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i Langston's Alar/ref 15615 Seventh Street, Lathrop, Cah ornhf 04 toter 5, 1995 Interim Secondary Source Removal A 11 orrit.rring Well Rqn lacemetrt flage 4 32 Excavation Approximately 4,500 tons of ox erburden soil was removed doNNn to approximately I 151/2 feet below ground level (b g l ) This material was stockpiled, with permission from the City of Lathrop, on their adjacent Public Works lot on the south side of"L" Street The overburden material was sampled and verged as "ND" (None Detected), and v„ill be used as bacl ftll material (Figure 7, Table 1 5, Appendix A) Before excavation proceeded, th;, groundwater monitoring wells were destroyed, by grouting, undet permit and ciilien ision of San Joaquin County Environmental I Health Department (SJCEHD) personnel on July 27, 1995 These wells were subsequently reitioved during the excavation activities, ex-edit For MWA, wluch was on the north edge of the excavation (Figure 3) On August 14, 1995, pothole trenching was conducted into the contaminated horizon to explore levels of contamination that might be encountered during further excavating Five locations were dug from 151/ to 20 feet b g 1 in a southeast to northwest direction Readings taken with a portable photo-ionization detector (PID) were over the instrument maximum of 2,1.00 parts/million (ppm) IAn LEL instrument was then utilized, and measured levels i anging frons 17 to 22% volatile hydrocai bon,, Based on the aforementioned data, ventilation equipment was installed and prepared for use, as needed, doting excavation activities A de-watering well was installed at the south end of the excavation to a depth of 27 feet b g 1 The well was plumbed with 2-inch PVC to six 10,000 gallon storage tanks on the west side of the site Due to the high cost of off-site disposal a carbon drum scrubbing, system was installed to treat the lit eater, and either store treated water in tanks or discharge directly to the sever system An Industrial Wastewater Discharge Permit was obtained from tl,e City of Lathrop Public Works Department (Appendix 11) After demonsti-it,ng the effectiveness of the treatment system (witnessed by SJCEHD personnel) by chemical analysts of stored treated water (Table 2 2, Appendix B), direct discharge to the sewer system was authorized A ' rate fee for discharge of wastr skater to the Publicly Owned Treatment Works (POTW) was negotiated with the City of Manteca Daily laboratory sampling was continued through the entire de-,,vatering event Approximately 200,000 gallons of>/ ' water was pumped and treated c:tyi;t I, this excavatien activity Excavation of contarnmated toil began on August l0, 1995 and continued through Septembet 22, 1995 During th;s interval, the average air temperature was 92° F