MOBILITY AND SOLUBILITY OF TOXIC METALS AND NUTRIENTS IN SOIL 15 YEARS AFTER SLUDGE APPLICATION

Citation
Mb. Mcbride et al., MOBILITY AND SOLUBILITY OF TOXIC METALS AND NUTRIENTS IN SOIL 15 YEARS AFTER SLUDGE APPLICATION, Soil science, 162(7), 1997, pp. 487-500
Citations number
43
Categorie Soggetti
Agriculture Soil Science
Journal title
ISSN journal
0038075X
Volume
162
Issue
7
Year of publication
1997
Pages
487 - 500
Database
ISI
SICI code
0038-075X(1997)162:7<487:MASOTM>2.0.ZU;2-D
Abstract
The increased use of sewage sludge as a fertilizer, combined with repo rts that large fractions of sludge-borne heavy metals cannot be accoun ted for several years after land application, indicates that more deta iled 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 rec ently at this site was measured by inductively coupled plasma (ICP) sp ectrometry analysis of acid digests as well as by neutron activation a nalysis (NAA). These two methods gave comparable results for most slud ge-applied heavy metals, but not for those elements that reside largel y 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 esti mated 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 t he topsoil. There is Limited evidence that about half of other heavy m etals such as Hg, Ag, and Au has also been lost. The ICP analysis of w ater extracts of the soils shows present concentrations of water-solub le Cu, Zn, and Ni in the sludge-amended topsoil are more than 10 times higher than those in the control soil. Ion-selective electrode measur ements confirm that a large fraction of soluble Cu appears to be in an organically complexed and mobile form, which leaches to shallow groun dwater. 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 ele mental 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 appli cation 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 upta ke and severe phytotoxicity symptoms in vegetable crops. In summary, a lthough the vertical metal concentration profiles failed to manifest s ubstantial movement of most trace metals, it appears that large fracti ons of certain metals applied in the sludge amendment have redistribut ed and moved out of the soil surface by physical-chemical or biologica l processes and that there is potential for groundwater and surface wa ter contamination.