Effect of season of application on the adhesion, retention and recontamination of herbage by potentially toxic metals and sewage sludge

Grassland in the United Kingdom can accept sewage sludge when, as a result of cropping restrictions, arable land cannot. However, one of the uncertainties associated with the application of sewage sludge to grassland is the potential exposure of the grazing ruminant to sewage sludge particulates adhered to the grass or deposited on the soil surface. The implications of elevated levels of sludge particulates and associated potentially toxic metals (PTMs) are the potential accumulation of PTMs into edible tissues of the grazing animal. Two experiments were performed to investigate the effect of season of application on the adhesion, retention and recontamination of herbage by sewage sludge and PTMs. In each experiment, three treatments were installed, an untreated control, application of sludge in the autumn and application of sludge in the spring. Sewage sludge particulates and PTMs were eluted sequentially 21 days after the application of sludge using water and detergent and the residual (tissue) concentrations were monitored. Total concentrations in each fraction and adhesion indices were calculated. Soil samples taken from the sward treatments after each application showed no elevated concentrations of PTMs or concentrations that approached the maximum concentrations allowed for soils amended with sewage sludge. The amounts of sludge adhered to herbage ranged from 114 to 360 g sludge DM/kg herbage DM. There was evidence of preferential retention of PTMs to herbage. However, during the 21-day no grazing period there was a general decline in concentration of all PTMs. Copper was retained on the herbage for greater periods of time compared to Zn, CD, Pb and Fe ( P < 0.01). The proportion of Pb recovered from the water phase increased during the 21-day no grazing period in comparison to Cu, Zn and Fe ( P < 0.001). It is concluded that the concentrations of PTMs adhered and integral to herbage as a result of surface application of sewage sludge are likely to fall to acceptable levels in well-managed permanent grassland.