WORK PLANS FILE 2 - Laserfiche WebLink

Home Browse Search

Geological recknles 1= Page 3 Additional Site Characterization Work Plan Project No.724.2 March 15,2005_ Need for Additional Site Characterization The "Conclusions & Recommendations" section of GTI's "Dual Phase Pilot Test" report contained the following points: o The five foot sampling methodology utilized at this site may have failed to intercept lithologic units of greater hydraulic conductivity. o The plume remains undefined down gradient of the MW-8/-108/-208 well cluster. These concerns indicate that additional site characterization is warranted. Another issue is that an off-site source of contamination exists that suggests the presence of a co-mingled plume. The active Shell service station located to the east across N. El Dorado Street has had a documented gasoline release and is listed on the State of California's Geotracker environmental database. Mr. Jeff Wong of the SJC/EHD supplied GTI staff with a copy of a July, 2003 groundwater monitoring report for the subject Shell station facility. The report indicates that the wells on the northern portion of the Shell site contain low levels of TPH-G (110- 2,500 ug/1). GTI staff accessed the GeoTracker database and retrieved the latest data for groundwater sampling at the Shell station (see Appendix B). The Valentine (Castle Motors) shallow well TPH-G data for the ls` quarter 2005 groundwater monitoring event for site are included in Figure 3. The adjacent site's monitoring well locations, TPH-G and MTBE data from their July 2004 event are included on the right side of Figure 3. The groundwater plume extends northeast from the` subject site to the northwest corner of the Shell station. Although a groundwater sample from boring SP-1 (see Figure 2) was recorded as containing 66 ug/1 TPH-G in 1997, the plume is currently undefined beyond the MW-8 well cluster and Shell wells MW-1 and MW-2. 2.0 PROPOSED INVESTIGATION GTI proposes using Cone Penetrometer Testing (CPT) in order to define the down gradient extent of the gasoline impacted groundwater. Three CPT borings (CPT 1 — 3) will be advanced to the northeast of the subject site as shown in Figure 4. The CPT locations are located in the predominant down gradient direction as shown in Figure 5 — Groundwater Gradient Rose Diagram. This spatial arrangement will augment the data from the MW- 8/108/208 well set and data from the Shell station wells MW-1 and MW-2. Shell wells MW- 3 and MW-4 currently define the southeasterly edge of the plume. Soil and groundwater samples will be obtained from the borings to update the site conceptual model as outlined below in Section 3.0. The proposed depth of the borings is 100 feet bgs. GTI believes that clay layers recorded in the 80 — 100 feet bgs depths at the site may inhibit vertical migration of the contamination.