Factors influencing heavy metal distribution in six Oklahoma benchmark soils

Six contrasting benchmark Oklahoma soils underlain by sedimentary rock were selected and sampled by horizon to parent material or a 2-m depth. Total Co, Cu, Ni, Pb, and Zn concentrations by horizon in total soil and clay fractions were determined by x-ray fluorescence. Total concentration of heavy metals in these soils is dependent on clay content. There are fivefold more Cu, twofold more Zn and Ni, and 1.5-fold more Co and Pb in the clay fraction than in the total soil. The clay fraction of surface horizons contains a greater concentration of Cu, Zn, and Co than subsoil horizons due to biogeochemical cycling of these micronutrients from the subsoil to the surface during melanization. Biogeochemical cycling reduces the total amount of heavy metal loss from surface to subsoil horizons by clay translocation but doesn't offset the illuviation of total heavy metals sorbed to clay. Higher levels of total heavy metals in each profile for each element are found in the B horizons than the A horizons (77% of the samples). Characterization of soil for total heavy metal content should include a sample from the B horizon especially in soil with higher clay content in the B than the A horizon. Soil parent material discontinuities cause changes in concentration of several heavy metals within two of the six soil profiles. Identification of soil parent material discontinuities is not a consistent factor causing change in heavy metal concentration within the soils studied.