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• Frequency of Groundwater Monitoring Quarterly <br /> Sampling Date March 6,2006 <br /> Is Free Product(FP) Present on Site No <br /> FP Recovered This Quarter NA <br /> Cumulative FP Recovered to Date NA <br /> Approximate Depth to Groundwater <br /> Upper 41 49 to 42 32 feet below ground surface (bgs) <br /> Groundwater Flow Direction Upper Predominately north and northeast, with some <br /> radial flow inward beneath the ARCO site <br /> Groundwater Gradient Upper 0 0070 ft/ft(onsite) and 0 0013 ft/ft(offsite) <br /> Approximate Depth to Groundwater <br /> Lower 42 15 to 42 33 feet below ground surface(bgs) <br /> Groundwater Flow Direction Lower Generally east-southeast, apparent groundwater <br /> high beneath the Knowles-Roek site <br /> Groundwater Gradient Lower 0 0093 ft/ft (onsite)and 0 0012 ft/ft(offsite) <br /> INTERIM REMEDIATION SYSTEM OPERATION AND PERFORMANCE <br /> Equipment Inventory Ozone Injection System (Manufactured by <br /> Operating Mode Operational _ <br /> • Reporting Period From January 10,2006 to March 21, 2006 <br /> Historical Highest GRO Concentration 55,000 pg/L(MW-6, 2002) <br /> Historical Highest Benzene Concentration 18,000 Ag1L(MW-8, 200 1) <br /> Historical Highest MTBE Concentration 85,000 Ag/L (MW-6, 2002) <br /> Highest GRO Concentration this Period 640 pg/L(MW-4) <br /> Highest Benzene Concentration this Period 5 5 Ag/L(MW-4) <br /> Highest MTBE Concentration this Period 5,700 Ag/L(MW-6) <br /> Number of System Shutdowns This Period 0 <br /> DISCUSSION <br /> Upper Zone Wells <br /> At the time of the first quarter 2006 monitoring event, groundwater elevations in the ARCO wells had <br /> increased between 146 and 1 66 feet since the previous monitoring event(December 8, 2005) Depth to <br /> water measurements were corrected to mean sea level (MSL) and used to construct a groundwater <br /> elevation contour map (Figure 3) At the former Top Filling site measurements were collected on <br /> February 28, 2006, and these wells were not used in contouring A north-northeast groundwater flow <br /> direction was generally observed east of Wilson Way, beneath the ARCO and Knowles-Roek properties <br /> Radial groundwater flow (near ARCO well MWA1) and northwest groundwater flow (between ARCO well <br /> MW-7 and the Knowles-Roek well MW-15A) were also observed Low average groundwater gradients <br /> (0 007 fVft onsite and 0 0013 ft/ft offsite) were observed, consistent with previous monitoring events <br /> . Northeast, east, and southeast groundwater flow directions have predominated during historical <br /> monitoring <br /> K 1Bp-Arco for URS154691Quarterly120061Arco_5469_1 Q06 docPage 3 of 6 5TRATU5 <br />