SITE INFORMATION AND CORRESPONDENCE_CASE 2

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Page I of 2 a • • Harlin Knoll [EH] From: Tim Cuellar[tcuellar@advgeoenv.com] Sent: Wednesday, January 02, 2008 1:06 PM To: Harlin Knoll [EH] Subject: Sierra Lumber Case No. 2 Harlin, Earlier this morning,we had discussed preparing a work plan for replacement wells at case No. 2. However, although additional ground water monitoring wells will be necessary to assess the extent of gas-impact, I believe it is a better idea to advance a series of CPT borings, gather data, and in the report of findings, recommend locations of additional wells for lateral and vertical assessment. In reviewing previous soil and ground water data, substantial soil impact is was reported in the tank pit(MW-15) between 20 and 40 feet at concentrations around 2,000 mg/kg; additionally, Alta's boring (B-3)detected soil- impact at 20 feet at 4,960 mg/kg. Based on the vertical soil profile for the site to depths of 65 feet, it is very likely that soil and ground water are impacted at depths greater than 65-70 feet. Therefore, a CPT boring to a depth of at least 100 feet in the area of MW-15 would go a long way in helping to identify deeper lithology, migration pathways,vertical extent and in properly locating additional monitoring well screen intervals. As you know, shallow and deep ground water monitoring wells (MW-10 and MW-14)were recently abandoned; shallow well MW-10 was typically an ND well while deep well MW-14 had consistently been impacted at concentrations between 2,000 and 4,000 ug/I. I believe you may have had some concerns with this data, largely due to the soil samples being ND during the installs of MW-10 and MW-14. However, I believe the ground water data MAY have been representative of conditions at 65 feet. In reviewing the boring logs, the soil samples at deeper depths from MW-14 all had strong HC odors. Additionally, I reviewed all monitoring well purge sheets for shallow wells MW-10 and MW-15 and compared to that of MW-14. The field parameters (pH, EC and temp.) collected at MW-14 appeared to be representative of deeper water conditions, and consistently had lower ground water temp's and lower EC values than that of MW-10 and MW-15. Prior to installing replacement wells of MW-10 and MW-15, 1 believe a CPT boring further south of this area would be beneficial in identifying the vertical extent of the ground water impact. There is currently limited data west of the former UST area. The furthest data point was Alta's boring B-6,which did have some limited impact at 15 feet bsg. Based on the recent ground water flow directions from quarterly ground water monitoring events, a CPT boring would be beneficial in an area west of the Alta boring B-8.Also, there is lack of assessment east of the tank pit.A CPT boring east of the former tank pit would be beneficial. Rather than prepare a work plan for re-installing wells MW-10 and MW-15, 1 would prefer to prepare a work plan for CPT borings.....................your thoughts? Thanks-tim Timothy I Cuellar 1/2/2008