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<br />A- 3.0 AQUIFER-TEST THEORY <br />Transmissivity is a definition of the ability of an aquifer to transmit water. <br />Common units of transmissivity are gallons per day per foot (gal/day/ft). A <br />transmissivity expressed in these units is the amount of water, in gallons per day, <br />that can flow through a vertical strip of aquifer one-foot wide extending the full <br />saturated height of the aquifer normal to the flow direction under a unit hydraulic <br />gradient. <br />Transmissivity must be adjusted by [he actual aquifer width and hydraulic <br />gradient to determine actual aquifer flow rates. <br />THEIS EQUATION <br />Theis, .in 1935, introduced his equation which describes a non-leaky, confined <br />aquifer. The following is a general definition of the Theis equation: <br />T = 114.6 Q W(u)/e <br />u = 2693 r°2 S/T [ <br />where: s = drawdown, in feet <br />Q =discharge, in gallons per minute (gpm) <br />W(u) = well function <br />= the integral from u to infinity of (e°-u)/u du <br />T = transmissivity, in gal/day/ft <br />u = well function variable <br />r = observation well radius from pumping well, <br />in fee[ <br />S = storage coefficient, <br />and t = time since pumping started, in min. <br />NOTE: "O" denotes exponentiation. <br />Pump test data are analyzed by matching the log-log plot of drawdown versus time to <br />Theis' type curve (W(u) vs. 1/u) and applying the above equations to the match. <br />Pages 92-98 of Ferris and others (1962) present a more thorough discussion of the <br />Theis equation. <br />The value of the integral expression for W(u) is given by the following <br />series: <br />W(u) _ -0.577216- In u + u - u°2/2.2! +u°3/3.3! ... <br />where all terms are as previously defined. <br /> <br />A-27 <br />REVISED FEB 13 '87 <br />