Accumulation of potentially toxic elements in the body tissues of sheep grazed on grassland given repeated applications of sewage sludge

Abstract

AbstractIngestion of sewage sludge with soil, together with herbage containing elevated concentrations of potentially toxic elements (PTEs) might pose a risk to the human food chain through the accumulation of PTEs in edible tissues of livestock grazed on pastures which have received repeated doses of sludge. The accumulation of PTEs in tissues of animals grazed on a permanent pasture which had received repeated applications of sewage sludge between 1981 and 1994 was studied over two consecutive grazing seasons by monthly serial slaughter. Blocks of six weaned lambs were allocated at random at the start of the grazing seasons to slaughter date. Blocks of lambs were also allocated at random to an adjacent untreated pasture and to initial slaughter groups. Animals grazed continuously for two periods of 181 days and 152 days in 1994 (experiment 1) and 1995 (experiment 2), respectively. An application of sewage sludge was made to the treated site between the two experiments. The concentration of cadmium (Cd) in soil samples taken to 25 cm depth from the treated site was close to the United Kingdom statutory limit of 3 mg Cd per kg dry matter (DM) and 1·67 times the limit at the soil surface. Concentrations of other PTEs in soil were below statutory limis. The concentrations of PTEs in herbage at the treated site increased as each grazing period advanced (P < 0·05). Mean concentrations of Cd, lead (Pb), copper (Cu) and zinc (Zn) were higher (P < 0·05) in both experiments for herbage at the treated site than at the untreated site. No differences were observed in the concentrations of PTEs in muscle tissue between lambs grazed on treated or untreated pastures. The concentration of Cd in kidneys of lambs grazed on the treated pasture increased as the grazing seasons progressed from 0·19 to 0·36 mg/kg DM (P < 0·01) in experiment 1 and from 0·03 to 2·57 mg/kg DM in experiment 2 (P < 0·001). There was no measurable accumulation of Pb into kidney in either experiment. Concentrations of Cu in kidney remained similar during experiment 1 but increased during experiment 2 in lambs grazed on the treated pasture only (P < 0·05). Similar trends to those for kidney were observed for Cd in liver in both experiments. The concentrations of Pb increased in liver for lambs grazed on the treated site in experiment 1 (P < 0·01) but not in experiment 2. The concentrations of Cu in lambs grazed on the treated pasture in liver decreased (P < 0·01) in experiment 1 from 139 to 28 mg Cu per kg DM. Liver concentrations of Cu in experiment 2 increased (P < 0·01) from 63·5 to 197 mg Cu per kg DM, possibly reflecting the application of sludge to the experimental site between the two experiments. The results indicated that soil and herbage concentrations of PTEs were elevated following repeated applications of sewage sludge to grassland. The accumulation of PTEs in edible body tissues was generally low, with the greatest being of Cd and Cu in the second experiment, possibly reflecting recent application of sludge.