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I ' in MW-9A through MW-12A, and from 60 to 65 feet bsg in wells MW8B through MW-I2B, with iblank casing extended to the surface Pilot boring EX-1 was completed as a ground water extraction ' well with 0 020-inch slotted screen installed from approximately 15 to 35 feet bsg and with blank casing extended to the surface A California state-licensed survey engineer from Baumbach and Piazza,Inc surveyed a i eference point on the top ofwells MW-3,MW-6,MW-7,MW-8A/B through ' MW-12A/B and EX-1 to the nearest 0 01-foot relative to a City of Stockton Bench Mark On 22 and 23 October 2002 monitoring wells MW-1,MW-2 and MW-4 were abandoned by utilizing ' hollow stem augers to drill out wells MW-1, MW-2 and MW-4 to the wells total depths Subsequently, each borehole was completely backfilled with a portland cement grout mixture to one to two feet below surface grade and the remaining areas were backfilled to surface with neat cement Between March 1992 and October 1999, seven ground water monitoring events were performed at the site, ground water monitoring has been conducted quarterly since November 2000 ' SITE CONCEPTUAL MODEL Based on the data developed to date, including all soil borings,monitoring wells installed and CPT ' soundings,AGE has developed a site subsurface model forthe release,migration and distribution of the contaminants in the subsurface as described below ' • The subsurface from site grade to approximately 120 feet bsg, based on the CPT data, is composed primarily of sand,silty sand and gravelly sand were encountered at depths between approximately 20 and 31 feet bsg,48 and 74 feet bsg and 113 and 117 feet bsg Silt,silty clay, ' clayey silt, sandy silt and clay were observed at all other depths Other site investigations suggest that tan to brown,damp to moist clay with silt and sand and ' sandy clay were encountered from approximately 5 feet to 15 feet bsg Tan to brown,moist to saturated clay, clayey sand and poorly graded sand was encountered from 15 to30 feet A distinct color change from tan or brown to gray was observed at approximately 20 feet bsg ' Olive to bluish gray,saturated silty to sandy clay was encountered from 3 5 feet to 47 feet bsg Gray, saturated poorly graded sand was encountered from 47 feet to 65 feet bsg o A coarse-grained interval of variable thickness and lateral extent occurring between 50 and ' 60 feet bsg, with a greater thickness in the former UST area and a laterally discontinuous coarse-grained interval at approximately 25 to 30 feet bsg ' These characteristics allowed consti uction of a site geologie/hydrogeologic model of the vadose zone extending from surface grade to approximately 20 or 25 feet bsg ' Hydrological Unit 1 (HUI), at the top of the saturated zone, extends from approximately 20 feet to 45 feet bsg and is comprised dominantly of silt, with apparently laterally discontinuous sand lenses within the upper 10 feet The unit is characterized by a varying ' range of pore pressures, from generally medium Iow between 20 and 35 feet bsg, to low between 35 and 40 feet bsg, to medium high between 40 to 45 feet bsg Advanced GeoEnvironmental,Inc