Ecological Risk Assessment Of Composted Sludge Applied In Yongding River Wasteland Afforestation Project

Municipal sludge composting is an effective approach for the utilization of sludge resources.Compared with direct agricultural application,wasteland afforestation exhibit additional advantages,which can avoid the human body exposure of sludge toxic substance through food chain.However,heavy metals and PAHs may accumulate in soil or permeate into surface and ground water through rainfall runoff,causing potential biological hazards.In this paper,afforestation sites and mixed tillage sites in wastelands of Daxing Yongding river downstream were studied.Three sludge composting products using two fertilization methods(facilities and mixed plowing)with different proportions(10%,20%,30%and 40%)and sludge composting application thickness(1 cm,3 cm and 4 cm)were studied to determine the migration and transformation pattern of environmental risk factors(heavy metals and PAHs)in different mediums(soil,surface water,ground water).The potential ecological risks of heavy metals and PAHs were investigated.Acceording to the continuous 1.5-year monitoring results,the accumulation of heavy metals and PAHs in soil and surface runoff showed a significant incremental tendency in higher sludge dosage.Meanwhile,exposure concentration of all environmental risk factors decreased as the fertilization time extended.During the monitoring period,concentrations of heavy metals in soil didn't exceed the restrict of level ? of Environmental quality standard for soils(GB15618—1995),and the concentrations of PAHs in soil didn't exceed the restrict of level ? of Environmental quality standard(Revised)(GB 15618-2008).Concentrations of all heavy metals except Hg met therestrict of level ? of Surface water environmental quality standards(GB3838-2002)and concentrations of all heavy metals met the restrict of level III of Groundwater quality standards(GB/T 14848-93).Soil single factor and potential ecological index evaluation determined that the major sources of potential ecological risks of wasteland afforestation were Hg and Cd.According to surface runoff single factor evaluation and comprehensive evaluation,Hg may cause certain ecological risks.Effect of sludge afforestation to heavy metal concentration of groundwater was negligible.The results of human health risk assessment showed that heavy metal exposure exhibited no carcinogenicity and carcinogenesis risks.From the heavy metal exposure perspective,the use of ginkgo forest under 20%of "mud" and white pine forest with one topdressing under 20%of "raw mud" is safe.The environmental ecological risks of mixed farming methods might be relatively high.The results of soil PAHs bioavailability evaluation showed that the probability of biological toxicity occurrence was less than 10%and could barely cause any negative biological effect.According to BaP toxicity equivalent method,no biological risks were detected except a few samples.The results of the risk quotient method showed that flatland afforestation proportion under 20%and mixed plowing thickness under 3 cm exhibited low risks.Only 6 low molecular weight PAHs were detected in surface runoff and groundwater,total TEQBaP of ?PAHs in surface runoff showed no ecological risks.The results of human health risk assessment showed that cumulative risk of life-long carcinogenic risk of soil are all less than 10-6,which could be regarded as safe.As for PAHs pollution,neither flatland afforestation nor mixed plowing sites exhibited environmental biological risks.