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C i CAPTURE ZONE CALCULATIONS '� 4 Cs. Based on the hydrological properties determined through the groundwater extraction test and ground water gradient commonly inferred for the site,AGE has calculated the capture zone for ground water extraction well when pumped at a rate of 5 gallons per minute. Modeling assumes an X axis parallel ' to the ground water flow direction and a Y axis perpendicular to the X axis through the pumping well. Positive x values extend upgradient and negative x extend down gradient. Assuming that: 1) steady state conditions were achieved during the pump test;2)the extraction well , fully penetrated a confined aquifer; 3)uniform horizontal flow of ground water prior to pumping and 1 that no water enters the aquifer at the base; 4) no dispersion - all contaminants travel on the stream lines; and 5)the aquifer is isotropic;the following equation give the maximum half width(YL) of the 0 capture zone at an infinite distance in the upgradient direction from the extraction well: k C YL = ± Q = (2Kbi) 'I rl F Cwhere: Q =the pumping rate-in cubic feet per day K= hydraulic conductivity in feet per day b= aquifer thickness i=hydraulic gradient C I The assumptions noted above are not fully met by the field conditions, the more serious being that t the test was conducted in a partially penetrated unconfined aquifer;the method should still give good results if drawdown in the well is 20% or less of the aquifer thickness (b). The drawdown was approximately 25% of the initially assumed aquifer thickness (32 feet, screen interval and overlying water). Correction of b by addition of % the capture zone width would put the drawdown to approximately 18% of b. Neither this aquifer nor any other is likely to be isotropic. The results of the pump test were as follows: Q = 5 gallons per minute or 962.5 cubic feet per day K= 94 feet per day b= 32 feet (without addition of% the capture zone width, sort of a chicken before the egg situation) Review of previous ground water monitoring events showed that the gradient on the site is commonly greater than 0.01, being as high as 0.016 on one occasion. For the purposes of this calculation, and average gradient of 0.01 is assumed. Putting these values in the above equation, one finds that: YL = = 16 feet i Thus, at an infinite distance directly upgradient from the extraction well, the capture zone would C A� I