The Health Risk Assessment for Pb and Cr Leachated from Fly-Ash Monolithic Landfill

As of 2004, nearly 200 thousand tons of fly ash monolith are created by Portland cement each year in Taiwan. The main purpose is to confine heavy metals to reduce the quantity leachated by precipitation. However, due to abnormal monolith fracture and poorly liner quality or exceeding usage than designed landfill capacity, serious groundwater pollution has been observed.

Therefore, this research focused on Pb and Cr leachated from monolithic landfill to assess the risk of groundwater pollution in the vicinity. The methodology combined water budget simulations using HELP model with fate and risk simulations using MMSOILs model for 5 kinds of landfill structures and 2 types of leaching models, and calculated the risk distribution over 400 grids in the down gradient direction of groundwater.

The results demonstrated that the worst liner quality will cause the largest leaching, but with the best quality liner the least leaching will be generated. The most significant exposure pathway is groundwater intake, which accounted for 98% of the total risk. Comparing Pb and Cr concentrations in the groundwater with the drinking water standards, only 14.25% of the total grids were found to be under 0.05mg/L of Pb, and over 96.5% of the total grids were in the safety range of Cr. The results indicated that we should pay greater attention to Pb leaching from fly ash monolithic landfills.

Without consideration of the parameters uncertainty, the cancer and non-cancer risk of Pb with the sanitary landfill method was 4.23E-07 and 0.63, respectively, both under acceptable levels. However, by considering the parameters uncertainty, the non-carcinogenic risk of Pb became 1.43, exceeding the acceptable level. Only under closed landfill method was the HQ below 1. The reason may be due to that there was high variability in the Pb concentration in the fly-ash monolith.