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i Former Beacon Station,No474 September 22,2005 Stockton California WorkPlanfor an Additional Subsurface Investigation SCOPE OF WORK Four additional monitoring wells, MW-18-through :MW-21, are proposed at, the locations shown on Figure 2. The cluster of two monitoring wells, MW-18 and MW-19, is proposed to further characterize the lateral distribution of petroleum hydrocarbons at the water table, and in'the intermediate aquifer interval approximately 90 ao 100 feet below surface grade (bsg), respectively, in the central portion of the site. These wells will provide data.points for . lateral groundwater plume characterization at these aquifer intervals and for monitoring plume response to remedial actions.and natural,attenuation. fli is Horizon's opinion that_a� "`deep aquifer interval welrat this-location-is riot-necessary-because-groundwater-.analytical {data from.-the.existing deep aquifer interval monitoring wells MW 13 through MW-1 located in the source area and along the perimeter of the site..indicate that the vertical extent; fo`f hydrocarbons,and specifically D C-AK in groundwater is delineated at the site: II' Monitoring wells MW-20 and MW-21 are proposed to characterize the groundwater conditions in the intermediate aquifer interval along the southern boundary of the site. Concentrations of DCA in this. interval were detected in hydropunch groundwater samples collected during the previous investigation: Current depth to groundwater at .the site is approximately 62 feet bsg. The well screen interval for the proposed water table monitoring well MW-18.shall be set at approximately 50 to 70 feet bsg. Intermediate aquifer levels proposed for monitoring are approximately 90 to 100 feet bsg. Selection of well screen intervals for the intermediate aquifer interval shall' be made on the basis of field screening and lithologic data collected during drilling and typically shall be 5 feet in length. . It is Horizon's opinion that a d tional Hells in the vicinity.of_monitoring-well-MW-10 are of:necessary-at this--time. The—data-that support this opinion are: groundwater monitoring data for monitoring well MW 10 indicate onlyrsteady,relatively low concentrations of total 1 petroleum hydrocarbons as gasoline (TPHg)have been regularly detected in this well; 1474%vp9-05 ProJecc-No. 1474.16 3 HORIZON.ENVIRONMENTAL INC.