Soil Leaching to Ground Water Evaluation for Total Petroleum Hydrocarbons (TPH) Guidance
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impact on contaminant transport, since, cosolvent contamination may accelerate primary contaminant transport. For
example, chlorinated hydrocarbon solvents (i.e., perchloroethylene) or alcohols may facilitate the movement of TPH
chemicals to greater depths than would result from transport by infiltrating rain water alone. Thus, the guidelines for
leaching factors, given below, can only be reliably applied at sites without substantial co-contamination by other
chemicals that might impact lower mobility compounds.
Secondary Features: The potential for discrete features in the unsaturated zone (both unconsolidated and consolidated
deposits) to act as conduits to the water table must be assessed qualitatively. The presence, character, and density of any
faults, fractures, joints, subsidence fissures, solution channels, significant sand seams, and other similar features should be
evaluated. In the presence of such features, generic methodology may not be applicable.
Soil Stratigraphy: To select the appropriate leaching values, the horizontal and vertical variation in soil properties and
stratigraphic units should be evaluated, including the continuous profile of the stratigraphic units beneath the property
and the thickness and lateral extent of each unit. The effects of stratification on saturated and unsaturated flow should
also be considered, in addition to any anthropogenic influences (e.g. sewer pipes, conduits for utilities, etc.) that may
impact the geology/hydrogeology and create preferential pathways for migration.
Soil Contaminant Concentrations Ceilings: For the purpose of using this guidance, soil ceiling concentrations have been
developed. These values are summarized in the Stage Sections.
Man-Made Deposits: The modeling approach used to develop this guidance assumed that the incident precipitation
infiltrates directly into natural soils. Man-made deposits, such as coal piles, ash or slag heaps, gravel piles or coal tar
staining, could potentially alter the chemistry of the infiltrating rain water (e.g., pH, hardness, organic and metallic
content), resulting in different rates of leaching than predicted by these models. Thus the presence of any surface feature
that allows infiltration through anything other than natural soils could alter the leaching process. If any such features are
present, then a site-specific analysis would be necessary to determine their effects on the leaching process. The leaching
factors developed in this guidance should be used with great caution at such sites.
Final Ground Water Concentrations The purpose of this guidance is to provide an estimate of the relationship between
soil concentrations of TPH components and the resulting contamination levels in the underlying ground water. While the
cleanup levels formulated in this guidance are based on the best scientific knowledge currently available, local
hydrogeologic conditions and variability in petroleum product compositions may result in ground water contamination
levels above those predicted from the residual soil concentrations. Thus, remediating soil to the predicted TPH levels may
not always result in the ground water remedial goals. In those cases, additional remediation would be necessary.
Performance goals for remedial actions involving TPH contaminated soil should ultimately be based on the ground water
concentrations rather than the soil concentrations.
Stage I, Basic Equilibrium between TPH Contamination and Ground Water
Stage I assumes that TPH contaminated soil is in direct contact and chemical equilibrium with ground water. The basic
equilibrium equation is as follow:
(Eq.1)
(Eq.2)
Where C
GW
= the ground water concentration, in mg/l, C
SOIL
= the soil concentration in mg/kg and LF
GW
= the leaching
factor. That leaching factor is itself a function of the soil parameters:
Where:
LF
gw
= leaching factor (kg/L)
ρ = dry bulk density, (gm/cm
3
)
θ
w
= fraction of water filled porosity
θ
a
= fraction of air filled porosity
K
d
= partitioning constant for the soil = Koc X Foc, (cm
3
/g)
H’ = Henry’s Law constant for the COC, (dimensionless)
[ ]
GW
w d a
3
LF
k H'
X
cm kg
gm L
=
+ +
⋅
⋅
ρ
θ ρ θ