MOBILITY AND SOLUBILITY OF TOXIC METALS AND NUTRIENTS IN SOIL FIFTEEN YEARS AFTER SLUDGE APPLICATION

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Abstract
The increased use of sewage sludge as a fertilizer, combined with reports that large fractions of sludge-borne heavy metals cannot be accounted for several years after land application, indicates that more detailed study of potential mobility of these metals in soils is needed. A field site that had received a single heavy application of municipal sewage sludge was re-investigated 15 years later to measure the degree to which toxic heavy metals as well as nutrient elements remained in the topsoil. Total elemental composition of soil samples collected recently at this site was measured by inductively coupled plasma (ICP) spectrometry analysis of acid digests as well as by neutron activation analysis (NAA). These two methods gave comparable results for most sludge-applied heavy metals, but not for those elements that reside largely in mineral lattice structures resistant to acid digestion. Based on elemental deficits calculated using soil chromium concentration as an indicator of the original sludge concentration in the soil, it is estimated that most of the sludge-applied Na, S, Ca, and Sr, about 40% of the Zn and Cu, and less than 30% of the Cd and P have been lost from the topsoil. There is limited evidence that about half of other heavy metals such as Hg, Ag, and Au has also been lost. The ICP analysis of water extracts of the soils shows present concentrations of water-soluble Cu, Zn, and Ni in the sludge-amended topsoil are more than 10 times higher than those in the control soil. Ion-selective electrode measurements confirm that a large fraction of soluble Cu appears to be in an organically complexed and mobile form, which leaches to shallow groundwater. To varying degrees, numerous other elements, including Cd, P, and S, have solubilities substantially higher than in the control soil after the 15-year period. Nevertheless, despite the fact that the elemental deficits are correlated to the distribution coefficients of the elements, the deficits can only be accounted for by leaching if those elements were more soluble in the soil immediately after sludge application than they are now. The Zn and Cd remaining in the topsoil (pH 6.5-7.0) after 15 years is plant-available, as indicated by excess uptake and severe phytotoxicity symptoms in vegetable crops. In summary, although the vertical metal concentration profiles failed to manifest substantial movement of most trace metals, it appears that large fractions of certain metals applied in the sludge amendment have redistributed and moved out of the soil surface by physical-chemical or biological processes and that there is potential for groundwater and surface water contamination.