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310 CMR 40.0000: MASSACHUSETTS CONTINGENCY PLAN

Section 40.0926: Identification of Exposure Point Concentrations and Other Data Criteria

  1. For each oil and/or hazardous material in each medium at each Exposure Point, an Exposure Point Concentration shall be identified and documented.
  2. Exposure Point Concentrations shall be determined or estimated in a manner consistent with the type and method of Risk Characterization which is being performed.
  3. In estimating the Exposure Point Concentration, the objective shall be to identify a conservative estimate of the average concentration contacted by a receptor at the Exposure Point over the period of exposure.
    1. Maximum concentrations shall be used to estimate an Exposure Point Concentration under the following conditions:
      1. evaluations of acute exposures;
      2. screening assessments that evaluate maximum exposure potential to streamline the assessment process; or
      3. evaluations of exposures for which the data available to characterize temporal variability or the spatial distribution of site concentration is limited, including when there is insufficient data to adequately characterize the effects of seasonal variation on groundwater contaminant concentrations.
    2. For chronic and subchronic exposures (other than for screening evaluations), the arithmetic average of site data is acceptable as an Exposure Point Concentration, provided either of the following criteria are met:
      1. for discrete or composite samples, the arithmetic average is less than or equal to the applicable standard or risk-based concentration limit, 75% of the data points used in the averaging procedure are equal to or less than the applicable standard or risk-based concentration limit, and no data point used in the averaging is ten times greater than the applicable standard or risk-based concentration limit; or
      2. a valid justification is provided indicating that the sample mean is unlikely to substantially underestimate the true mean of the concentration of oil or hazardous material at the Exposure Point. Such a demonstration should include, but need not be limited to, consideration of the observed distribution of the data, sampling strategy (including frequency, density, and potential biases), graphical representation of analytical results, and/or statistical analyses.
    3. For chronic and subchronic exposures (other than for screening evaluations), the use of maximum concentrations or the 95 percentile upper confidence limit on the mean, whichever is lower, shall be used to estimate an Exposure Point Concentration when the criteria specified in 310 CMR 40.0926(3)(b) are not met. In such cases, the sample size is likely to be insufficient for the simple arithmetic average to estimate the true value with reasonable  confidence and there is a considerable probability of substantially underestimating the mean.
  4. In determining the concentrations to compare to Upper Concentration Limits, the objective shall be to provide a conservative estimate of the average concentration within the site, and the average concentration within any Hot Spots within the site. A conservative estimate of the average concentration should be developed in accordance with 310 CMR 40.0926(3).
  5. In determining the concentrations to evaluate Hot Spots, the objective shall be to provide a conservative estimate of the average concentration within the Hot Spot. A conservative estimate of the average concentration should be developed in accordance with 3 10 CMR 40.0926(3).
  6. Except as provided in 310 CMR 4 0.0926(7), Exposure Point C concentrations shall be developed using analytical data gathered during the site investigation at the Exposure Point as the primary line of evidence, as described in 310 CMR 40.0924.
  7. Fate and transport models generally accepted by the environmental modeling community may be used in conjunction with other site information and data and/or in cases where direct sampling of a media of concern is not possible or appropriate.
    1. For indoor air, sub-slab soil vapor data and/or conditions may be used to:
      1. estimate or aid in the estimation of Exposure Point Concentrations in the event that it is not possible to distinguish disposal site-related contamination at the Exposure Point from interior sources at ongoing commercial and/or industrial operations or interior building materials contaminated by past commercial or industrial operations; or
      2. where appropriate, to rule out an indoor air Exposure Pathway.
    2. For indoor air, fate and transport models shall not be used to estimate future Exposure Point Concentrations in the indoor air of buildings that have not been constructed.
    3. For groundwater, current groundwater contaminant concentrations and site hydro-geologic conditions may be used to estimate future Exposure Point Concentrations in groundwater and/or surface water.
    4. For soil, current soil concentrations and site-specific factors such as infiltration rate of precipitation and soil characteristics may be used to estimate the future leaching potential of soil contaminants.
    5. Such models shall be clearly documented and incorporate input parameters to provide a conservative estimate of the Exposure Point Con centration pursuant to 310 CMR 40.0926(3).
  8. No exposure potential exists (the Exposure Point Concentration may be set equal to zero) for those sites described at 310 CMR 40.0924(2)(b)3. if the following conditions are met and documented based on data collected at the disposal site:
    1. Demonstration of source elimination or control at the disposal site as described in 310 CMR 40.1003(5);
    2. Demonstration of diminishing contaminant concentrations throughout the horizontal and vertical extent of the plume;
    3. Demonstration that contaminant concentrations are not detected at or above analytical limits appropriate for a GW-1 area at the downgradient edge of the plume, at least 1,000 feet from the Public Water Supply well; and
    4. The demonstrations pursuant to 310 CMR 40.0926(8)(b) and (c) are confirmed by a minimum of two years of quarterly groundwater monitoring conducted after the termination of any Active Remedial  System and after the achievement of such contaminant concentrations.