MCP Category GW-2 Standards (310 CMR 40.0974(2)) apply to groundwater that is considered a potential source of indoor air contamination. These standards apply to groundwater that is both shallow and below an occupied building. The specific regulatory criteria used to determine the applicability of the GW-2 standards are described at 310 CMR 40.0932(6).
The volatilization of oil or hazardous material from contaminated groundwater and its infiltration to indoor air has proven to be a significant exposure pathway at some c.21E sites. Historically the transport of radon gas into indoor air has received a great deal of attention, but it is only recently that this migration pathway has been examined for common volatile organic contaminants. Journal articles (Johnson and Ettinger, 1991; Little et al., 1992) provide discussions of this pathway and develop predictive models for its assessment. Numerous regulatory support documents (USEPA, CTDEP, MIDEP, CO DEP) discuss the use of vapor transport models to evaluate specific environmental conditions.
The model used to develop the MCP GW-2 standards calculates a unique attenuation factor ( Î±, or "alpha" ) for each chemical, assuming highly permeable soil with low moisture content. The attenuation factor relates the indoor air concentration (Ci) to the soil-gas concentration at the surface of the groundwater (Csg): Î±= Ci/Csg.
The GW-2 standards are developed using a variation of the USEPA spreadsheets for the Johnson and Ettinger (1991) Model for Subsurface Vapor Intrusion into Buildings (http://www.epa.gov/oswer/riskassessment/airmodel/johnson_ettinger.htm) to determine the attenuation factor (Î±). The attenuation factor is used in combination with MADEP indoor air risk assessment equations to generate target groundwater concentrations, as described below.
The sequential approach taken to develop the MCP GW-2 standards is as follows:
Standard toxicity information, risk assessment methodologies and odor thresholds (when available) are used to identify concentrations in indoor air associated with:
|2.||The lowest of these three values is carried through the process.|
|3.||A background concentration in indoor air is identified, if available (Workbook: MCP Toxicity.xls, sheet: Toxicity, column: AE).|
|4.||The higher of the values identified in steps 2 and 3 is carried through the process.|
|5.||A vapor transport model is used to estimate a concentration in groundwater that would result in the target indoor air concentration identified in step 4.|
|6.||A ceiling concentration of 0.005% (50,000 Âµg/L) is noted (Workbook: MCP Toxicity.xls, sheet: Toxicity, column: BP).|
|7.||The lower of the concentrations identified in steps 5 and 6 is carried through the process.|
|8.||A Practical Quantitation Limit (PQL) for an appropriately sensitive analytical method is identified (Workbook: MCP Toxicity.xls, sheet: Toxicity, column: AO).|
|9.||A background concentration in groundwater is identified, if available (Workbook: MCP Toxicity.xls, sheet: Toxicity, column: AD).|
|10.||The highest of the three values identified in steps 7, 8 and 9 is chosen.|
|11.||The value identified in step 10 is rounded to one significant figure. This value is adopted as the MCP GW-2 standard.|
This process is diagramed:
Indoor Air Target Level - Noncancer
Workbook: MCP GW.xls, sheet: GW-2, column B
[OHM]air-nc = Indoor air target concentration, noncancer (Âµg/m3)
RfC = Reference Concentration (Âµg/m3)
HI = Target Hazard Index (unitless)
IFia-nc = Inhalation Factor, noncancer, indoor air
C1 = Conversion Factor (Âµg/mg)
Workbook: MCP GW.xls, sheet: GW-2 Exposure, cell G18 for noncancer, cell G26 for cancer
IFia = Inhalation Factor, indoor air (Âµg/mg)
AP = Averaging Period (years)
C2 = Conversion Factor (days/year)
EF = Exposure Frequency (days/year)
EP = Exposure period (years)
Indoor Air Target Level - Cancer
Workbook: MCP GW.xls, sheet: GW-2, column C
[OHM]air-ca = Indoor air target concentration, cancer (Âµg/m3)
ELCR = Target Excess Lifetime Cancer Risk (unitless)
URair = Unit Risk, air (Âµg/m3)-1
IFia-ca = Inhalation Factor, cancer, indoor air
(See Equation above.)
Target Groundwater Value
Workbook: MCP GW.xls, sheet: GW-2, column P
[OHM]target-gw = Target concentration in groundwater (Âµg/L)
[OHM]target-air = Target concentration in indoor air (Âµg/m3)
d = DEP dilution, degradation factor (dimensionless)
Î± = Attenuation factor "alpha" (dimensionless)
HLC0 = Dimensionless Henry's Law Constant
C = Conversion Constant (L/m3)
The GW-2 standards are based on the inhalation of indoor air in a residential setting. Similar to drinking water exposures, exposure is assumed to occur daily over the exposure period.
The attenuation factor depends upon a large number of factors, including:
- depth to groundwater
- concentration of the material in groundwater
- partition coefficients
- groundwater flow
- building structure
- building ventilation rate
The MCP GW-2 standards were derived using chemical-specific attenuation factors (alpha, or Î±) estimated using for sandy soil (permeability of 9.9E-08 cm2).
|Exposure Frequency (EF)|
|1-8||365||Days/year||Residents are assumed to occupy a home continuously.|
|Exposure Period (EP)|
|1-8||7||Years||30 years is approximately the 90 th percentile value for residence time. (USEPA, 1997)|
|Averaging Period (AP)|
|1-8||7||Years||Equal to EP for noncancer risk, a lifetime for cancer risk.|
|LF||- Depth below grade to bottom of enclosed space floor||183||cm|
|LWT||- Depth below grade to water table||213||cm|
|TS||- Average soil/groundwater temperature||10||Â°C|
|hA||- Thickness of Soil Stratum A||213||cm|
|hB, hC||- Thickness of Soil Strata B and C||0||cm|
|pbA||- Soil dry bulk density||1.5||g/cm3|
|nA||- Soil total porosity||0.43||unitless|
|- Soil water-filled porosity||0.06||cm3/cm3|
|Lcrack||- Enclosed space floor thickness||15||cm|
|- Soil-bldg. pressure differential||40||g/cm-s2|
|LB||- Enclosed space floor length||961||cm|
|WB||- Enclosed space floor width||961||cm|
|HB||- Enclosed space floor height||488||cm|
|w||- Floor-wall seam crack width||0.1||cm|
|ER||- Indoor air exchange rate||0.45||1/h|
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