Migration of heavy metals in soil strata below and around a simple dump of MSWs

Field and laboratory studies are carried out to investigate the degree of soil contamination below and around a simple dump of municipal solid wastes in Huainan. The field program includes borehole investigation, soil and water sampling and measurement of water levels. The soil and water samples are collected to laboratory to determine the contents of Cu, Pb, Zn and Cr and their speciation in the soil strata. The distributions of heavy metal content along both the vertical and horizontal directions are obtained. The test results show that the contents of Cu, Pb and Cr in the strata do not exceed the second-level limits of the national soil quality standard, and the content of Zn in the shallow soil layer exceeds the second-level limit and is smaller than the third-level limit. After 17 years of operation, the heavy metals of Cu, Pb, Cr in leachate have migrated to the depth of more than 2 m, i.e., breaking through the planting soil stratum. The migration depth of Zn is more than 3 m at the bottom of the waste dump. The percentage of exchangeable heavy metals over the total content is measured to be lower than 1% even for the soil stratum in contact with the leachate. The percentage of organically-bonded heavy metals at the bottom of the waste dump was observed to be significantly greater than the background value. With the increase of depth, the speciation of heavy metals is close to the background value. The silty clay and clay layer at the depth greater than 2 m, with low permeability and a thickness of more than 7 m, is found to act as an effective barrier to the heavy metals.