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Corrective Action Plan Page 18 Zone of Influence of W&1] Army. Estimation of the radius of the zone of influence of a pumped well is subject to large uncertainties This is because, under the ideal conditions that must be assumed when applying the available mathematical models, the cone of depression formed in the water table is predicted to expand indefinitely as pumping continues In actuality, as pumping time increases, continuing radial expansion of the cone of depression slows to close to an infinitesimal rate and is limited by the flux of regional, groundwater flow which will eventually balance the rate as which water is being removed from the subsurface Other effects that influence the size of a well's zone of influence are recharge (from precipitation or water courses) and the presence of low permeability strata in the far field of the well Despite the uncertainties, practical and conservative calculations can be made to check that the proposed groundwater extraction system will have an adequate zone of influence when the system is operating under normative conditions This has been done as follows Using the same pumping test data and rules of proportionality as before, it is estimated that pumping for 14 days from a single well at the proposed normative rate of 10 gpm will draw down the water level in the casing by 7 31 ft The computed value of the logarithmic slope of the time-drawdown plot, As, under those conditions is 0 61 Assuming a well efficiency of 80%, the drawdown of the water table in the formation adjacent to the 12 in diameter well bore (i a at a radius of 0 5 ft from the center of the well) at that time is predicted to be 5 85 R For a given well and pumping rate, the logarithmic slope of the distance-drawdown plot (where distance is measured radialy from the center of the well) is twice the logarithmic slope of the time-drawdown plot (Ref 3) For the case considered here, where the well has been steadily pumped at 10 gpm for 14 days and the drawdown at a radius of 0 5 ft from the center of the well is 5 85 ft , As = 2 x 0 61 = 1 22 and the drawdown, s, at any other radius, r, from the center of the well is given by the relationship s = 5.85 - 1.22 (Log r- Log 0.5) Eq 1 where s and r are in feet Differentiation yields ' ds/dr=-3.32/r Eq 2 where ds/dr is the gradient of the water table within the cone of depression produced by pumping the well P g The gradient of the undisturbed groundwater table beneath the site is typically + 0 003 ft /ft to the north (Ref 9 ) If, for the present purposes, the zone of influence of a recovery well is defined as the area within which the gradient of groundwater flow toward the