§ 250.308. Soil to groundwater pathway numeric values.
(a) A person may use the soil-to-groundwater pathway numeric values listed in Appendix A, Tables 3B and 4B, as developed using the methods contained in paragraph (1), (2) or (4), may use a concentration in soil at the site which does not produce a leachate in excess of the MSC for groundwater contained in Appendix A, Tables 1 and 2, when subjected to the Synthetic Precipitation Leaching Procedure (Method 1312 of SW-846, Test Methods for Evaluating Solid Waste, promulgated by the U.S. EPA), or may use the soil-to-groundwater pathway soil buffer criteria in subsection (b) or may use the soil-to-groundwater pathway equivalency demonstration in subsection (d).
(1) A value which is 100 times the applicable MSC for groundwater identified in § 250.304(c) or (d) (relating to MSCs for groundwater), expressed as milligrams per kilogram of soil.
(2) For organic compounds, a generic value determined not to produce a concentration in groundwater in the aquifer in excess of the MSC for groundwater as calculated by the equation in paragraph (3).
(i) For soil not in the zone of groundwater saturation, the generic value shall be calculated by the equation in paragraph (3).
(ii) For soil in the zone of groundwater saturation, the generic numeric value is 1/10th of the generic value calculated by the equation in paragraph (3).
(3) The equation referenced in paragraphs (1) and (2) is the following:
MSCS = MSCGW ((Koc * foc) + θw/ρb) DF
where: MSCS (mg/kg) = the generic value for a regulated substance in soil
MSCGW (mg/L) = MSC of a regulated substance in groundwater
Koc (L/kg) = organic carbon partition coefficient for a regulated substance
foc = fraction of organic carbon in soil (default value = 0.0025)
θw = water-filled porosity of soil (default value = 0.2)
ρb (kg/L) = dry bulk density of soil (default value = 1.8 kg/l)
DF = dilution factor (default value = 100)
(4) For inorganic regulated substances, a generic value determined not to produce a concentration in groundwater in the aquifer in excess of the MSC for groundwater as calculated by the equation in paragraph (5) and listed in Appendix A, Table 4B.
(i) For soil not in the zone of groundwater saturation, the generic value shall be calculated by the equation in paragraph (5).
(ii) For soil in the zone of groundwater saturation, the standard is 1/10th of the generic value calculated by the equation in paragraph (5).
(5) The equation referenced in paragraph (4) is the following:
MSCS = MSCGW (Kd + θw/ρb) DF
where: MSCS (mg/kg) = the generic value for the inorganic regulated substance in soil
MSCGW (mg/L) = MSC of the inorganic regulated substance in groundwater
Kd (L/kg) = soil to water partition coefficient for the inorganic regulated substance
θw = water-filled porosity of soil (default value = 0.2)
ρb (kg/L) = dry bulk density of soil (default value = 1.8 kg/L)
DF = dilution factor (default value = 100)
(b) The soil-to-groundwater pathway soil buffer is the entire area between the bottom of the area of contamination and the groundwater or bedrock and shall meet the following criteria:
(1) The soil depths established in Appendix A, Tables 3B and 4B for each regulated substance.
(2) The concentration of the regulated substance cannot exceed the limit related to the PQL or background throughout the soil buffer.
(3) No Karst carbonate formation underlies or is within 100 feet of the perimeter of the contaminated soil area.
(c) The soil-to-groundwater pathway soil buffer distances were developed by using the following equations.
(1) The following equations were used iteratively for each layer of soil for each time increment in a total time period of 30 years and pertain to a soil column where the first 4 feet of soil are contaminated with a regulated substance at a concentration of 10,000 mg/kg.
where: Csoil(it) (mg/kg) = concentration of regulated substance in soil at layer i at time increment TI
Csoil(i(t-1)) (mg/kg) = concentration of regulated substance in soil at layer i at time increment immediately preceding TI
Csoil(i+1)(t-1) (mg/kg) = concentration of regulated substance in soil at layer immediately above layer i at time increment immediately preceding TI
R (cm/yr) = recharge rate of water infiltrating soil (default value = 33 cm/yr)
DI (feet) = thickness of each layer i of soil (default value = 1 foot)
ρb (kg/L) = dry bulk density of soil (default value = 1.8 kg/L)
360 (month-cm/yr-ft) = conversion factor
TI (months) = time duration of each time increment
θW = water-filled porosity (default value = 0.2)
Caq(i(t-1)) (mg/L) = concentration of regulated substance in soil pore water at layer i at time increment immediately preceding TI
Caq(i+1)(t-1) (mg/L) = concentration of regulated substance in soil pore water at layer immediately above layer i at time increment immediately preceding TI
Kd (L/kg) = soil to water partition coefficient for the regulated substance where five different contaminant-specific values are used: 2.5, 5, 10, 100 and 1,000.
Kd values of organic regulated substances are based on the following equation:
Kd = (foc)(Koc)
where: foc = fraction of organic carbon in soil (default value = 0.005)
Koc (L/kg) = organic carbon partition coefficient for a regulated substance
(2) The buffer distances listed for regulated substances in Tables 3B and 4B were determined by the Department using these equations to model the distance that the regulated substance travels from the bottom of the first 4 feet of contaminated soil through the soil column in 30 years at a concentration at or above 1 µG/L in the water infiltrating the soil.
(d) For any regulated substance, an equivalency demonstration may be substituted for the soil-to-groundwater numeric value throughout the site and the soil-to-groundwater pathway soil buffer if the groundwater is below the MSC value or the background standard prior to remediation. This equivalency demonstration shall include the following:
(1) Fate and transport analysis of the regulated substance from the deepest point of contamination in the soil through unsaturated zone soil and shall include the use of soil-to-water partition coefficients. The analysis shall demonstrate that the regulated substances will not migrate to bedrock or the groundwater within 30 years at concentrations exceeding the greater of the groundwater MSC or background in groundwater as the endpoint in soil pore water directly under the site.
(2) In addition to sampling required for attainment of the inhalation or ingestion numeric values for soils up to 15 feet, as applicable, reporting and monitoring for eight quarters that shows no exceedances of the greater of the groundwater MSCs or of the background standard for groundwater beneath the contaminated soil and no indications of an increasing trend of concentration over time that may exceed the standard.
Authority The provisions of this § 250.308 amended under sections 104(a) and 303(a) of the Land Recycling and Environmental Remediation Standards Act (35 P.S. § § 6026.104(a) and 6026.303(a)); and section 1920-A of The Administrative Code of 1929 (71 P.S. § 510-20).
Source The provisions of this § 250.308 amended January 7, 2011, effective January 8, 2011, 41 Pa.B. 230; amended November 19, 2021, effective November 20, 2021, 51 Pa.B. 7173. Immediately preceding text appears at serial pages (383331) to (383332) and (355289) to (355290).
Cross References This section cited in 25 Pa. Code § 250.304 (relating to MSCs for groundwater); 25 Pa. Code § 250.305 (relating to MSCs for soil); 25 Pa. Code § 250.310 (relating to minimum threshold MSCs); 25 Pa. Code § 250.312 (relating to final report); 25 Pa. Code § 250.604 (relating to fate and transport modeling requirements for exposure assessments); and 25 Pa. Code § 271.1 (relating to definitions); and 25 Pa. Code § 287.1 (relating to definitions).
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