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Jeff Wong 4PRION San Joaquin County Environmental Health Department 30 October 2019 Page 3 These data indicate that concentrations of TPHg, benzene, and MTBE are below cleanup objectives, while TPHd concentrations range from less than to slightly greater than cleanup objectives. 3. Long-term monitoring of concentrations and plume trends indicate the lateral extent of the plume is stable (Orion, 2019). As described in Stantec's 23 September 2016 work plan for ozone system removal, downgradient petroleum hydrocarbon concentrations remained stable over a 4-year period of post-remedial rebound monitoring following shutdown of the ozone systems in 2012. Stantec concluded that ozone injection was no longer necessary within the downgradient plume and the RWQCB concurred (RWQCB, 2016). These conditions indicate that the risks of lateral contaminant migration from upgradient source areas are generally low. 4. There is limited potential for vertical contaminant migration between water- bearing zones A and B in the vicinity of the 12 wells proposed for grouting. / 8 of the 12 wells are screened within the A-Zone and therefore do not pose a risk of providing a conduit for vertical contaminant migration to deeper water- bearin zones. 4 of the 12 wells are screened within the B-Zone. Petroleum ess dense than water and is only moderately soluble which is generally unfavorable for vertical migration. As summarized above, impacts within the A-Zone are generally low in the vicinity of the former ozone injection wells. Based on the properties of petroleum and the relatively low magnitude of impacts within the A-Zone, the risks of contaminant migration between the A- and B-Zones are generally low. Based on petroleum hydrocarbon impacts in the B-Zone and deeper water-bearing zones, monitoring results also indicate that the barrier between the A- and B-Zones is not complete and vertical migration has already occurred in the vicinity of former source areas. 5. The wells proposed for destruction were installed relatively recently and as a result, are in good condition and able to effectively hold sealant material. The wells were installed in 2003 by Cascade Drilling, a national contractor and C-57-licensed driller. The wells were constructed according to current industry and regulatory standards including a 2.5-feet-thick hydrated bentonite seal emplaced above the screened intervals. The condition of the wells indicates that the risk of degradation of the seal materials is generally low. Boring logs for the wells proposed for destruction by grouting are included in Attachment B. S:\Clients\52STTC\LTR\LTR002.docx 10/30/19