TRACE-METAL MOVEMENT IN AN AERIC OCHRAQUALF FOLLOWING 14 YEARS OF ANNUAL SLUDGE APPLICATIONS

The potential for movement of sewage sludge-borne trace metals beyond the zone of soil incorporation poses a real concern for society. To address this concern, we sampled an Aeric Ochraqualf (Blount Series) after 14 yr of massive, sludge additions (765 Mg ha-1 cumulative sludge applications, dry wt. basis). Cadmium, Zn, and Cu concentrations in each genetic horizon to a depth of 1.0 m were determined by atomic absorption spectroscopic analyses of 4.0 M HNO3 extracts. In addition, ped surfaces (cutans) and intra-ped material (s-matrix) from prismatic structural units of the lower B and upper C horizons were analyzed to determine the partitioning of trace metals in this highly structured profile. Cadmium concentrations in the subsoil of sludge-treated areas were consistently and significantly higher (approximately 0.4 mg kg-1) than levels present in the control areas. Similarly, Zn concentrations in the 0.32 to 0.51 m region (Bt1) were increased (approximately 12 mg kg-1) with sludge applications; no increases in Cu levels were observed. Significantly more Cd (0.08 mg kg-1) was present in cutans removed from ped surfaces in the upper B horizon than was observed in the s-matrix for sludge-treated areas. Cutans also contained more sludge-borne Zn than the associated s-matrix. The evidence suggests that small amounts of sludge-borne Cd and Zn moved out of the tillage zone into the subsoil of this highly structured agricultural soil over a 14-yr period of massive sludge additions.