Soil Cleanup Levels Protective of Groundwater
Kristy J. Hendrickson, Landau Associates, Inc.
Introduction
The Model Toxics Control Act (MTCA) Cleanup Regulation (Chapter 173-340 WAC) defines cleanup
level as “the concentration of a hazardous substance in soil, water, air, or sediment that is determined to
be protective of human health and the environment under specified exposure conditions” (WAC 173-340200). A cleanup level together with the location at which the cleanup level must be met, called the point
of compliance, are the two primary components of the cleanup standard. Other regulatory requirements
that apply to the site because of the type of action or location of the site are also included in the cleanup
standard.
Under MTCA, soil cleanup levels must be at least as stringent as applicable state and federal laws and
must be protective of terrestrial ecological receptors, direct human contact, and groundwater. For many
contaminants, the concentration protective of groundwater is lower than the concentrations protective of
direct human contact and terrestrial ecological receptors and, therefore, determines the soil cleanup level.
This presentation will focus on determination of soil concentrations that are protective of groundwater.
There are two criteria for protection of groundwater. The first criterion is that the soil concentration will
not cause contamination of groundwater at a level greater than the groundwater cleanup level. The
second criterion is that the soil concentration will not result in a non-aqueous phase liquid (NAPL) in or
on groundwater.
MTCA (WAC 173-340-747) provides several methods for establishing a soil concentration that will not
cause groundwater contamination above an acceptable level. The amount of site-specific information
needed and the time required to use these methods range from the fixed parameter three-phase model,
which requires no site-specific information and can be used to quickly calculate protective soil
concentrations, to leaching tests and alternative fate and transport models, which require site-specific data
and more time and effort to calculate protective soil concentrations. For sites where the cleanup level has
a significant effect on the selection or cost of a cleanup alternative, use of site-specific data, a leaching
test, an alternative fate and transport model, or an empirical demonstration may be cost effective.
Three-phase Partitioning Model
The easiest method for calculating soil concentrations that will not cause an exceedance of groundwater
cleanup levels is the fixed parameter three-phase partitioning model using default input parameters. No
site-specific measurements are needed to calculate a cleanup level using this model. The three phases
considered are soil, water, and air so this model is not applicable when NAPL is present. The partitioning
model is a simple linear soil/water partitioning equation that relates the concentration of a contaminant in
soil to soil leachate concentrations in the contamination zone, based on simplified assumptions. These
assumptions include:




The source is infinite; a steady-state concentration is maintained in the soil.
The contamination is uniformly distributed from the surface to the top of the aquifer.
There is no contaminant attenuation as it migrates through the soil column.
The soil/water partitioning equilibrium is instantaneous and linear.
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



The aquifer is unconfined and unconsolidated, homogeneous, and isotropic.
The receptor point in groundwater is at the downgradient edge of the source.
There is no contaminant attenuation in the aquifer.
There are no NAPLs present.
The three-phase partitioning equation is:
C = C (UCF) DF [K + ((w +a H )/b)]
s
w
d
cc
Where:
Cs
Cw
UCF
DF
=
=
=
=
Kd
w
=
=
a
=
Hcc
b
=
=
Soil concentration (mg/kg)
Groundwater cleanup level established under WAC 173-340-720 (μg/L)
Unit conversation factor (1 mg/1,000 μg)
Dilution factor (dimensionless: 20 for unsaturated zone soil, 1 for saturated zone
soil)
Distribution coefficient (L/kg, chemical-specific)
Water-filled soil porosity (ml water/ml soil; 0.3 for unsaturated zone soil, 0.43
for saturated zone soil)
Air-filled soil porosity (ml air/ml soil; 0.13 for unsaturated zone soil, 0 for
saturated zone soil)
Henry’s law constant (dimensionless, chemical-specific)
Dry soil bulk density (1.5 kg/L)
Distribution coefficient values and Henry’s law constants are chemical-specific. Distribution coefficients
for inorganic constituents or soil organic carbon-water partitioning coefficients for organic constituents
and Henry’s law constants are available in Washington State Department of Ecology’s (Ecology) CLARC
database (https://fortress.wa.gov/ecy/clarc/CLARCHome.aspx). There are specified default values, as
shown above, for the other input parameters. Hun Seak Park of Ecology developed an excel workbook
(MTCASGL11.0) that calculates the concentration of a specific hazardous substance in soil that is
protective of groundwater using this equation (http://www.ecy.wa.gov/programs/tcp/tools/toolmain.html)
or it is relatively straightforward to set up your own spreadsheet to calculate cleanup levels using the
three-phase model equation.
The resulting soil concentration from this equation using the default input parameters provides a
concentration that is intended to be protective of groundwater under most circumstances and conditions.
Consequently, it is overly conservative in some circumstances. If a relatively small increase in the soil
cleanup level (within an order of magnitude or so) is important to your cleanup, it may be advantageous
to use the variable parameter three-phase partitioning model [WAC 173-340-747(5)]. The model is the
same as the one described above, but instead of using default values for the distribution coefficient, soil
bulk density, soil volumetric water content, soil air content, and dilution factor, site-specific values for
one or more of these may be used in the calculation.
For nonionic hydrophobic organic contaminants, the distribution coefficient is equal to the soil organic
carbon-water partitioning coefficient (Koc) times the soil fraction of organic carbon (foc). The Ecology
default value for the soil fraction of organic carbon is 0.1%; many soils have f oc values greater than that.
Organic carbon fraction is a fairly inexpensive analytical test so, at some sites, it makes sense to collect
samples of uncontaminated soil from below the root zone to determine foc and calculate a site-specific
distribution coefficient for use in calculating the soil cleanup level. For example, an increase in foc from
0.1% to 0.2% would approximately double the calculated protective soil concentration for
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benzo(a)pyrene. The effect of an increase in foc is smaller for organic compounds with lower Koc values,
however; thus the calculated protective soil concentration for vinyl chloride would change by only about
6% if the foc increased from 0.1% to 0.2%. Alternatively, a site-specific distribution coefficient may be
determined, with Ecology approval, from measurements of contaminant concentrations in soil and soil
pore water or groundwater, or from batch equilibrium tests. A site-specific distribution coefficient may
also be derived from the scientific literature.
Soil bulk density values and soil volumetric water content may be measured using American Society for
Testing and Materials (ASTM) Method 2049 and 2216, respectively, or other methods approved by
Ecology. The soil air content may be estimated by calculating the soil porosity and subtracting the
volumetric water content.
The default dilution factor for unsaturated soil is 20 and for saturated soil is 1. A site-specific dilution
factor may be derived using the equations in WAC 173-340-747(5)(f). A site-specific dilution factor for
unsaturated soil is most likely to be greater than 20 for small contaminated areas, less than 0.5 acres.
Four-phase Partitioning Model
The four-phase model is used for sites where NAPL is present. This model is based on three equations, a
conservation of volume equation, the four-phase partitioning equation, and a molar density equation. The
model is much more complex than the three-phase model, with multiple equations and unknowns. The
equations are solved simultaneously using iteration. The four-phase model is typically used for
petroleum, with analysis for petroleum fractions and associated hazardous substances. As for the threephase model, either default or site-specific values may be used for input parameters. Hun Seak Park of
Ecology developed an excel workbook (MTCATPH11.0) that calculates the concentration in soil that is
protective
of
groundwater
using
the
four-phase
model
(http://www.ecy.wa.gov/programs/tcp/tools/toolmain.html).
Leaching Tests
The Synthetic Precipitation Leaching Procedure (SPLP) or Toxicity Characteristic Leaching Procedure
(TCLP) may be used to establish soil concentrations protective of groundwater for arsenic, cadmium, total
chromium, hexavalent chromium, copper, lead, mercury, nickel, selenium, and zinc. For situations where
conditions are not acidic and the metals of interest are more soluble under alkaline conditions, the SPLP
should be used. For metals in wood waste, municipal solid waste landfills, in high sulfur content mining
wastes, or other situations with pH <6, TCLP should be used. To show the soil concentration is
protective of groundwater, the leaching test effluent concentration for cadmium, lead, and zinc, must be
less than or equal to ten times the groundwater cleanup level. For arsenic, total or hexavalent chromium,
copper, mercury, nickel, and selenium, the leaching test effluent concentration must be less than or equal
to the groundwater cleanup level to show the soil concentration is protective of groundwater.
The SPLP or TCLP may be used to establish soil concentrations protective of groundwater for hazardous
substances other than the metals identified in the previous paragraph with Ecology approval. Alternative
leaching tests may be used for the metals identified in the previous paragraph or for other hazardous
substances with Ecology approval.
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Alternative Fate and Transport Models
Fate and transport models other than the three-phase or four-phase partitioning models may be used with
Ecology approval. An alternative model may be useful at sites where the assumptions for the partitioning
model are not appropriate.
Empirical Demonstration
Site-specific data may be used to demonstrate that soil concentrations will not cause an exceedance of
groundwater cleanup levels. The advantage of using an empirical demonstration is that it reflects actual
site conditions rather than relying on conservative assumptions to establish protective soil concentrations.
The criteria in WAC-173-340-747(9) for an empirical demonstration include:


The measured groundwater concentration is less than or equal to the groundwater cleanup
level.
The measured soil concentration will not cause an exceedance of the groundwater cleanup
level in the future.
- A sufficient amount of time has elapsed for migration of hazardous substances from soil
into groundwater to occur
- Site characteristics are representative of future site conditions.
At some sites where the horizontal and vertical extent of contamination has been characterized, there is a
significant depth of clean soil between the maximum depth of contamination and groundwater, and
sufficient time has elapsed for migration to have occurred, it may be possible to demonstrate that existing
soil concentrations will not cause an exceedance of groundwater cleanup levels using soil concentrations
at various depths, including concentrations from below the maximum depth of contamination. An
example of a situation where this may be possible would be a site in eastern Washington, where the
uppermost groundwater is deep, the maximum depth of hazardous substances is fairly shallow, and the
release occurred many years ago. For example, if lead from a release 40 years ago is present in soil from
the surface to 6 ft below ground surface and the groundwater is at a depth of 60 ft, soil concentrations at
depths of 2 ft, 4 ft, 6 ft, and 8 ft should be adequate to show the changes in concentration with depth, and
demonstrate that contaminants have not migrated and are not likely to migrate to groundwater at
concentrations that would cause an exceedance of groundwater cleanup levels. The regulations, however,
specify that groundwater concentrations be determined as part of an empirical demonstration and some
Ecology site managers may want to see groundwater data.
Residual Saturation
The second criterion for establishing a soil concentration that will not cause an exceedance of
groundwater cleanup levels is that the soil concentration will not result in a NAPL in or on groundwater.
Residual saturation is the amount of NAPL that will be held in the soil pore spaces by capillary forces; at
concentrations greater than residual saturation the NAPL will migrate and may reach groundwater.
Ecology provides default values for various petroleum products in Table 747-5 (WAC 173-340-900).
The values vary from 1,000 mg/kg for weathered gasoline to 4,000 mg/kg for mineral oil. There are also
several other methods for determining residual saturation included in WAC 173-340-747(10). One
method is an empirical demonstration. An empirical demonstration includes collection of site-specific
soil and groundwater data to show there is not currently and will not be in the future NAPL on or in
groundwater. A site-specific value for residual saturation may be determined from site-specific
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measurements using methods approved by Ecology. Laboratory measurements or theoretical estimates
[such as those calculated as described in A Review of Immiscible Fluids in the Subsurface: Properties,
Models, Characterization and Remediation, James W. Mercer and Robert M. Cohen, Journal of
Contaminant Hydrology, Vol. 6 No. 2 (Sept. 1990) pg 107-163] are to be verified using site data.
Residual saturation values vary with the type of NAPL and the soil type. Residual saturation values are
higher for more viscous products and, for a given product, increase as soil becomes more fine-grained.
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