The potential for contamination of soil and surface waters from sewage sludge (biosolids) in a sheep grazing study, Australia

The application of biosolids to agricultural lands can have both beneficial and harmful effects. Due to the potential for the contamination of surface and ground water by nitrates, metal contaminants and pathogens from biosolids, careful guidelines have to be established for their use in land application. Guidelines must be based on actual data from trials carried out under appropriate environmental conditions. Biosolids in the form of dewatered sludge cake (dewatered biosolids, DWB) were applied at 0, 30, 60 and 120 dry t/ha to three types of soils in a sheep grazing trial at Goulburn. These rates of applications were high (usual rates are 10–15 dry t/ha) and the biosolids used also had higher concentrations of metal contaminants than the biosolids applied to farms. Surface and subsurface movement of nutrients and metals were monitored in the runoff water and soil profile, respectively. The biosolids reduced runoff and increased surface retention of rainfall. Data collected over one and a half years show very low concentrations of metals in the runoff waters. The concentrations measured are not considered to be of any concern to environmental pollution. The nitrate concentration in the runoff water from the biosolids-treated plots were higher than from the control plots. There was movement of the plant nutrients Zn and Cu in the upper 30 cm of the soil profile. There was a significant movement of nitrates down the soil profile to a depth of 50 cm in the duplex soils and 70 cm in the sandy Red Earths in one and a half years. Subsurface lateral movement of nitrates was observed in one duplex soil on the 60 and 120 dry t/ha treatments and was significant in the higher rate. In summary, surface and subsurface movement of nitrate and some metals were detected in the grazing trial at Goulburn. However, the actual amounts of these plant nutrients were low and almost negligible in treatments applied at rates of 30 dry t/ha.