SITE INFORMATION AND CORRESPONDENCE_1993-2003

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Cori L. Condon - 2 - 29 August 1997 Discussion: The following are numerical water quality limits for the VOCs detected in off-site wells: PCE Cal/EPA and USEPA 10-6 cancer risk estimate 0.7 µg/1 California and USEPA Primary MCL 5 TCE Cal/EPA 10-6 cancer risk estimate 2.3 µg/1 USEPA 10-6 cancer risk estimate 3 California and USEPA Primary MCL 5 1,2-DCA Cal/EPA 10-6 cancer risk estimate 0.5 µg/1 USEPA 10-6 cancer risk estimate 0.4 California Primary MCL 0.5 USEPA Primary MCL 5 Cancer risk estimates are calculated from published cancer potency factors assuming 70 kg body weight and 2 liters/day water consumption. Pursuant to an agreement between staff of OEHHA, DTSC, the State Water Board's Division of Clean Water Programs and the Central Valley Regional Board, Cal/EPA cancer risk estimates have precedence over USEPA values because the Cal/EPA values are the basis of formal California regulatory decisions. MCLs for TCE and PCE and USEPA's MCL for 1.2- DCA are not purely health-based limits because they were set equal to analytical quantitation limits that were widely achievable at the time that each MCL was established. . Using 106 Cal/EPA cancer risk levels, lifetime cancer risk estimates may be calculated from the maximum VOC concentrations measured in samples of off-site wells as follows: PCE: (6.3 µg/l)_ (0.7µg/1) = 9 additional cases in 106 persons exposed or 9 x 10-6 TCE: (3.1 µg/1)- (2.3 µg/1) = 1.3 additional cases in 106 persons exposed or 1.3 x 10-6 1,2-DCA: (20. µg/1)_ (0.5 µg/1)= 40 additional cases in 106 persons exposed or 4 x 10-5 Actual cancer risks would be lower than these estimates if persons were exposed to lower concentrations than the maximum detected levels or if they were exposed for a shorter period than the 70-year lifetime value used to derive the published cancer potencies. Actual cancer risks would be higher if concentrations of VOCs increase with time from plume advancement. Arsenic was also detected in off-site private wells. The California and USEPA Primary MCL of 50 µg/1 does not reflect cancer risk posed by exposure to arsenic. The Proposition 65 regulatory level for arsenic is 5 µg/1. The USEPA 10-6 lifetime cancer risk estimate for arsenic is 0.02 µg/1. A lifetime cancer risk estimate calculated from the highest measured arsenic concentration of 49 µg/1 would be (49 µg/1)_ (0.02 µg/1)=2450 additional cases in 106 persons exposed or 2.5 x 10-3. The ATSDR report contains errors and omissions. • Page 33. The summary table of VOC concentrations in off-site private wells does not include data for 1,2-DCA. • Page B-3, $ 3. ATSDR's calculated lifetime cancer risk from arsenic exposure to on-site workers of 6.16 x 10"4 is not within the Superfund residual risk range of 104 to 10-6, as claimed.