Trace Metal Movement in an Aeric Ochraqualf following 14 Years of Annual Sludge Applications

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Abstract
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 (∼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 (∼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.

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