GROUNDWATER - Water Quality Engineering in Natural Systems - page 180

flow zone(s) as the suspected contaminants. Historical
records of land use at the site should always be consid-
ered, and any relevant historical data should be used in
establishing background concentrations.

under conditions of great uncertainty, and that system
design relies heavily on conceptual models, screening
level calculations, empiricism, heuristics, experience,
monitoring, and refinement. The effectiveness of reme-
diation strategies is dependent on knowledge of con-
taminant sources, since only when sources are properly
identified can the optimal remediation strategy be
selected. Following sound design practice does not
guarantee success, but certainly provides a higher pro-
bability of success.

5.8.2 Remediation Strategies

Remediation strategies are designed to accomplish
well-defined remediation goals. There are a wide variety
of remediation strategies, and the appropriate strategy
for any particular situation depends on several factors,
including the hydrogeology of the site, the nature of the
contaminants, the distribution of contaminants in the
subsurface, the target levels, the time frame for cleanup,
exposure of cleanup workers and the public, technical
feasibility, and economic considerations. A particular
concern in groundwater remediation is that significant
uncertainties can exist in subsurface characterization,
fate and transport models, and the performance of
various remediation strategies. Under such circum-
stances, consideration should be given to conducting a
probabilistic risk assessment in which the trade-off
between uncertainty and cost of remediation is taken
into consideration.
Remediation strategies can be grouped as those that
(1) remove residual nAPL contaminants in the soil and
groundwater that serve as sources of contamination,
referred to as source zone treatment , (2) target the treat-
ment of groundwater containing dissolved contami-
nants to restore the groundwater to target levels,
referred to as aquifer restoration , or (3) prevent further
migration of the contaminant referred to as migration
prevention . Some remediation strategies belong to more
than one group, and several commonly used remedia-
tion strategies are listed in Table 5.15. It is important to
note that remediation strategies are generally designed

5.8.2.1 Free Product Recovery. most regulatory
agencies require the removal of any mobile and pump-
able imiscible free product liquid that can be collected
in monitoring wells. In the case of LnAPLs, free product
is usually found within the capillary zone just above the
water table, although seasonal fluctuations in the water-
table elevation can cause free product to become
trapped below the water table. In the case of DnAPLs,
pure product is usually found at the bottom of the
aquifer. To recover LnAPL free product, a recovery well
is typically screened in the zone containing the free
product and the LnAPL is pumped out of the well.
However, it is important to note that free product thick-
ness in the recovery well does not give a true indication
of the free product thickness in the aquifer. The reason
for this is that the nAPLs are distributed at varying
saturation levels within the capillary fringe, which con-
tains mostly groundwater. Installation of a well screen
allows the nAPL to seep into the well from the entire
capillary fringe and above, filling the well up to the level
where there is significant amounts of nAPL in the sur-
rounding porous medium. Since the pores in the aquifer
material surrounding the well are not fully saturated
with nAPL, the depth of nAPL in the well gives a false
indication of the depth of nAPL in the surrounding

TABLE 5.15. Groundwater Remediation Strategies

Objective

Source Zone Aquifer migration

Technology Treatment Restoration Prevention

Free-product recovery •

Excavation and disposal •

Soil vapor extraction •

Bioventing •

Air sparging • •

Air sparging cutoff trenches •

Pump-and-treat • • •

Bioremediation •

•

In situ reaction walls

In situ containment •

natural attenuation • •