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CLOSURE REQUEST 5-31-94 0 GROUND WATER FLOW DIRECTION AND VELOCITY The ground water gradient for this site historically fluctuates from flowing to the northeast to the southeast, with occasional recharge events forming a ridge that trends from MW7 to MW5 and then towards MW6, see Figure 4B (Ground water gradient 2/16/94) . A ground water velocity for the higher K values (MW6 to MW1) range from 0 34 to 0 22 feet per day with the lower K values (MW2, MW3 , MW5) ranging from 0 . 01 to 0 . 09 feet/day was calculated ; from the ground water gradient (I) of 0 . 003 and an effective porosity (Ne) that ranges between 10 and 20%, see Tables 1A . 1F Appendix I V=KI/ne The velocities calculated from the slug test indicate that the ground water is presently moving through the more conductive portion of the aquifer at an estimated rate of 124 feet a year and along the boundaries of 33 feet per year CONTAMINANT MIGRATION Under normal conditions, hydrocarbon compounds do not travel through the ground at the same rate as water The rate of migration is slowed by retention of the compound molecules by organic carbon in the soil The amount this slows the migration of the compounds is called the retardation factor (Kd) . (Fetter, 1988 , 399-406) The Kd is affected by the amount of Organic carbon (Oc) in the soil and the compound' s tendency to move from the water into the soil, the soil-water partition coefficient (Koc) The Kd can be estimated from the Oc times the Koc Kd = Oc x Koc The retarded velocity can be found by the following formula vc = v/ [1+ (Pb/n) (Kd) ) Pb = bulk density = 1 45 gm/cm3 (SB14 Q 25 feet) n = volumetric moisture content -- 0 3 Assuming 0 . 14 %s Oc in soil (brown silty clay) COMPOUND Koc Kd vc n-octane* 6800 9 5 24 ft/year benzene 97 0 14 74 ft/year toluene 242 0 34 47 ft/year ethylbenezene 622 0 87 24 ft/year xylene 552 0 7752 26 ft/year *n-octane closely approximates the aliphatic portions of gasoline range hydrocarbons pagel9