Wyoming Administrative Code
Agency 020 - Environmental Quality, Dept. of
Sub-Agency 0011 - Water Quality
Chapter 17 - STORAGE TANKS
Part J - ENVIRONMENTAL RESTORATION STANDARDS FOR LEAKING STORAGE TANK REMEDIATION ACTIONS
Section 17-41 - Soil Environmental Fate and Transport Evaluation
Current through September 21, 2024
(a) Conceptual organic compound fate and transport model.
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%.
K oc = chemical specific organic carbon partition coefficient, mL/gm.
where,
t = time of 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.
where,
Z = thickness of soil containing three (3) percent or greater organic carbon, cm.
K ' d = adsorption coefficient in the top two (2) feet of soil which is equal to the measured fraction of organic carbon, foc, times the Koc value
Kd = soil adsorption coefficient in the remaining soil column calculated from Equation 7, mL/gm.
[RHO] ' = bulk soil density of soil containing three (3) percent or greater organic carbon, gm/cm3.
[THETA]' = volumetric soil moisture content at field capacity of soil containing three (3) percent or greater organic carbon, mL/cm3.
The parameter, Z , takes into account natural organic carbon which 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 foc is greater than three (3) percent. If the uppermost two (2) foot zone contains less than three (3) percent natural organic carbon, the Z portion of the time of travel calculation drops out, thus 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.
where,
K = biodegradation rate constant, 0.693/T1/2, 1/yr.
T1/2 = 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 maximum contaminant level, 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/gm. Where more than one Kd value is used for two (2) or more different organic carbon soil types, use the lowest individual Kd value.
Equation 10 establishes the leaking storage tank site soil remediation concentration for each organic chemical compound which could be allowed to remain in soil without threatening degradation of groundwater quality even if groundwater seasonally passes through the contaminated zone.
(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 which 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 which 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.
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 Kd, [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.
Because subsurface organic carbon content below one foot is expected to approach a very low number in Wyoming soils, contaminant travel time is calculated by:
where,
t = time of 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 one foot or greater in thickness, individual soil type specific values for Kd , [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 twelve (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.
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.
[LAMBDA] = 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, up-gradient location immediately near the leaking storage tank site shall be collected and analyzed for the appropriate constituent. Soil metal remediation is not required for concentrations that are below natural background concentration(s).
(c) Final Storage Tank Cleanup Concentration. The final numerical leaking storage tank site 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, and the environmental fate and transport considerations. The final numerical leaking storage tank site soil cleanup concentration value for metals, inorganic compounds, and total petroleum hydrocarbons shall be the lower numerical value of: the environmental fate and transport calculation and 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.
