ARCHIVED REPORTS_MONITORING WELL INSTALLATION REPORT - SITE ASSESSMENT WORK PLAN

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1 ' Historical Background:gr d. River City Petroleum and Connell Motor LIR Truck Page 10 of 13 depth-discrete intervals (hydrogeologic units) to monitor vertical and lateral changes in hydraulic ' gradient,contaminant concentrations and migration directions within each unit.Each hydrogeologic unit was defined based on lithologic information provided by laboratory analyses and description of soil samples and from cone penetrometer testing,as detailed in the AGE-prepared Quarterly Report, ' First Quarter 2004, dated 14 May 2004. Eight hydrogeologic units (HUs)were defined: • Shallow-screened(40 to 60 feet bsg)monitoring wells MW-1 through MW-17 were screened ' through the first hydrogeologic unit(HU1); • Intermediate-upper-screened (75 to 85 feet bsg) wells MW-18 through MW-21 were ' screened in the second hydrogeologic unit (HU2); • Intermediate-lower-screened (90 to 105 feet bsg) wells MW-22 through MW-29 were screened in the third and fourth hydrogeologic units HU3 and HU4; • Deeper-screened(125 to 135 feet bsg)wells MW-31 through MW-33 were screened in the fifth hydrogeologic unit (HU5) from 110 feet to 145 feet bsg; ' • A depth-discrete well in the former UST area, MW-34, was screened in the sixth hydrogeologic unit (1-rU6) at 145 to 150 feet bsg. ' • Recent data collected from soil boring MW-35 indicates a seventh hydrogeologic unit(HU7) from 225 feet to 245 feet bsg comprised of silt and clay, and a lower eighth hydrogeologic ' unit (HU8) from approximately 245 feet bsg comprised of sand. 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 for the release, migration and distribution of the varying contaminants in the subsurface as described below: ' • The subsurface from site grade to approximately 240 feet bsg is composed primarily of fine-grained soil with a coarse-grained interval of variable thickness and semi-consistent lateral distribution. A major coarse-grained sand and gravel interval in the vadose zone occurs between 20 and 35 feet bsg;a laterally continuous coarse-grained sand interval occurs at approximately 75 to 85 feet bsg and approximately 150 feet bsg; a vertically and laterally significant sand/gravel unit begins at 165 to 185 feet bsg, and is present across the site to a ' depth of 240 feet bsg. A laterally discontinuous, yet significantly thick sand unit extends from 100 feet to 110 feet bsg. • Overall high pore-pressure was documented in the CPT sounding of the coarse-grained sand below the water table. The remaining stratigraphy of the site is comprised of silt with clay inter-channel fill areas. A laterally continuous clay, locally silt at CPT9, unit is present ' beginning at 120 feet bsg across most of the distribution of the contaminate plumes. Advanced GeoEnvironmental,Inc.