Estimating Transmissibility with One Drawdown

Abstract In an effort to find a method of determining aquifer transmissibility with an abbreviated pumping test the basic equations governing the Theis non‐equilibrium method of determining aquifer constants were examined. They show the relationships of the coefficients of storage and transmissibility. Rearranged in terms of T (transmissibility) and equated to eliminate that parameter, the expression is rewritten as image The left side of the equation can be computed from existing tables. All parts of the right side of the equation can be measured during a test except S, the coefficient of storage. Assuming a value for it and measuring distance from the pumping well, drawdown, time, and pumping rate, uW(u) can be computed. The single value of u corresponding to this can be determined from tables or a curve, and the calculation of transmissibility using the available data is the simple final step. Computations were made over a wide range of actual and hypothetical examples, and it was found that for a range in assumed values of coefficient of storage of 4,000 times, the range in coefficient of transmissibility was between 1.52 and 3.22 times. The method was tested using actual well data with the conclusion that it is reasonably reliable if used in an unconfined aquifer after about three hours of pumping and making drawdown measurements in the pumping well or in an observation well less than 25 feet from the pumping well. Calculations involving a confined aquifer suggest that the method is applicable within a shorter time and over larger distances from the pumping well