Environmental Engineering Reference
In-Depth Information
3.3.2.8 Use of Laboratory Batch Tests to Measure
K
d
in Soil
Contaminants residing in the vadose zone may leach to underlying groundwater from percolation
of rainfall, l ood water, or applied irrigation. The degree to which contaminants are prone to dis-
solution, desorption, leaching, and migration to the saturated zone is the subject of the various
estimation methods described in this section. Fate and transport models often rely on published
values of
K
oc
and assumed organic matter contents (
f
oc
) in soil to estimate contaminant retardation
and partitioning between sorbed and aqueous phases. This approach can signii cantly underpredict
the sorption potential of a contaminant and overpredict its mobility in soil or groundwater, when
compared with soil-column leach testing.
*
Laboratory batch tests can be used to quantify the mobility of contaminants in soil and to
estimate the initial concentration of the contaminant in soil leachate and the i nal concentration as
the contaminant enters the saturated zone. Batch tests involve placing a small quantity of soil in
buffered, deionized water, agitating the soil slurry for a set period of time, and measuring the solu-
ble fraction of the contaminant. A minimum of three soil samples is generally needed to validate
batch test data for each area investigated (HDOH, 2007). The ratio of the mass of a contaminant that
remains sorbed to the contaminant mass that goes into solution is referred to as the contaminant's
desorption coefi cient or
K
d
value (see additional discussion of
K
d
—also called the distribution coef-
i cient, but most often called the partition coefi cient—in
Sections 3.3.2.3
,
3.4.1
, and
3.4.2
).
The key parameter in soil leaching models is the contaminant's
K
d
value. Lower
K
d
values imply
greater contaminant mobility in soil and a greater leaching potential. Contaminants with
K
d
values
less than 1.0 are considered to be highly mobile, whereas contaminants with
K
d
values greater than
20 are considered essentially immobile and do not pose a signii cant leaching concern. Measured
values of 1,4-dioxane are typically less than 1.0, as discussed in Section 3.4.2.
The contaminant concentration that partitions into leachable soil moisture can be calculated by
using the soil-specii c
K
d
value in an equilibrium partitioning equation:
(
)
È
˘
Ê
ˆ
Q+Q¥ ¢
H
Ê
1mg
ˆ
w
a
CC
=
¥
Í
K
+
˙
¥
,
(3.37)
Á
˜
Á
˜
r
1000
m
g
total
leachate
d
Ë
¯
Í
Ë
¯
˙
Î
b
˚
where
C
total
is the total concentration of chemical in the soil sample (in mg/kg),
C
leachate
is the
dissolved-phase concentration of chemical (in
μ
g/L),
K
d
is the estimated or measured distribution
coefi cient (in L/kg),
Θ
w
is the water-i lled porosity (in L/L or volume of water per volume of soil),
Θ
a
is the air-i lled porosity (in L/L or volume of air per volume of soil),
H
′
is the Henry's law
constant at 25°C [in (
g/L), that is, micrograms of chemical per liter of vapor divided by
micrograms of chemical per liter of water], and
μ
g/L)/(
μ
ρ
b
is the soil bulk density (in kg/L).
The i nal concentration of the contaminant in groundwater as a consequence of its leaching is
estimated by assuming a groundwater-leachate dilution factor, DF, usually assumed to be 20 for
sites less than 0.5 acre in size (USEPA, 2001b). The DF is the volume of affected groundwater
divided by the volume of leachate.
Where site-specii c hydrogeological data are available, a site-specii c DF can be calculated
according to the following equation (USEPA, 2001b):
Ê
Ki d
IL
¥¥
ˆ
DF
=+
Á
1
,
(3.38)
˜
Ë
¥
¯
where
K
is the aquifer hydraulic conductivity (in m/year),
i
is the regional hydraulic gradient,
d
is the assuming mixing zone depth (default is 2 m),
I
is the surface-water ini ltration rate
*
Personal communication with Roger Brewer, PhD, State of Hawaii Department of Health.
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