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of Pb with the particulate fraction of organic carbon (r
2
= 0.42), Pb did not correlate
with any element. These results indicated that most of the trace metals could be
associated to particles such as metallic oxy-hydroxides and for Ni and Cu, clays
minerals or siliceous particles. Pb seemed to be linked partially to particulate
organic matter.
As filtration of runoff through a 0.45 mm filter can result in colloidal materials
being measured as dissolved, in series partitioning was performed on four samples.
The whole protocol described above was applied to the two first samples. No sig-
nificant drop in concentration for trace metals was observed applying the 0.22-mm,
100 and 30 kDa cut-offs. The 0.05-mm cut-off seemed to slightly influence the
concentration of Zn and organic carbon but could hardly be related to a specific
particle size. Finally, the applied protocol was composed of four cut-offs (8, 1.2 and
0.45 mm and 5 kDa). Three fractions were considered to compare the results: the
dissolved fraction <5 kDa (dissolved species, very small colloids such as fulvic
acids); the colloidal fraction with sized particles between 5 kDa and 8 mm and the
particulate fraction composed of particles above 8 mm. Size fractionation for the
May 16, 2006 rain event was selected for illustration because the mass balance was
especially accurate for trace metals (Fig.
2
). Al, Fe, Cr and Pb were predominantly
associated with particles greater than 8 mm in size. Colloidal-bound metals repre-
sented only a few percent of the distribution. Zn, Ni, Cu were also mostly particu-
late-bound but 3-15% of these trace metals was colloid-associated. Na, Ca and K
were present in runoff waters mainly as dissolved species (75-90%), Mg being
equally fractionated between the fractions <5 and >5 kDa.
Fig. 2
Distribution of major and trace elements in the particulate, colloidal and dissolved frac-
tions for a spring rain event (May 16, 2006)
