Accumulation of potentially toxic elements by sheep given diets containing soil and sewage sludge. 1. Effect of type of soil and level of sewage sludge in the diet

Increasing amounts of sewage sludge will be applied to agricultural land over the next 10 years as a result of the prohibition of its disposal to the sea. The addition of potentially toxic elements (PTEs) to the soil via sewage sludge is controlled by European legislation designed to limit the accumulation of PTEs in soil which could give rise to toxicity to plants or livestock. However the possibility exists that direct ingestion of sewage sludge and soil together with grazed herbage may result in accumulation of PTEs in body tissues. To assess the extent of accumulation of PTEs by direct ingestion of soil and sewage sludge 12 groups of housed weaned lambs were given diets ad libitum comprising dried grass (900 g/kg total diet dry matter (DM)) and three different soils (100 glkg total diet DM). Soil was replaced by dried digested sewage sludge at levels ofO (level 0), 75 (level 1), 150 (level 2) and 300 g/kg soil DM (level 3). Voluntary intake of DM was markedly depressed by the inclusion of sewage sludge in the diet (on average by 24 g DM per g sewage sludge DM addition). There was no effect of sewage sludge on diet apparent digestibility. Live-weight gain was depressed (P < 0-001) by the addition of sewage sludge to the diet from 236 glday (level 0) to 141 glday (level 3). Liver and kidney weights were also reduced (P < 0-01). The apparent availability coefficients for Cd, Pb and Cu increased with increasing level of sewage sludge in the diet fP < 0-05), as did their concentrations in the liver and kidney (P < 0-01). Concentrations of Cd and Pb in liver increased from <0-07 mg/kg DM and <0-40 mg/kg DM (level 0) respectively to 0-61 mg/kg DM and 4-60 mg/kg DM (level 3) respectively at the end of the trial. Similarly the concentrations of Cd and Pb in kidneys increased from 0-19 mg/kg DM and <0-56 mg/kg DM (level 0) respectively to 0-80 mg/kg DM and 7-10 mg/kg DM (level 3) respectively by the end of the trial. No increases were observed in concentration s of Cd or Pb in muscle tissue. The main effect of type of soil on concentrations of PTEs in body tissues was not significant The daily rate of accumulation ofPb in kidney ranged from 1-1 to 51-5 uglg daily tissue DM growth during the first 57 days of the experiment and from 0-33 to 6-78 /xg/g daily tissue DM growth between day 57 and day 112. A decrease in the second period was also observed for Cd, with accumulation in kidney ranging from 0-31 to 4-44 fig/g daily tissue DM growth during the first 57 days and from 0-21 to 1-44 /jg/g daily tissue DM growth between day 57 and 112. Concentrations of Pb in liver of lambs given the highest level of sludge approached the statutory limit set for human food. The results indicate that in relation to accumulation of PTEs in liver and kidney there would appear to be little margin of safety with respect to the current United Kingdom statutory limits for the concentrations of Cd and Pb in sludge-amended soils. Confirmation of these results is required in the grazing situation.