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Site Background Information: Den's Auto Repair Page 6 of 8 ANALYTICAL RESULTS OF MONITORING WELL WATER SAMPLES The ground water monitoring network has been monitored since September 1997. Over that time span, the highest concentrations of dissolved hydrocarbons have been detected in well MW-1A,� located on the western edge of the site.This location is generally the p-gradien poin f the site and the source of the. dissolved hydrocarbons is not readily apparent or re ated o contemporarily Y removed UST set. le— The lateral extent of dissolved hydrocarbons is defined to the east by well MW-3(A,B) and tot e--_ 6 south by MW-5 and MW-6;is defined somewhat west of well MW-1 A by samples from CPT-4.The shallow lateral extent of dissolved hydrocarbons is defined in the shallow zone by samples from \ r,.,,'CPT-3. However, the deep lateral extent is not defined to the north. Additional off-site wells were recommended, however, these wells have not been installed. S!i IPNKrhe vertical extent of dissolved hydrocarbons is defined on-site by deep wells MW-4 and MW-7. HYDROGEOLOGIC MODEL Based on the CPT results, AGE developed a preliminary subsurface model of the site sediment stratigraphy comprised of three informal.zones, a vadose (unsaturated) zone and three hydrologic units. Hydrologic units may be defined as vertical intervals of soil having significant lateral extent and similar hydrologic characteristics (i.e. hydraulic.conductivity, transmissivity, etc.) over their vertical and horizontal extents. A `flow unit' may be defined as a hydrologic unit, and is characterized by significantly greater hydraulic conductivity values than overlying and underlying units. A flow unit is capable of serving as a preferred migration route, and could potentially allow migration of large contaminant masses over great distances. r-- The model presented is interpretational and will be modified as more data is collected. The four vertical zones identified are as follows(depths are approximate and can vary by several feet across the site): �---- Vadose Zone-Extending from surface grade to 25 feet bsg,the depth to the water table,the vadose zone is composed predominantly of fine-grained material,with a coarse-grained sand unit identified between 25 feet and 30 feet bsg at CPT-3.,off-site.The zone is characterized by low pore pressures. Hydrologic Unit 1_(HU1) - At the top of the current saturated zone, extends from 25 feet to approximately 45 feet bsg and is comprised predominantly of clay;interbedded on the southwest and on the northeast with clayey silt and silt. This interval is characterized by generally low pore pressures. Advanced GeoEnvironmental,Inc. I