Geoscience Reference
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amendments applied on clay loam and silt clay loam, Hafida et al. (
2007
)considered
that pore occlusion by biologically mediated formation of neutral sugars, uronic
acids, and lipids contributes to aggregate stability. As a consequence, it can be
concluded that addition of organic amendments to soils causes increases in pore
surface roughness and pore occlusion, which contribute more to the formation of
stable aggregates than to growth of surface wetting angles. In agricultural practice,
sludges are repeatedly applied to the land surface, and organic residues continue to
accumulate, providing a quasicontinuous source of energy for microbial populations.
As a result, we can expect that sludge-induced changes in the microstructure of
WSA is an irreversible process on the scale of human lifetimes.
Clogging of pores by suspended particles, following disposal of sludges on the
land surface and their vertical transport during rainfall or irrigation, is another
process that causes changes in the microstructure of the soil-subsurface domain
and on its water transmission properties. The dominant mechanism of clogging
depends on the physical and chemical properties of the soil-subsurface domain,
the composition of the wastewater or sludge, the method of application on land
surface, and the timescale involved. In the short term, physical clogging of pores
by suspended solids in the upper part of the soil layer forms a surface mat; this is a
major feature that reduces the hydraulic conductivity. The process of filtration at
depth leads to a reduction in soil hydraulic conductivity, but at a slower rate than
that of clogging of surface pores. The stability of clogging, and thus its irreversible
impact, should also be considered in terms of microbial habitats and activity
during drying-wetting cycles in the unsaturated zone.
Soil column laboratory experiments by Vinten et al. (
1983
) showed the relative
amounts of deposition of suspended solids in the top 50 mm, following leaching of
three different soils with about 100 cm of wastewater (characterized by 98 mg/L
suspended solids) (Fig.
18.41
). In silt loam soil, the water was filtered before use,
giving a concentration of 38 mg/L. The soil type strongly influences the rate of
clogging. In the sandy soil, the hydraulic conductivity decreases much slower than
that of silt loam, despite the filtering of the effluent before application.
On-site wastewater systems are used widely around the world as an alternative
to centralized treatment facilities. Septic tanks connected to subsurface trenches,
used in on-site water systems, form point sources for accumulation of organic
matter components in the soil-subsurface domain. These components clog pores
and cause a decrease in water transmission properties. The kinetics of suspended
particle deposition in a porous medium significantly affect pore clogging, as shown
by Amitay-Rosen et al. (
2005
) using noninvasive MRI measurements (Fig.
18.42
).
Some studies have revealed biochemical processes under on-site wastewater
disposal systems, with associated clogging and decrease in infiltration. McKinley
and Siegrist (
2010
) analyzed the clogging layer (1.5 cm thickness) in an on-site
wastewater disposal system in a sandy loam soil (Golden, Colorado), finding that
most organic materials accumulated in the upper 0.7-cm segment. The concentra-
tions of total organic carbon (TOC), humic acid (HA), and fulvic acid (FA), as well
as of polysaccharides, found in the clogging layer are shown in Fig.
18.43
. In this
particular site, the less humified carbonaceous materials (polysaccharide and fulvic
