Kouli_etal_2008_Groundwater modelling_BOOK.pdf - Pantelis ...

182
Z. Yu, Y. Huang, A. Baron et al.
Excavation, fixation, and soil venting are available techniques for removing contaminants
in the unsaturated zone and soil. Generally, for small spills, excavation and removal is the
most cost effective solution. In these cases, contaminated materials can be excavated and
either treated or stored in other safe places. Fixation and encapsulation methods can be used
to reduce contaminant leaching. Soil venting is only used to remove volatile organic
compounds from the unsaturated zone and is based on the principle of enhancing
volatilization by the aspirating gases to create subsurface vapor circulation.
When contaminants enter the groundwater, the problems become more complicated.
Interceptor systems such as drains and trenches are used to collect contaminants close to the
water table. Pump-and-treat systems have been widely used to remediate various
contaminated sites, but, because of their inefficiency in mass removal, they are currently used
largely as hydraulic barriers. When combined with physical barriers, these systems can
extract contaminated groundwater from the subsurface and intercept aqueous contaminant
plumes. The extracted contaminated groundwater must then either be treated or directed to
nearby streams (if it will not affect the stream water quality significantly). Air stripping can
be used to remove the volatile contaminants in the extracted contaminated groundwater,
granular activated carbon can be used to remove dissolved organic contaminants, and
biological systems can be used to remove biodegradable contaminants.
Because of the nature of various contaminants (e.g. NAPLs) and their associated
chemical reactions and physical processes, many of them cannot be extracted quickly.
Contaminated water can be removed and replaced with clean water, but undissolved
contaminants remaining in subsurface media in the saturated zone will slowly re-contaminate
the groundwater. The result is that very long periods of time are required for pump-and-treat
to be effective and for contaminated water in the saturated zone to reach a required drinking
water standard or other cleanup goal. Complete restoration of the groundwater is often
impossible with pump-and-treat. The limitations of the system were not fully realized until
1989 (NRC, 1994). Due to our poor understanding of contaminant-related processes, the
majority of sites that used only pump-and-treat systems have not been restored to drinking
water standards. The feasibility of contamination cleanup with pump-and-treat systems
depends on site-specific contaminant chemistry, site geology, and the quantity and duration of
contamination. In light of these factors and the long cleanup times required, pump-and-treat
systems alone are often impractical for restoring the groundwater quality. Alternatives are
required.
Various enhanced pump-and-treat and alternative technologies have been developed and
used in the last decade. Pulsed and variable groundwater pumping allows the contaminant
concentration to build up so that contaminants can be removed with higher efficiency. Many
NAPL contamination problems are dealt with by directly pumping the nonaqueous phase,
which tremendously increases mass removal. For LNAPLs, this requires installation of
floating skimmer system pumps in recovery wells. Soil vapor extraction can be coupled with
groundwater pumping to extract organic contaminants by flushing air in the unsaturated zone
near a created cone of depression. Similar to soil vapor extraction, air sparging uses circulated
air to remove volatile contaminants, although sparing is carried out in the saturated zone, and
involves injection of air, not solely removal. The air stream carrying contaminants from a
sparged saturated zone must be captured by well-designed soil vapor extraction systems. In
situ bioremediation systems inject stimuli (e.g., oxygen) to stimulate subsurface
microorganisms (e.g., bacteria) to biodegrade hydrocarbon contaminants in the saturated zone