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 benef icial and harmful effects. Due to the potential for the contamination of surface and ground water by nitrates, metal contaminants and pathog ens from biosolids, careful guidelines have to be established for thei r use in land application. Guidelines must be based on actual data fro m trials carried out under appropriate environmental conditions. Bioso lids in the form of dewatered sludge cake (dewatered biosolids, DWB) w ere 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 hig h (usual rates are 10-15 dry t/ha) and the biosolids used also had hig her concentrations of metal contaminants than the biosolids applied to farms. Surface and subsurface movement of nutrients and metals were m onitored in the runoff water and soil profile, respectively. The bioso lids reduced runoff and increased surface retention of rainfall. Data collected over one and a half years show very low concentrations of me tals in the runoff waters. The concentrations measured are not conside red to be of any concern to environmental pollution. The nitrate conce ntration in the runoff water from the biosolids-treated plots were hig her than from the control plots. There was movement of the plant nutri ents Zn and Cu in the upper 30 cm of the soil profile. There was a sig nificant movement of nitrates down the soil profile to a depth of 50 c m in the duplex soils and 70 cm in the sandy Red Earths in one and a h alf 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 ni trate and some metals were detected in the grazing trial at Goulburn. However. the actual amounts of these plant nutrients were low and almo st negligible in treatments applied at rates of 30 dry t/ha. (C) 1998 Elsevier Science B.V. All rights reserved.