Numerical Groundwater Standards

Numerical Groundwater Standards. Groundwater is expected to meet the water quality standards equivalent to the Maximum Contaminant Level (MCL) of any constituent, as established by the Environmental Protection Agency (EPA). Table 1 is a list of those chemicals for which EPA has promulgated MCL's. As EPA adopts additional or different MCL's, this table will be revised and updated accordingly. For chemicals with no established MCL, the water quality standard shall be calculated using the procedure outlined in this section. A. Carcinogens (1) A water quality standard may be calculated from Risk-Specific Doses (RSD's) developed according to EPA Guidelines for Carcinogen Risk Assessment. The RSD is an upper bound estimate of the average daily dose of a carcinogenic substance that corresponds to a specified excess cancer risk for lifetime exposure. The standards calculated are derived from the following basic formula: RSD = (R/q1) x (W/I) Equation (1) Where: RSD = the Risk Specific Dose, or standard for the toxicant of interest; R = the specified risk level (e.g. 10-6); q1 = the carcinogen slope factor (CSF) in (mg/kg/day)-1 developed by the Carginogen Assessment Group (CAG) of the EPA, Office of Health & Environmental Assessment, or the EPA's Carcinogen Risk Assessment Verification Endeavor (CRAVE) Workgroup; W = the assumed weight of the exposed individual; and I = the intake amount for a given time period. (2) For purposes of calculating groundwater quality standards, it is assumed that the weight of the exposed individual (W) will be 70 kg & that the intake rate (I) will be 2 liters/day over a lifetime. Therefore, equation (1) is reduced to: RSD = 35 x R/q1 Equation (2) (3) Except as provided in Paragraph E of this section, the standard calculated from Equation (2) shall correspond to a risk level (R) of no less than 10-6 for Class A & B carcinogens, or 10-5 for Class C carcinogens. B. Systemic Toxicants (1) A water quality standard may be calculated from Reference Doses (RfD's) developed according to EPA accumulated data describing noncarcinogenic end points of toxicity. The RfD is an estimate of the daily exposure an individual (including sensitive individuals) can experience without appreciable risk of health effects during a lifetime. The standards calculated are derived from the following basic formula: C = (RfD) x (W/I) x (RSC) Equation (3) where: C = concentration for the toxicant of interest; RfD = Reference Dose in mg/kg/day; W = the assumed weight of the exposed individual; I = the intake amount for a given time period; and RSC = Relative Source Contribution, or the fraction of the overall exposure contributed by ingestion of water over the lifetime of an individual. (2) For purposes of calculating groundwater quality standards, it is usually assumed that the weight of the exposed individual (W) will be 70 kg and that the intake rate (I) will be 2 liters/day over a lifetime. Therefore, Equation (3) is reduced to: C = 35 x RfD X RSC Equation (4) The Relative Source Contribution (RSC) may vary widely with each application of Equation (4). Again, for purposes of calculating a groundwater quality standard, it should be assumed that ingestion from drinking water contributes a minimum of 20% of the overall exposure of a specific contaminant over the lifetime of an individual. If, however, there is information indicating that ingestion represents a higher fraction of the overall exposure, the RSC value may be adjusted, but in no case should it exceed 80%. C. TOXICANTS WHICH ARE BOTH CARGINOGENS & SYSTEMICALLY TOXIC Some toxicants may be both carginogenic and systemically toxic. In such cases, the lower of the two values as calculated by Equations (1) - (4) shall be the standard. D. DETECTION LIMITS In cases where the calculated standard is below the current analytical detection limit, the standard shall be the detection limit. E. ALTERNATIVE STANDARDS (1) For remedial purposes only, the Commission on Environmental Quality may establish an alternative standard (AS) in lieu of the calculated standard, as long as: (a) the AS established is based upon human health criteria; and (b) the AS does not exceed a lifetime cancer risk level of 10-4. (2) Environmental, technological, and economic factors, as well as consistency with EPA regulations and guidance may be considered in establishing an AS. (3) An AS may be site specific or for a group of remedial sites with similar characteristics.