Movement of Heavy Metals below Sewage Disposal Ponds

Coarse‐textured soils below sludge and effluent disposal ponds at two sewage treatment plants were studied. The concentrations of Zn, Cd, Cu, Cr, and Ni in the soils at various depths were determined to investigate the downward movement of these heavy metals below the two types of disposal ponds. Concentrations of acid‐extractable metals (4N HNO3) were greater under disposal ponds than offplant control sites at both treatment plants. Metal enrichment was evident to depths as great as 3 m under some ponds. The depth and degree of metal enrichment depended on the specific pond type and the treatment plant. Metal concentrations were greater under anaerobically digested sludge‐holding ponds than effluent ponds. Concentrations of the metals in saturation extracts of soils beneath the ponds were much greater than those typically found for California soils. The distributions of metals with depth were closely related to the changes in chemical oxygen demands of soil samples with depth. This suggests that the metals have moved as soluble metal‐organic complexes.