Agriculture Reference
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was not expected to be solubilized over a reasonable period of time on natural
conditions [8]. For the four heavy metals, the exchangeable fractions with high
bioavailability and mobility were increased averagely about 2.0-10.1%, which
indicated the increase of direct risk of Pb, Cd, Cr, and As to agricultural
production and soil/groundwater system. The amounts of non-residual
fractions represent the amounts of potentially active trace elements [25]. In
general, the high proportion of trace elements in non-residual fractions of the
soils may suggest great contribution of anthropogenic elements. The non-
residual fractions of Pb, Cd, Cr, and As in the original topsoil averaged
47.07%, 48.50%, 17.06% and 5.66%, respectively. The non-residual fractions
of Pb, Cd, Cr, and As in topsoil after leaching tests averaged 75.2%, 65.4%,
77.7% and 66.8%, respectively. The increase of non-residual fractions
elucidated the increase of potential risk of Pb, Cd, Cr, and As to soil
environment.
C ONCLUSION
The topsoil of two farmland sites in northeast China were found
moderately or heavily contaminated with one or more of four elements (Pb,
Cd, Cr and As). Several major isomers of BHC and DDT were all detected in
the two soils. In the migrate process of heavy metals from root, stem, leaf to
fruit, and their concentrations were diluted and diffused. The experiments of
adsorption and column leaching of heavy metals and organchlorine pesticides
in topsoil indicated that the two soils had greater adsorption capacity of heavy
metals (Pb, Cd, Cr and As) than organochlorine pesticides (DDT and BHC).
For the two soils, the adsorption capacities of Pb and Cd were greater than that
of Cr (VI) and As (V). The migration rate of Cr (VI) and As (V) in topsoil was
much greater than that of Pb and Cd and the migration rate of Cr (VI) was
greater than that of As (V). Due to the combined effect of adsorption and
degradation, ʱ-BHC migrated slowly in topsoil. It could be found from the
results of adsorption and leaching tests that Pb and Cd had greater threat to
agricultural production. Cr (VI) and As (V) had greater threat to groundwater
system. Adsorption, degradation and prohibition of organic pesticides made its
less threat to agriculture production and soil/groundwater system. Speciation
analysis of heavy metals showed that human input made the increased
concentrations of exchangeable heavy metals with high bio-availability and
mobility.
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