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 ( α, 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): α= Ci/Csg.

General Methodology

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 ( to determine the attenuation factor (α). 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:

  • 20% of an allowable intake based on non-cancer health effects (Workbook: MCP GW.xls, sheet: GW-2, column: B)
  • an excess lifetime cancer risk equal to one-in-one million (10-6) (Workbook: MCP GW.xls, sheet: GW-2, column: C), and
  • a 50% odor recognition threshold (Workbook: MCP GW.xls, sheet: GW-2, column: F).
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 µg/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:

Diagram of GW-2 Development Methodology


Indoor Air Target Level - Noncancer

Indoor Air Target Level - Noncancer Calculation 1
Workbook: MCP GW.xls, sheet: GW-2, column B

Parameter     Description

[OHM]air-nc = Indoor air target concentration, noncancer (µg/m3)

RfC = Reference Concentration (µg/m3)

HI = Target Hazard Index (unitless)

IFia-nc = Inhalation Factor, noncancer, indoor air

C= Conversion Factor (µg/mg)

Indoor Air Target Level - Noncancer Calculation 2
Workbook: MCP GW.xls, sheet: GW-2 Exposure, cell G18 for noncancer, cell G26 for cancer

Parameter        Description

IFia = Inhalation Factor, indoor air (µg/mg)

AP = Averaging Period (years)

C2 = Conversion Factor (days/year)

EF = Exposure Frequency (days/year)

EP = Exposure period (years)

Indoor Air Target Level - Cancer

Indoor Air Target Level - Cancer Calculation 1
Workbook: MCP GW.xls, sheet: GW-2, column C

Parameter        Description

[OHM]air-ca = Indoor air target concentration, cancer (µg/m3)

ELCR = Target Excess Lifetime Cancer Risk (unitless)

URair =  Unit Risk, air (µg/m3)-1

IFia-ca =  Inhalation Factor, cancer, indoor air
(See Equation above.)

Target Groundwater Value

Target Groundwater Value Calculation 1
Workbook: MCP GW.xls, sheet: GW-2, column P

Parameter                Description

[OHM]target-gw = Target concentration in groundwater (µg/L)

[OHM]target-air = Target concentration in indoor air (µg/m3)

d =  DEP dilution, degradation factor (dimensionless)

α = Attenuation factor "alpha" (dimensionless)

HLC0 = Dimensionless Henry's Law Constant

C = Conversion Constant (L/m3)

Input Parameters

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 α) estimated using for sandy soil (permeability of 9.9E-08 cm2).

Exposure Assumptions:



Exposure Frequency (EF)
1-8365Days/yearResidents are assumed to occupy a home continuously.
Exposure Period (EP)
1-87Years30 years is approximately the 90 th percentile value for residence time. (USEPA, 1997)
Averaging Period (AP)
1-87YearsEqual to EP for noncancer risk, a lifetime for cancer risk.


Johnson & Ettinger Model Assumptions:
Parameter, DescriptionValueUnits


LF- Depth below grade to bottom of enclosed space floor183cm
LWT- Depth below grade to water table213cm
TS- Average soil/groundwater temperature10°C
hA- Thickness of Soil Stratum A213cm
hB, hC- Thickness of Soil Strata B and C0cm
pbA- Soil dry bulk density1.5g/cm3
nA- Soil total porosity0.43unitless
Soil water-filled porosity symbol - Soil water-filled porosity0.06cm3/cm3
Lcrack- Enclosed space floor thickness15cm
Soil-bldg. pressure differential symbol - Soil-bldg. pressure differential40g/cm-s2
LB- Enclosed space floor length961cm
WB- Enclosed space floor width961cm
HB- Enclosed space floor height488cm
w- Floor-wall seam crack width0.1cm
ER- Indoor air exchange rate0.451/h