Long-Term Leaching of Trace Elements in a Heavily Sludge-Amended Silty Clay Loam Soil

Analysis by ICP-MS of shallow groundwater collected at a field site in New York that had been heavily loaded with sewage sludge more than 15 years earlier revealed elevated concentrations of Cu, Zn, Sr, Rb, Mo, Cd, As, Cr, Ni, Sb, W, Ag, Hg, and Sn compared with a nearby control site. Enhanced leaching of some elements from this near-neutral, fine-textured (silty clay loam) soil could be explained by exchange of soil-bound elements by components of the added sludge. For most of the heavy metals, however, increased leaching was a response to the high metal loadings in the soil, probably facilitated by higher dissolved organic matter in the leachate. Laboratory-determined distribution coefficients, KD, for the metals in newly prepared sludge/soil mixtures were lower than KD values of the field-aged sludge-treated soil, suggesting that metal mobility may have been substantially higher shortly after sludge application than many years later. Cumulative losses of certain trace elements from the topsoil have been estimated relative to Cr, a comparatively immobile element. These suggest that relative long-term losses range from 20 to 80%, with the order being: Sr, Mo, Sb > Ni, Cd, Cu > Zn, Ag. Generally, those elements with the smallest KD values (most soluble) measured recently in the soil had the largest loss estimates. However, present leaching loss rates are too low to explain the estimated relative losses of several of these elements from the topsoil over the 15 or more years since sludge application.