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06124/2662 15 42 2094671118 AGE STOCKTON PAGE 15 CAPTURE ZONE CALCULATIONS Based on the hydrological properties determined thi ough the gr ound water extraction test and ground water gradient commonly infer ed for the site,AGF has calculated the capture zone for gi ound water exti action well when pumped at a i ate of 5 gallons pei minute Modeling assumes an X axis parallel to the ground Water flow direction and a Y axis perpendicular to the X axis through the puncipmg. well Positive x values extend upgradient and negative x extend down gradient Assume ng that 1) steady state conditions wer a achieved during the pump test,2)the extraction well fully penetrated a confined aquifer,3)uniform horizontal flow of groundwater prior to pumping and 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 folio wing equation gyve the maximum half width(YL) of the capture zone at an infinite distance in the upgradient direction from the extraction well YL = ± Q — (2Kbi) where Q = the pumping rate in cubic feet pei day K= hydi aulnc conductivity in feet pet day b= aquifer thickness i =hydraulic gi adient The assumptions rioted above are not fully met by the field conditions, the more serious being that the test was conducted in a partially penetrated unconfined aquifei,the method should still give good results if drawdown in the well is 20°10 of less of the aquifer thickness (b) The drawdown was approximately 25% of the initially assumed aquifer thickness (32 feet, sci een interval and overlying water) Correction of b by addition of V2 the capture zone width would put the drawdown to approximately 18% of b Neither this aquifer nor any other is likely to be isoti opic The results of the pump test were as follows Q — 5 gallons per minute of 962 5 cubic feet per day K= 94 feet pei day b= 32 feet (without addition of/� the capture zone width, sort of a chicken before the egg situation) Review ofprevious gi oundwatei monitoring events showed that the gi adient 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 gi adient of 0 01 is assumed Putting these values in the above equation, one finds that Y, = + 16 feet Thus, at an infinite distance directly upgi adient from the exti action well, the capture zone would