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
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.