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Stantec Addendum to Work Plan for Vapor Probe Destruction, Well Destruction, and Well Installation ARCO Service Station No. 2186 January 26, 2009 Soil Vapor Probe Destruction Instead of pressure grouting to the total depth, the existing soil vapor probes (VP-1 through VP-3) will be over-drilled to the total depth and will be grouted from the total depth to the ground surface using neat cement via a tremie pipe. The over-drilling of soil vapor probes VP-1 through VP-3 will be supervised by Stratus. A licensed drilling company will complete the soil vapor probe destruction work following the California Well Standards bulletin 74-90 Part 111, Section 19 and 23. Installation of Two Shallow Wells Two on-site soil groundwater monitoring wells .(MW-1 R and MW-3R) screened across the shallow groundwater bearing zone are proposed to be installed at the approximate locations shown on Figure 1. The proposed. well locations-may change in the field due to unexpected site conditions such as a hard pan layer or Imitations in equipmentnt placement;'etc. Prior to drilling, the soil borings will be air-knifed to at least five ft bgs. The soil borings for MW-1 R and MW-3R will be advanced to approximately 58 ft.bgs, instead-of_55 ft bgs, using a drill rig equipped with eight-inch diameter, continuous flight, hollow-stem augers.- The ugers:The hollow-stem auger will be advanced to_30.ft_bgs, during which continuous coring will begin I -.r� � x - - �_ -and endo the total boring depth. An attending geologistwill fog_cuttinds'_rid sample-in accordance with American Society for Testing and Materials {ASTM)'`D-2488 Standard Practice for Description and Identification of Soils (Visual-Manual Procedure) under the direction of a State of California Professional Geologist or Professional Engineer. Soil cores will be screened in the field for the presence of volatile organic compounds,using.a-photo-ionization• detector.(PID):-;All down-hole drilling equipment will be decontaminated with high pressure steam following -the—completion of each soil boring. Down-hole sampling equipment will be decontaminated between samples using an Alconox detergent solution and a tap water rinse. The shallow wells will be constructed of two-inch.diameter,.*flush-treaded, schedule 40 .poly vinyl chloride-(PVC.) well casing with,20 ft of 0.020-inch.factoryyslo ed well screen set at depths of 38 to 58 ft-bgs,' instead of 35-to 55 ft bgs.`-A sand filter pac will be placed in the annular i space surrounding the entire screened interval, and will extend approxi mately one.to=two ft ! above the screen interval. A two to three-foot thick _bentonite annular seal will be'placed above the filter pack-and will.be�h.ydrated., The'-remainder of'the`annular space will be filled with neat I cement and/or grout from the top of the bentonite seal to within 12 inches ofAhe_-surface. The well will be secured wit ."lock ing7well cap'`and c pie d with aterfight traffic-rated welllf box installed at grade. A detail of the well completion is presented in Figure 2.—_JA .5wff 0 a Installation of One Deep Well One on-site soil groundwater monitoring well (MW-1D) screened across the deep groundwater bearing zone is proposed to be installed at the approximate location shown on Figure 1. The proposed well location may change in the field due to unexpected site conditions such as a hard pan layer or limitations in equipment placement, etc. J Prior to drilling, the soil boring will be air-knifed to at least five ft bgs. The soil boring for MW-1 D will initially be advanced to approximately 65 ft bgs using a drill rig equipped with an eight-inch diameter, continuous flight, hollow-stem auger as a pilot hole. Afterward, the soil boring will be _ 2186 Addendum to Work Plan for Vapor Probe Destruction, Well Destruction, and Well Installation.doc 2