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Mr. Maurice Benson <br /> December 30, 2009 <br /> Page 2 <br /> groundwater program operation and maintenance costs and to minimize personal <br /> liability from sampling potable wells on private residences. Although the report <br /> proposes the significant reductions in sampling frequencies there is no evaluation <br /> presented on the impact to human health or safety considerations of <br /> implementing the reduced sampling. <br /> Off-Site guard well clusters are proposed to.be installed in flow paths located <br /> between known COC plumes and off-site PWs. The guard well clusters are <br /> proposed to be sampled at least twice per year to help provide an early warning if <br /> COC plumes migrate further off-site. If groundwater data from the guard well <br /> clusters show that a plume may be migrating, the flow path will be identified if <br /> possible and may result in changes to the sampling frequencies of each PW as <br /> deemed necessary. <br /> DTSC is unaware of any comprehensive analysis determining the guard well <br /> cluster locations relative to the known COC plume pathways and if the guard <br /> wells are appropriately screened in the specific hydrologic zones represented by <br /> the PWs. In many cases potable well construction logs do not exist or have not <br /> been procured by DESJC or the County, making it difficult to assure that the <br /> guard wells provide a high degree of comparability. Due to: the uncertainties in <br /> representative guard well location and screen intervals; uncertainties and <br /> variability in off-site potable well intervals; seasonal variability in groundwater <br /> flow directions; and, the likelihood of unknown preferential groundwater <br /> pathways, DTSC is not supportive of the significant reduction in sampling <br /> frequencies proposed in the Contingency Plan. DTSC requests that DESJC <br /> provide a more comprehensive analysis identifying how the proposed off-site <br /> guard well locations and screen intervals are representative of the off-site potable <br /> wells known to be in the COC plumes flow paths. DESJC should consider a <br /> phased approach for reductions in the PWs sampling frequencies rather than the <br /> significant cutbacks proposed. <br /> Specific Comments <br /> 1.) Page 2-1, section 2.2, 4`h sentence; The risk assessment for chloromethane <br /> accounted for the inhalation and ingestion pathways but did not identify whether <br /> the dermal pathway was considered. A risk assessment accounting for a <br /> residential use of tap water would include dermal contact as an exposure <br /> pathway accounting for bathing or washing hands as an example. The report <br /> should identify if the residential use of tap water risk calculations for <br /> chloromethane considered the dermal exposure pathway, and if not, should be <br />