MCP category GW-3 Standards (310 CMR 40.0974(2))) apply to all groundwater in the Commonwealth, as stipulated in the MCP at 310 CMR 40.0932(2). The GW-3 standards can be regarded as the minimum standards for sites evaluated using MCP Risk Characterization Method 1.
The GW-3 standards are intended to provide some protection against the migration and eventual discharge of groundwater contaminants to surface water at concentrations that could pose a significant risk of harm to aquatic organisms. Risk-based "target values" in surface water are modified by two dilution/attenuation factors to estimate an allowable concentration in groundwater. The two dilution attenuation factors are intended to conservatively account for dilution within the receiving waterbody (Dsw) and attenuation in the groundwater as the contaminant migrates towards the waterbody (Dgw).
The sequential approach taken to develop the MCP GW-3 standards is as follows:
1. Risk-based target values in surface water are identified in accordance with an established hierarchy, as described in section 2.3.3.
2. The risk based target value in surface water is multiplied by a dilution/attenuation factor, Dgw to account for dilution between the groundwater and the receiving surface water body
3. The risk-based target value is surface water is multiplied by a dilution/attenuation factor, Dsw to account for dilution of the groundwater in the receiving surface water body.
4. The lower of the concentrations identified in steps 5 and 6 is carried through the process.
5. A Practical Quantitation Limit (PQL) for an appropriately sensitive analytical method is identified (Workbook: MCP Toxicity.xls, sheet: Toxicity, column: AO).
6. A background concentration in groundwater is identified, if available (Workbook: MCP Toxicity.xls, sheet: Toxicity, column: AD).
7. The highest of the three values identified in steps 7, 8 and 9 is chosen.
8. The value identified in step 10 is rounded to one significant figure. This value is adopted as the MCP GW-3 standard.
This process is diagramed:
Selection of Target Values in Surface Water
The term "target values" refers to the risk-based concentrations on which the GW-3 Standards are based. The target values were chosen in accordance with the following hierarchy:
•If EPA has published National Recommended Water Quality Criteria (NRWQCs), the target value is based on one of the criteria. Preference is given to the lower of the marine and fresh water Criterion Continuous Concentration (CCC, or chronic criterion). If no CCC is available, a Criterion Maximum Concentration (CMC, or acute criterion) is selected.
•If no CCC or CMC is available, the study for which the lowest toxicity value is reported in the AQUIRE database is identified. After reviewing the study to ensure that the results are valid and applicable, the lowest observed effect concentration (LOEC) from the study (if reported) is selected. If a LOEC was not reported, an EC50 (effects concentration for 50% of the test organisms) or LC50 (lethal concentration for 50% of the test organisms) is selected as the basis for the target value.
•If all of the above were unavailable, Suter and Tsao (1996) is reviewed to identify a toxicity study that provides a toxicity concentration suitable for use as the basis for the GW-3 standard.
•If no source of toxicity data is available for the substance in question from the sources identified in the preceding steps, then the references previously selected as sources of toxicity data for other substances were reviewed. If any these contain toxicity information for the substance in question, it is used as a basis for the GW-3 standard for that substance.
•If an acute (for this project, study duration < 7 days) value is selected, the acute concentration is divided by 10 to estimate a chronic value.
For target values selected according to bullets (2) through (4) above, the lowest observed effect concentration (LOEC), or equivalent value from the study is selected if reported in the study. If a LOEC was not reported, an EC50 (effects concentration for 50% of the test organisms) or LC50 (lethal concentration for 50% of the test organisms) is selected as the basis for the target value. The LOEC is given preference over the LC50 because it generally will provide a more accurate estimate of the threshold concentration at which effects can be expected to begin occurring.
The references for the technical studies that are the basis for the surface water benchmark levels are listed in the spreadsheet calculating the GW-3 standards file size 1MB (Workbook: MCP GW.xls, sheet SW Target, column AO). A copy of each reference is available for review in each DEP Regional Office and in Boston. Contact Tom Angus (Thomas.Angus@state.ma.us, 617-292-5513) for more information.
Target Groundwater Level
Workbook: MCP GW.xls, sheet: GW-3, column F
[OHM]target-gw = Target groundwater concentration (Âµg/L)
[OHM]eco-sw = Identified ecologically-based surface water criterion (Âµg/L)
Workbook: MCP GW.xls, sheet: SW Target, column AC
Dgw = Groundwater-to-surface water dilution factor (dimensionless)
Workbook: MCP GW.xls, sheet: GW-3, column E
Dsw = Dilution factor in the receiving surface water body (dimensionless)
Workbook: MCP GW.xls, sheet: GW-3, column D
The chemical-specific ecologically-based surface water criteria are listed in spreadsheets (Workbook: MCP GW.xls, sheet: SW Target, column AC) with references provided (Workbook: MCP GW.xls, sheet: SW Target, column AO). Copies of the original literature are available for review, if needed. Please contact Thomas Angus (Thomas.Angus@state.ma.us, 617-292-5513) for more information.
The following table lists the dilutions factors applied in the calculations.
|dimensionless||Koc < 103|
|dimensionless||103< Koc < 106|
|dimensionless||Koc > 106|