020-1 Wyo. Code R. §§ 1-41 - Soil Environmental Fate and Transport Evaluation

A soil environmental fate and transport evaluation shall be completed. The evaluation shall estimate the potential for soil to contaminate groundwater at levels exceeding STP groundwater restoration standards.

(a) Conceptual Organic Compound Fate and Transport Model.

(i) The model is based on the following assumptions:

(A) A finite amount of soil contamination exists at variable depths beneath a leaking storage tank site. It may extend from the surface to below the groundwater table, or it may be confined to a discrete zone. There is an uppermost aquifer beneath the site that is not adequately protected by an impermeable barrier between the contaminated soil and the aquifer. Percolating rainfall or snow melt moves through the contaminated soil, mobilizes some of the contamination as a leachate and carries the contamination towards the aquifer. A portion of the contamination remains strongly adsorbed to the soil. The portion of the contaminants that are not permanently adsorbed are available for biodegradation and a limited amount of leaching.

(B) The point of compliance for protecting groundwater quality is directly below the contaminated soils at the surface of the aquifer.

(C) The rate of leaching from the soil has reached a steady state.

(D) The soils beneath the leaking storage tank(s) represent the only source of contamination to the groundwater.

(E) Vapors emanating from the contaminants in the soil are moving primarily upwards to the ground surface, and there is no perched saturated zone above the contaminated soils. Based on existing program experience, the potential does exist for some lateral movement of contaminant vapors; however, this movement is not the primary direction.

(F) A leachate plume beneath the contaminated zone has not yet reached the groundwater table.

(ii) The model for calculating soil cleanup concentrations involves a set of mathematical equations designed to calculate soil remediation concentrations. The equations have been modified and simplified to make it possible to calculate soil cleanup concentrations using as much site-specific data/information as possible. The site-specific data used in the equations should be available from the subsurface investigations and are preferred over using the default values.

(iii) The equations are a mathematical expression of the conceptual model. The organic contaminant concentration in the soil is reduced by a fractional amount that has been biodegraded by natural bacteria in the soil system. Therefore, a biodegradation factor, e^-kt, has been included in the evaluation process. Because the biodegradation factor will reduce the amount of contaminant available for leachate generation, the soil cleanup concentration can be adjusted upward by a calculated amount. The amount, which is adsorbed, is calculated using the chemical-specific adsorption coefficient, K_d.

(iv) The adsorption coefficient, K_d, is calculated from the following equation using site-specific data:

Equation 7:

K_d = (f_oc)(K_oc)

where:

foc	=	Site-specific fraction of organic carbon, mg organic carbon/mg soil in the uncontaminated subsurface site soil. Normal range of foc in Wyoming soils is 0.1-3%. If a site-specific foc value is not determined, use a default value of 0.1%.
Koc	=	Chemical specific organic carbon partition coefficient, mL/gm.

(v) The conceptual model discussed above is represented by the following series of equations with further explanation, as necessary:

(A) Determine travel time to reach groundwater table, t.

(I) Subsurface soil contamination separated from the groundwater table by more than 1 foot of depth is calculated as follows. Because subsurface organic carbon content below 1 foot is expected to approach a very low number in Wyoming soils, the following contaminant travel time equation has been developed:

Equation 8:

Click here to view image

where:

t=	Time for contaminant(s) to travel from the bottom of the contaminated zone to the groundwater table, yrs.
d=	Depth to the groundwater table from the bottom of the contaminated zone(s), cm.
[THETA]=	Volumetric soil moisture content(s) at field capacity, mL/cm3.
0.5 =	50% infiltration rate for precipitation (worst case).
[ALPHA]=	Average annual precipitation, cm/yr.
[RHO]=	Bulk soil density, gm/cm3.

(II) If more than one soil type exists at a contaminated site or remediation project location where the organic carbon content differs by 0.5% or greater and the different soil type is 1 foot or greater in thickness, individual soil type specific values for K_d, [THETA], and [RHO] shall be used in the time of travel calculation for each soil type. Further, the individual values for depth, d, to the groundwater table from the bottom of each contaminated soil type zone shall be used in the calculation. If the depth, d, from the bottom of the contaminated soil type zone to the groundwater table is less than 12 inches, this method for determining contaminated soil remediation concentrations is not valid. In these cases, cleanup of contaminated groundwater will govern the satisfactory remediation of contaminated soil within this 12-inch interval. The final time of travel, t, is the sum of the individual soil-type segments.

(III) Surface contamination extending from the ground surface to depths greater than 2 feet. In order for the following equation to be used, the subsurface soil within the 2-foot distance shall contain at least 3 percent total organic carbon, otherwise Equation 8 applies for the time of travel calculation. If using two different K_d values for different soil organic carbon concentrations, the equation is derived as follows:

Equation 9:

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where:

Z =	Thickness of soil containing 3 percent or greater organic carbon, cm.
K'd= Kd=	Adsorption coefficient in the top 2 feet of soil, which is equal to the measured fraction of organic carbon, foc, times the Koc value. Soil adsorption coefficient in the remaining soil column calculated
[THETA] '=	from Equation 7, mL/gm. Bulk soil density of soil containing 3 percent or greater organic carbon, gm/cm3.
[RHO] '=	Volumetric soil moisture content at field capacity of soil containing 3 percent or greater organic carbon, mL/cm3.

The parameter, Z, takes into account natural organic carbon that may be present at the ground surface, and it may extend for a limited vertical distance [0-60 cm (0-24 inches)] into the ground. Development of site-specific soil adsorption coefficient isotherms may be required for complex surface environments where f_oc is greater than 3 percent. If the uppermost 2-foot zone contains less than 3 percent natural organic carbon, the Z portion of the time of travel calculation drops out, leaving Equation 8 to apply for the time of travel calculation. This portion of the calculation provides a mechanism to account for higher surface contaminant adsorption by naturally occurring organic carbon within this zone.

(B) Calculate the soil remediation concentration for the biodegradation potential, C_s,org, for the organic compound(s) using Equation 10:

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where:

k=	Biodegradation rate constant, 0693/T½, 1/yr
T½ =	Half-life for the specific chemical substance in groundwater in years
t=	Contaminant travel time to reach groundwater table, yrs
Cst,org =	Organic compound drinking water MCL, or state DWEL, mg/L
Cs,org =	Soil cleanup concentration for organic chemical compound, mg/kg
Kd =	Soil adsorption coefficient calculated from Equation 7, mL/gmWhere more than one Kd value is used for two or more different organic carbon soil types, use the lowest individual Kd value

Equation 10 establishes the site soil remediation concentration for each organic chemical compound that could be allowed to remain in soil without threatening degradation of groundwater quality even if groundwater seasonally passes through the contaminated zone.

(vi) The soil saturation limit is the contaminant concentration at which soil pore air and pore water are saturated with the chemical and the adsorptive limits of the soil particles have been reached. Above this limit, the contaminant may be present in the free phase. Equation 11 is used to calculate the soil saturation limit for each organic chemical at the site:

Equation 11:

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(b) Conceptual Metal, Inorganic Compound, and Total Petroleum Hydrocarbon Fate and Transport Model.

The conceptual model for metals, inorganic compounds, and total petroleum hydrocarbons (TPH) assumes that these substances are distributed in subsurface soils around, or below, the level of a storage tank that had contained leaded regular gasoline or a hazardous substance. Some of these substances will be mobilized in percolating rainfall or snow melt and may be transported to the groundwater table as a leachate. That portion of these substances that remains adsorbed to the soil particles is determined by the adsorptive properties of both the substance and soil. It is calculated using the adsorption coefficient, Kd. The factor, e^[LAMBDA]t is used as a leaching rate factor in this model to determine the rate at which leachate is released from the contaminated soil.

The conceptual model for metals, inorganic compounds, and TPH is represented by the following series of equations:

(i) Determine the leaching rate constant, X ,

Equation 12:

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where:

[LAMBDA] =	Leaching rate constant, 1/yr.
[ALPHA](alpha) =	Average annual precipitation, cm/yr.
[THETA] =	Volumetric soil moisture content at field capacity, mL/cm3.
[RHO] =	Bulk soil density, gm/cm3.
Kd=	Soil metal, inorganic compound, or TPH adsorption coefficient, mL/gm.
[TAU] =	Thickness of contaminated soil seam, cm.

If more than one soil type exists at a contaminated site where the organic carbon content differs by 0.5% or more and the different soil type is one foot or greater in thickness, individual specific soil type values for K_d, [THETA] and [RHO] shall be used in the leaching rate constant calculation for each soil type. The final leaching rate constant, [LAMBDA], is the sum of the individual soil type segments.

(ii) Calculate travel time to reach groundwater table, t.

Subsurface soil contamination separated from the groundwater table by more than 1 foot is handled in the following way:

Because subsurface organic carbon content below 1 foot is expected to approach a very low number in Wyoming soils, contaminant travel time is calculated by:

Equation 13: Click here to view image

where:

t	=	Time for contaminant to travel from the bottom of the contaminated zone to the groundwater table, yrs.
d	=	Depth to the groundwater table from the bottom of the contaminated zone, cm.
[THETA]	=	Volumetric soil moisture content at field capacity, mL/cm3.
0.5	=	50% infiltration rate for precipitation (worst case).
[ALPHA]	=	Average annual precipitation, cm/yr.
[RHO]	=	Bulk soil density, gm/cm3.

If more than one soil type exists at a contaminated site where the organic carbon content differs by 0.5% or greater and the different soil type is 1 foot or greater in thickness, individual soil type specific values for , K_d , [THETA], and [RHO] shall be used in the time of travel calculation for each soil type. Further, the individual values for depth, d, to the groundwater table from the bottom of each contaminated soil-type zone shall be used in the calculation. If the depth, d, from the bottom of the contaminated soil-type zone to the groundwater table is less than 12 inches or groundwater travel fluctuates this distance, this method for determining contaminated soil remediation concentrations is not valid. In these cases, cleanup of contaminated groundwater will govern the satisfactory remediation of contaminated soil within this 12-inch interval. The final time of travel, t, is the sum of the individual soil type segments.

(iii) Calculate the soil remediation concentration for the leaching potential of the metal, inorganic compound, or TPH using the following derived equation:

Equation 14: Click here to view image

where:

Cs,inorg=	Soil cleanup concentration due to metal, inorganic compound, or TPH leaching potential, mg/kg.
Cstm=	Environmental standard concentration, primary MCL, or state DWEL, mg/L.
=	Chemical leaching rate, 1/yr.
t=	Contaminant travel time to reach groundwater table, yrs.
Kd=	Soil metal, inorganic compound, or TPH adsorption coefficient, ml/gm.

The soil cleanup concentration for metals, inorganic compounds, or TPH is determined by evaluating the above calculations and the natural background concentration. Information concerning the natural subsurface concentration may be available from either:

(1) a subsurface investigation report, or

(2) site-specific subsurface soil samples from an uncontaminated, upgradient location immediately near the leaking storage tank site. Soil metal remediation is not required for concentrations that are below natural background concentration(s).

(c) Final Storage Tank Cleanup Concentration. The final numerical soil cleanup concentration for organic chemical compounds shall be the lower numerical value of the total petroleum hydrocarbon concentration, the human health risk assessment, the soil saturation concentration, or the environmental fate and transport considerations. The final numerical soil cleanup concentration value for metals, inorganic compounds, or total petroleum hydrocarbons shall be the lower numerical value of the environmental fate and transport calculation or the human health risk assessment component. The goal of the final cleanup concentration(s) is to ensure that the remedial action will result in an acceptable cleanup for organic chemical compounds, inorganic compounds, TPH, and metals.

Notes

020-1 Wyo. Code R. §§ 1-41

Adopted, Eff. 6/29/2018.