SITE INFORMATION AND CORRESPONDENCE FILE 2

Home Browse Search

Velma Masterson 749 E. Charter Way, Stockton Page 2 of 3 be inadequate and cannot concur with the recommendation in the FS or lack of recommendation in the AFS . EHD also considers WP-SVE to be inadequate and cannot approve it. CGG proposes purging the wells of three pore-volumes of vapor prior to sampling. Provide EHD with details on the size of the area to be purged of three-pore volumes and how long the purging will take . The calculations should be submitted for review. It has been EHD's experience that it is common practice to collect several soil vapor samples during an extended test to analyze concentration trends over time; but a rational for collecting only one soil vapor sample per well was not provided by CGG . A consistently increasing contaminant concentration trend would be interpreted differently than stable or decreasing trends. The proposed analytical method utilizing Summa canisters and EPA Method TO-3 may be more accurate than previous results , but using historically utilized methods for sampling and laboratory analysis methods may provide data more comparable to the historical data collected from the SVE system . In reviewing our file information , EHD found only one lab report for vapor samples taken in 1997. There was no description of SVE system protocols for monitoring and sampling , or calculations showing the amount of removed contaminant mass. Submit a report to EHD by 01 March 2006 on the operation of the SVE system that includes a description of the vapor sampling protocol utilized for the SVE system , copies of all laboratory results for vapor samples and some example removed mass calculations . The WP-MW is also inadequate and not approved . The purpose of this work plan was to install a deep monitoring -well to verify the results from the CPT groundwater samples and to hopefully delineate the vertical extent of impacted groundwater. EHD had pointed out in the 27 September 2005 letter that an unsampled sand/gravel interval was identified in the CPT log at 132 to 135 feet below surface grade (bsg), which CGG did not address in the WP-MW ; the sand unit may be the deepest significant soil unit that may serve as a lateral migration pathway for contaminants. EHD considers the silty clay, clayey silt, silt and 'stiff fine grained' soil in the proposed 150 to 160 feet bsg interval to be a less likely significant lateral migration pathway for contaminants . Other potential lateral migration pathways above 100 feet bsg and below the currently monitored depths can be identified on the CPT log . The selected interval , based on the data , should be picked to optimize the value of the data to be collected from it, i .e. , delineating the vertical extent of the plume and checking a potential migration pathway, rather than merely selecting the deepest point sampled to date . It is possible that some of the analytical results for the deeper CTP groundwater samples may be the result of cross contamination of the samples . The proposed well should be located in the presumed down-gradient direction from the suspected main source area — the former UST pit. A very major problem with the WP-MW is the confusing proposed well construction details without a clarifying well diagram provided as part of the work plan . According to the WP-MW , the well is to be screened from 150 to 160 feet bsg (which would be a 10- foot screen), with "0 . 02 slotted casing in the bottom 20 feet of the well (which would be a 20400t screen ) and blank casing from 30 feet to the surface" (a 30400t blank casing? ). The work plan further states "a transitional seal will be placed from 25 to 22 feet which will consist of hydrated bentonite and that from 148 feet to the surface the