SITE INFORMATION AND CORRESPONDENCE

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Former Cheaper Store Number 37 Page 2 15 East Grant Line Road October 30, 2008 Tracy, California 95376-2720 MW-7 during the September 2008 sampling event were reported to contain only MTBE at concentrations less than one microgram per liter. The EHD will reconsider a monitoring well down gradient of MW-7 if concentrations of contaminants of concern increase significantly in groundwater samples collected from MW-7, or other data are presented that demonstrates a need for such a well. The EHD does not approve the installation of two proposed monitoring wells on the west side of Holly Drive, one screened in "Zone B" and one screened in "Zone C"; nor does the EHD approve the installation of two proposed monitoring wells screened within "Zone C", one located adjacent to the current underground storage tanks and one located adjacent to MW-7. Instead, the EHD believes two monitoring wells should be installed between soil boring CPT-2 and monitoring well MW-5, one screened within "Zone B" and one screened within "Zone C". Groundwater results from these two wells will either confirm that impacted groundwater is in "Zones B and C", or was simply cross contamination that occurred during the collection of grab groundwater samples CPT2-41 and CPT2-76. The EHD will reconsider installing additional wells if it is shown that groundwater collected from"Zones B and C"is impacted. ATC proposes to construct monitoring wells screened in "Zones B and C" using conductor casing to minimize the potential for vertical migration of contaminants. Although the work plan verbally describes the general construction of the conductor-casing wells, it does not explain in detail the dimensions of the conductor casing nor the drilling method to be used to construct the wells. Also, Figures 5 and 6 of Attachment 1 of the work plan do not illustrate the presence of conductor casing in the well construction diagrams. San Joaquin County requires monitoring wells to be constructed following the Well Standards in accordance with San Joaquin County Ordinance Code Section 9-1115.6. Well Standards' Section 13.9.2 specifically states that the monitoring well borehole diameter shall be at least four inches greater than the outside diameter of the conductor casing. The inner diameter of the permanent conductor casing shall in turn be at least four inches greater than the outside diameter of the well casing. Please resubmit an addendum to the work plan by December 31, 2008, that includes the drilling method to be used for installing the two conductor-casing monitoring wells, along with specific dimension for the borehole, conductor casing, and well casing. Include well construction diagrams with the dimensions in the work plan addendum. During the review of mass calculations,the EHD identified the following problems: • The method for obtaining average concentrations was explained verbally but was not demonstrated. As a result, the EHD was unable to duplicate most of the soil and groundwater concentrations. • The formula for the mass calculation was not presented; instead, ATC demonstrated the multiplication of rows of constants and variables, without explanation, for each plume segment. • Rather than estimate the weight mass of impacted soil, ATC used the weight of water for the same volume of impacted soil (62.43 pounds per cubic foot [lb/ft3]). Noting that soil is more dense than water, the EHD commonly uses a weight of 110 to 125 lb/ft' for mass estimates,but will use grain density measurements, if available.