HEAVY METAL MOVEMENT IN METAL-CONTAMINATED SOIL PROFILES

Abstract

Heavy metal movement in soil profiles is a major environmental concern because even slow transport through the soil may eventually lead to deterioration of groundwater quality. In this study, three metal-contaminated soils (Fuquay, Dothan, and Clarendon) were selected from cropland where a high-metal flue dust had been applied annually for 6 years to raise soil pH, with application ending 4 years before sampling. One uncontaminated soil (Tifton) from the same physiographic area was also sampled as a control. Soil samples were collected in 15-cm increments from the surface to 105 cm in depth. Total contents of Zn, Cd, and Pb in the soil samples were determined. To better understand metal movement in relation to metal fractions in the soil profile, soil samples were also extracted sequentially for exchangeable (EXC), organic matter (OM), Mn oxide (MNO), amorphous Fe oxide (AFEO), crystalline Fe oxide (CFEO), and residual (RES) fractions. The Clarendon soil had more Zn, Cd, and Pb than the Dothan and Fuquay soils because of a higher application of flue dust. The metal concentrations in the contaminated topsoils were much higher than that in the Tifton (control) topsoil, whereas the differences in the subsoils were minimal except for the Zn in the Clarendon soil. High concentrations of Zn were found in the Clarendon subsoil (96 mg kg-1 at 45-60 cm). A large portion of Zn in the subsoil was in the EXC fraction, which suggests that the Zn migrated in this soil profile. In the Fuquay and Dothan soils, movement of Zn was not evident. Except for slight movement of Cd in the Clarendon soil profile, there was no movement of Cd and Pb in these soil profiles. Cadmium and Pb had accumulated in the topsoils probably because of their affinity with the OM fraction. The application of high-Zn flue dust to coarse-textured soils low in organic matter may induce Zn movement in soil profiles.