GROUNDWATER - Water Quality Engineering in Natural Systems - page 179

aquifer matrix act as contaminant sources, since the
residual nAPL produces a dissolved plume as water
either infiltrates past the nAPL in the vadose zone or
flows past the nAPL in the saturated zone.
Residual sources of groundwater contamination typi-
cally exist in the overlying soil, and so remediation
of contaminated groundwater typically requires both
source removal in the soil and cleanup of contaminated
groundwater. The portion of the soil or aquifer matrix
containing the source nAPL is commonly referred to as
the source zone , and the groundwater containing dis-
solved contaminant is commonly referred to as the con-
taminant plume . Since LnAPLs and DnAPLs behave
much differently in soils and aquifers, remediation strat-
egies generally reflect these differences.
most sites contaminated with LnAPLs involve the
release of petroleum hydrocarbons that are aerobically
degradable by indigenous microorganisms. This natural
degradation typically limits the growth of dissolved
LnAPL plumes. Current conceptual models of biodeg-
radation assume that significant biological transforma-
tion does not occur in the residual nAPLs but occurs in
the dissolved phase.
Spilled gasoline is frequently encountered in prac-
tice, and the components of gasoline that are usually of
most concern are benzene, ethylbenzene, toluene, and
xylene (BTEX), which collectively make up about 10%
of gasoline by weight. The BTEX compounds, along
with fuel additives, are the most soluble constituents of
gasoline.

soil contaminants in the United States and worldwide;
a typical RGV for benzene is 1 mg/kg, and toluene,
ethlybenzene, and xylene have typical RGVs on the
order of 100 mg/kg. Target levels for total petroleum
hydrocarbons (TPHs) in soils are typically in the range
of 10-100 mg/kg.

5.8.1.2 Saturated Zone. The saturated zone is the
region below the water table where all pores are com-
pletely filled with groundwater. Target levels for ground-
water remediation in the saturated zone are usually
related to: (1) maximum contaminant levels (mCLs)
or maximum contaminant level goals (mCLGs) for
drinking water, (2) risk-based considerations, and/or
(3) resource protection goals. Usually, maximum con-
taminant levels or risk-based target levels are used.
mCLs are derived from acceptable risk levels by
assuming direct consumption of the water by humans,
while risk-based target levels take into account poten-
tial mechanisms for exposure of human or ecological
receptors, the uses of the groundwater, the distance
between the contaminant source zone and water-
supply wells, and the attenuation that occurs between
the contaminant sources and water-supply wells. Other
exposure pathways include inhalation of vapor and
discharges to surface water bodies. Risk-based target
levels can either exceed or be less than mCLs, depend-
ing on the site-specific conditions and the acceptable
risk of illness resulting from exposure to toxic com-
pounds originating at the contaminated site. Accept-
able risks of illness caused by carcinogenic chemicals
are typically in the range of 10 −5 -10 −6 . Remediation of
subsurface contamination based on risk analysis is
called a risk-based corrective action (RBCA). In con-
trast to using either mCLs or risk-based considerations,
resource protection goals apply to groundwaters where
no contamination is acceptable, and in these cases,
target levels are either zero or background concentra-
tions. Resource protection goals are usually the most
difficult to achieve.
Target cleanup levels for groundwater can be applied
either at the boundary of a compliance zone or every-
where in the groundwater. Cleanup everywhere in the
groundwater is always preferred but is not always prac-
tical, and compliance zones are sometimes negotiated.
Compliance zones are similar to mixing zones in surface-
water discharges, where contaminant concentrations
may exceed target levels within the compliance zone.
In general, background concentrations should be
determined on a case-by-case basis since these values
are likely to be site and depth specific. For a small site,
the best method for determining background concen-
trations is usually installing several upgradient wells,
and, ideally, these wells should be screened in the same

5.8.1 Remediation Goals

Remediation goals are the bases for the formulation of
remediation strategies. The key components of remedia-
tion goals are: (1) definition of target (contaminant con-
centration) levels, (2) delineation of a compliance zone,
and (3) a statement of the time frame within which
compliance will be achieved. Remediation goals are
typically defined separately for contaminated soils in
the vadose zone and contaminated groundwater in the
saturated zone.

5.8.1.1 Vadose Zone. The vadose zone is the unsatu-
rated region between the ground surface and the water
table and contains the soils that overly the groundwater.
Target levels for contaminants in soils are generally set
by regulatory agencies and are usually based on the
likelihood of human exposure and the risks to human
health. These target levels are commonly referred to as
regulatory guidance values (RGVs). Risk-based RGVs
are typically related to child ingestion, inhalation, and
dermal exposure. Benzene, toluene, ethylbenzene, and
xylenes are among the five most commonly regulated