietermaritzburg. The disposal site has an area of 57 ha. Soil samples were collected from 0-10, 10-20, 20-30, 30-40 and 40-50 cm depths from five transects on the land. A reference transect not treated with sludge was included. Concentrations of Zn, Cu, Ni, Cr, Pb, Cd, Hg and As were determined after extraction with aqua regia, NH4NO3, DTPA and the TCLP. Indigenous vegetables growing voluntarily on this area were harvested and analysed for heavy metals to determine the risk that the people consuming them are exposed to. Geo-spatial distribution maps showed that Zn, Cu, Ni, Cr, Pb, Cd, Hg and As were higher than the total investigative level (TIL), total maximum threshold (TMT) and only Zn, Cr, Pb and Cd had exceeded maximum permissible limits (MPL) while the other elements were approaching the MPL, with some transects being more contaminated than others, and with variation between different points within the same transects. Irrigation of sewage sludge results in substantial buildup of heavy metals in soils. Heavy metals such as Zn, Cr, and Cu had not moved much beyond 30 cm, whereas for Pb, Ni, Hg and As had moved up to 50 cm depth from the aqua regia extractions. Arsenic levels in the control were as high as in the disposal land and sometimes even higher than some other transects. Results from DTPA and NH4NO3 extracts indicated that available heavy metal were also high in the available form indicating a high possibility of metal up take by plants. Indigenous vegetable tissue composition of these metals also exceeded MPL set by FAO. Amaranths took up the greatest amounts of these heavy metals followed by tomato plant, with Solunum nigrum and pumpkin lower level, which also exceeded MPL, with less in the fruit. Zn was the most taken up by these indigenous vegetables. The findings of this work suggest that soils and plants on the long-term sewage sludge disposal land are now heavily polluted and that there is an urgent need for remediation of these soils in DWWW.