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Mr. Robert C. DOss•P.E. • <br /> Page 4 <br /> June 29, 1990 <br /> Sieve analyses of water producing strata in site wells should be <br /> done for materials coarser than clays or silts. The same sieve <br /> analysis can be used to design the filter pack (sand pack) for <br /> wells which are screened across the same geologic unit. <br /> Offsite background wells located unaffected areas and up-gradient <br /> of the site will be needed. MW-2 should not be considered a <br /> background well since it is onsite and near, or in, a <br /> contaminated area. <br /> A continuous ground water monitoring program should be <br /> implemented consisting of monthly water level measurements and <br /> collection of ground water samples. Monthly water level and <br /> analytical data is needed for at least one year to evaluate the <br /> seasonal ground water level and contaminant variations. <br /> On page 49 of the Report, it is indicated that ground water and <br /> soil samples containing elevated levels of organic material were <br /> diluted so that the levels fell within the linear range of the <br /> analytical instrument. Dilution of samples containing extremely <br /> elevated levels of target constituents is allowable for these <br /> constituents only. Diluted samples should not be analyzed for <br /> any other constituent(s) since the results would be erroneously <br /> low. An undiluted portion of the original sample should be used <br /> for analysis of all other non-elevated target constituents. <br /> Target Constituents Only the target constituents which have been <br /> historically associated with Town Gas waste were included in the <br /> analytical program. A site specific set of target constituents <br /> is needed. Enclosed are two tables giving analytical methods <br /> which' should be run on soil and ground water samples. The <br /> results . from these tests can be used to determine the <br /> constituents of concern for the Stockton facility. <br /> Off-site Areas of Concern The preliminary RI concentrated only <br /> on property currently owned by PG&E. Historical data provided <br /> indicated past gas plant operations on property now owned by <br /> Morton Paint Company. PG&E should investigate this parcel, as <br /> well as others, which were at any time part of the former <br /> manufactured gas plant facility. <br /> The Mormon Slough is located directly south and adjacent to the <br /> site. Ground water monitoring results indicate that ground water <br /> from under the site is flowing in a southerly direction toward <br /> the Mormon Slough. PG&E should valuate the possibility that <br /> contamination from the site may be impacting the water quality in <br /> the Mormon Slough. <br />