Environmental Engineering Reference
In-Depth Information
There are two basic processes by which contaminants move from the
earth's surface through soils and groundwater. These processes are diffu-
sion and mass flow (advection and dispersion).
Diffusion and mass flow are affected by properties of the contaminant,
the soil, the intermediate vadose zone (the area below a crop root zone and
above the permanent water table) and the aquifer; climatological factors and
vegetation patterns:
• Properties of contaminants that determine their movement and
potential threat to water quality include water solubility, any ten-
dency to adhere to soil materials, persistency, and toxicity.
• Properties of soil, the intermediate vadose zone, and the aquifer that
affect rate of contaminant movement include infiltration characteris-
tics, pore size distribution, microbial population density and diver-
sity, organic matter content, total porosity, ion exchange capacity,
hydraulic properties, pH, and redox status.
• Climatic factors include temperature, wind speed, solar radiation
and frequency, intensity, and duration of rainfall.
• Vegetation may act as a sink for contaminants by uptake or assimila-
tion, thus reducing the amount of contaminant available for trans-
port to groundwater.
All these properties interact to determine the rate and amount of move-
ment of contaminants in soils and groundwater. Groundwater contami-
nation proves to be most challenging from assessment and remediation
perspectives as it depends on both the nature of contaminants and regional
hydrogeology.
Once in the subsurface environment as part of the aquifer, contaminants
are transported either in the dissolved phase or bound to nanocolloid par-
ticles, thus resulting in a contaminant plume away from the source zone.
The plume composition varies with time and distance as its size increases.
Based on the plume composition at a particular contaminated site, it is con-
venient to separate the plume into three regions (a) a near-field or source
region, (b) a transition zone, and (c) a far-field or dissolved plume region.
Rather than distance from the contaminant source, the criterion employed
to designate these regions is the chemistry of the contaminant mixture (Rao
et al., 1996).
The rate at which contaminants move in groundwater may vary between
fractions of a cm to a few cm per year, forming under certain idealized condi-
tions, an elliptical plume of contamination with well-defined boundaries. In
a recent study conducted by CRC CARE in Adelaide, Australia, a trichloro-
ethylene plume was found to extend nearly 300 m away from the source zone
despite the groundwater flow being slow, only 5 cm per year (Chadalavada
et al., 2011). Where geological formations include fractured rocks, some of
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