Geoscience Reference
In-Depth Information
fraction. Turner et al. (
2004
) suggested that the soils may have two size fractions
of colloidal MRP: a large fraction (0.2-1.0 lm) associated with Al and Fe, which
contains phosphate adsorbed to fine clays, and a smaller fraction (0.3-3 nm)
probably associated with Ca and Mg (suggesting the presence of calcium phos-
phate and magnesium phosphate minerals). Large concentrations of colloidal MRP
in runoff suggest significant phosphate mobility, which may lead to phosphate
enrichment of surface water and negative effects on water quality, or greater
vertical phosphate mobility into the subsurface.
12.4.2 Transport in the Subsurface
Colloid-facilitated transport of organic compounds occurs frequently in the sub-
surface. Organic compounds with very high K
d
values (and low solubility) are
virtually immobile in the subsurface. As such, their presence in the groundwater
suggests that they migrate through the subsurface together with colloidal materials
rather than as dissolved solutes. Karathanasis et al. (
2005
), in an experiment
carried out on undisturbed soil monoliths, investigated the ability colloidal fraction
of organic biosolid applied to the soil as agricultural amendments to be transported
through the soil-subsurface cross section. Water-dispersible colloids fractionated
from lime-stabilized biosolids (LSB), aerobically digested biosolids (ADB), and
poultry manure biosolids (PMB) were applied to the soil monoliths, and effluents
were monitored for heavy metal and colloid concentration over 16-24 pore vol-
umes of leaching. Breakthrough curves for Cl
-
and colloids eluted from three soils
experiments are presented in Fig.
12.43
. Colloid breakthrough curves generally
were irregular but in all cases showed a slower breakthrough than the conservative
Cl
-
tracer, indicating significant interaction with soil during leaching. Irregularity
of the breakthrough curves may be also affected by different preferential flows in
each soil.
Early experimental tests on vertical transport of pesticides through soils dem-
onstrated this phenomenon (Vinten et al.
1983
). Figure
12.44
demonstrates ver-
tical transport of an herbicide (paraquat) and a very persistent insecticide (DDT)
adsorbed on suspended materials through various soil columns leached with
aqueous solution. Figure
12.44
a shows the results of an experiment using
14
C-
labeled paraquat adsorbed on a clay mineral (Li-montmorillonite) suspension
through a soil column. When the suspension medium was distilled water, 50 % of
the pesticides penetrated beyond 12 cm. Under these conditions, clay remains
dispersed and pesticide is readily transported through the soil. However, for a
suspension medium with an electrolyte concentration of 1 mM CaCl
2
, paraquat
remains in the upper 1-cm layer. The high calcium concentration results in rapid
immobilization of the clay in the soil through flocculation, and consequently, little
pesticide transport occurs.
The redistribution of DDT with depth also was tested, in the presence of organic
suspended solids from sewage effluents. Figure
12.44
b shows a range of behaviors
