Movement of Heavy Metals in Sewage Sludge‐Treated Soils

Abstract

Abstract A controlled column study was conducted to evaluate the potential for Cd, Cu, Ni, and Zn movement through three reconstructed soil profiles from surface applied sewage sludge. Anaerobically digested sewage sludge was mixed into the top 15 cm in either liquid or air‐dried form three times for a total rate of 476 metric tons/ha. The soil columns were leached with 5 m of Colorado River water over a 25‐month period. After leaching, columns were sectioned and analyzed for metal movement by a complete metal mass balance. Analysis of the soils below the sludge‐soil layers indicated that essentially no metals moved below the depth of incorporation. The mass balance calculation for the metals in the sludge‐soil layers confirmed that movement had not occurred, in that close to 100% of the added metals were recovered in the sludge‐soil layers. Although no metal movement was observed, a situation conducive to metal movement was noted. Greatly lowered pH of the soil profiles beneath the sludge‐soil layers was attributed to the mineralization and nitrification of the added organic N. A subsequent pH reduction in the sludge‐soil layer could increase the solubilization of the metals and induce movement in the soil profiles, but this was not observed within the time frame of this study.