Parameter Estimation Guidelines for Risk-Based Corrective Action ...

Groundwater Services, Inc.
Continued
Definitions for Cross-Media Transfer Equations
D s
eff
eff
D ws
Effective diffusivity in vadose zone soils:
eff ⎡ cm 2 ⎤
D s ⎢ ⎥
⎣⎢
s
⎦⎥ = θ Dair as
3.33 ⎡
+ Dwat ⎤
2 ⎢ ⎥
θ T ⎣⎢
H
⎦⎥
⎡ 3.33
θ ws
⎤
⎢ 2 ⎥
⎣⎢
θ T ⎦⎥
Effective diffusivity above the water table:
D
eff
ws
⎡
⎢
cm ⎣ s
2
⎤
⎥ = hcap
+ h
⎦
⎡ h
⎢
⎣⎢
D
(
cap
v ) eff
+
cap
h
D
v
eff
s
d Lower depth of surficial soil zone (cm)
d s Thickness of affected subsurface soils
D air Diffusion coefficient in air (cm 2 /s)
D wat Diffusion coefficient in water (cm 2 /s)
ER Enclosed-space air exchange rate (l/s)
f oc Fraction of organic carbon in soil (g-C/g-soil)
H Henry’s law constant (cm 3 -H 2 O)/(cm 3 -air)
h cap Thickness of capillary fringe (cm)
h v Thickness of vadose zone (cm)
I Infiltration rate of water through soil (cm/year)
k oc Carbon-water sorption coefficient (g-H 2 O/g-C)
k s Soil-water sorption coefficient (g-H 2 O/g-soil)
L B Enclosed space volume/infiltration area ratio (cm)
L crack Enclosed space foundation or wall thickness (cm)
L GW Depth to groundwater = h cap + h v (cm)
L s Depth to subsurface soil sources (cm)
P e Particulate emission rate (g/cm 2 -s)
U air Wind speed above ground surface in ambient mixing
zone (cm/s)
V gw Groundwater Darcy velocity (cm/s)
⎤
⎥
⎦⎥
−1
eff
D crack
eff
D cap
W
δ air
δ gw
η
Effective diffusivity through foundation cracks:
eff ⎡ cm 2 ⎤
D crack ⎢ ⎥
⎣⎢
s
⎦⎥ = θ 3.33 ⎡
Dair acrack
+ Dwat ⎤ ⎡
2 ⎢ ⎥ ⎢
θ T ⎣⎢
H
⎦⎥
⎣⎢
Effective diffusivity in the capillary zone:
eff ⎡ cm 2 ⎤
D cap ⎢ ⎥
⎣⎢
s
⎦⎥ = θ 3.33
Dair acap ⎡
+ Dwat ⎤
2 ⎢ ⎥
θ T ⎣⎢
H
⎦⎥
3.33
θ wcrack
2
θ T
⎡ 3.33
θ wcap
⎤
⎢ ⎥
2
⎢
⎣
θ T ⎥
⎦
Width of source area parallel to wind, or groundwater flow
direction (cm)
Ambient air mixing zone height (cm)
Groundwater mixing zone thickness (cm)
Areal fraction of cracks in foundations/walls
(cm 2 -cracks/cm 2 -total area)
θacap Volumetric air content in capillary fringe soils
(cm 3 -air/cm 3 -soil)
θ acrack Volumetric air content in foundation/wall cracks
(cm 3 -air/cm 3 total volume)
θ as
Volumetric air content in vadose zone soils
(cm 3 -air/cm 3 -soil)
θ T Total soil porosity (cm 3 -pore-space/ cm 3 -soil)
θwcap Volumetric water content in capillary fringe soils
(cm 3 -H 2 O/cm 3 -soil)
θ wcrack Volumetric water content in foundation/wall cracks
(cm 3- H 2 O)/cm 3 total volume)
θ ws Volumetric water content in vadose zone soils
(cm 3 -H 2 O/cm 3 -soil)
ρ s Soil bulk density (g-soil/cm 3 -soil)
τ Averaging time for vapor flux (s)
⎤
⎥
⎦⎥
FIGURE 2. CROSS-MEDIA PARTITIONING EQUATIONS IN THE RBCA SPREADSHEET SYSTEM
LATERAL TRANSPORT FACTORS
During lateral transport within air or groundwater, COC concentrations in the flow stream will be
diminished due to mixing and attenuation effects (see Figure 1). Site-specific attenuation factors
characterizing COC mass dilution or loss during lateral transport can be estimated using the air
dispersion and groundwater transport models provided in the RBCA Spreadsheet System. Equations for
the steady-state analytical transport models incorporated in the RBCA spreadsheet are shown on
Figure 3. Equations LT-1 and LT-2 correspond to the Domenico 3-D groundwater solute transport model
and the standard gaussian air dispersion model, respectively. The user must provide information
regarding COC properties and transport parameters (flow velocities, dispersion coefficients,
retardation factors, decay factors, etc.), as required for the selected contaminant transport model.
Procedures for definition of the contaminant source term for the groundwater solute model (Equation LT-
1) are illustrated on Figure 4. Key assumptions of these lateral transport models are detailed in the
Tier 2 RBCA Guidance Manual (Connor et al, 1995).
NGWA Petroleum Hydrocarbons Conference 6 Parameter Estimation Guidelines
Houston, Texas, November 1996
for RBCA Modeling