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10 October 2003 <br /> f AGE-NC Project No. 98-04;68 <br /> Page 2 of 4 <br /> bsg. The prevailing ground water flow direction in the vicinity of the site is estimated to be north to <br /> northeast at a static, maximum gradient of 0.02 feet/feet. Approximately 2,014,027 gallons'of <br /> impacted ground water with 647 grams/0.65 kilograms (converted to 1.4 lbs of MTBE) represents <br /> the plume to remain at the site. <br /> GROUND WATER VELOCITY(ASSUMED) <br /> Q= %2 K(ht,' ht22)/1 x <br /> assuming: . <br /> 1)hydraculic gradient equals slope of the water table <br /> 2) ground water flow direction is horizontal <br /> 3) contours of equipotential lines are vertical <br /> Q =,1/2 K (ht12 _ h'2 2)/1 X W <br /> Q=aquifer flow rate. <br /> K=hydraulic conductivity(presumed) = 100 gallons-day/feetZ (SAND) <br /> htj'­height of water table iia well one above base of aquifer <br /> ht22=height of water table in-well two above base of aquifer <br /> l =.distance along the flow path between two points,of measurement ; <br /> W=width of affected area(aquifer) <br /> 1/2 (100 gallons-day/feet ) (ht,2 - hf22.= 110.06-102.86)/100 feet x 40 feet <br /> 50 x 0.072 x 40= 144 pgd-is the estimated ground water flow velocity. <br /> As shown above a slow ground water flow is possible at the site.The"maximum release of TP13 was <br /> at the former UST and dispenser area (MW-3). MTBE was not detected in well MW-3•during the <br /> last sampling event. Thus, the maximum MTBE release was at the former UST area and migrated <br /> downgradient, from well MW-3 to.well MW-4 and MW-6, eventually to MW-9.and i\/IW-10 over <br /> time. The maximum MTBE concentration at the site is presumed to be 1,100pg/l at well MW-4 <br /> during the September 1999 sampling event. This concentration then degraded to 16 µg/l over 1260 <br /> days (Appendix A). The distance the dissolved MTBE plume has traveled is 200 feet, from the <br /> former UST locatiori to well MW-10. The principle mechanism is presumed to the ground water <br /> flow and channeiization or stratigraphy of the site. The degradation of MTBE over the investigation <br /> of the site is presumed to 98.5% at well MW-6. The total degradation from UST,MW-4, MW-6 to <br /> well MW-10 is approximately 97.5%.However,the bio-degradation of MTBE on.the site is not well <br /> documented, therefore the degradation or attenuation observed in well MW-4 and MW-6 should be <br /> classified dispersion. <br /> A highh percentage of attenuation by dispersion was observed at the site. The presumed attenuation <br /> of the dissolved MTBE.'at well MW-10, down-gradient, is assumed to.be Jess than 1260 days <br /> • <br />