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Addendum to Work Plan For Soil Boring And Well Installation February 17, 1999 To satisfy the directives in the SJCPHS-EHD letter,GR proposes to modify the scope of work proposed in the GSI Work Plan by drilling additional soil borings adjacent to wells MW-2 and MW-5, modifying the proposed construction of the groundwater monitoring well across Pacific Avenue,and installing an additional shallow-screened groundwater monitoring well between wells MW-7 and MW-11. This scope of work will be implemented by performing the scope of work in GSI's original Work Plan with the following modifications: • In addition to the boring adjacent to well V W-2 proposed by GSI, GR will also drill soil borings adjacent to wells MW-2 and MW-5. These borings will be advanced to a depth of 55 feet bgs or until field evidence of hydrocarbon impact(odor,discoloration,elevated photoionization readings, etc.)are no longer observed. If the borings are advanced deeper than 55 feet bgs(the depth of the existing monitoring wells), the borings will be completed as 2-inch diameter deep-screened air sparge wells. The actual well construction details(Figure 3)will depend of the depth of the boring and field conditions, and will be determined at the time of drilling. However, well construction details will be reviewed with SJCPHS-EHD personnel prior to well completion,and the wells will be constructed per SJCPHS-EHD and California Well Standards(Department of Water Resources, Bulletins 74-81 and 74-90). Groundwater samples will be collected from these deep-screened air sparge wells prior to initiating air sparging. • If significant hydrocarbon impact is observed in the borings but is not observed to extend below 55 feet bgs,these borings will be backfilled to a depth less than 55 feet bgs and completed as 2-inch diameter air sparge wells. The actual well construction details will depend on field conditions,and will be determined at ,the time of drilling. The proposed air sparge well construction details presented in Figure 3 will be modified according to field conditions and reviewed with SJCPHS- EHD personnel. • If the boring drilled adjacent to well MW-2 is not advanced deeper than 55 feet bgs(and therefore is not completed as a deep-screened air sparge well)and is not competed as a shallow-screened air sparge well, it will be backfilled to approximately 40 feet bgs and completed as a shallow-screened groundwater monitoring well. if the boring adjacent to well MW-2 is completed as an air sparge well, a second boring will be drilled in this area to approximately 40 feet bgs and completed as a shallow-screened groundwater monitoring well. The shallow groundwater monitoring well will be constructed with 20 feet of well screen (extending from 20 to 40 feet bgs) so that it intersects the existing water table. Proposed shallow-well construction details are presented on Figure 4. The well proposed for the east side of Pacific Avenue in the original GSI Work Plan will be constructed as a shallow-screened groundwater monitoring well(Figure 4). • Because the groundwater gradient at the site has fluctuated with time between north-northeast and east-northeast, a shallow-screened groundwater monitoring well will be installed between wells MW-7 and MW-I 1 (Figure 2). • Soil and groundwater samples will be collected and analyzed as described in the original GSI Work Plan. 140124.04 2