T = 1500 * Q/s (for an unconfined aquifer) The following equations can be used to estimate transmissivity per (Driscoll, 1986): The higher the transmissivity, the greater the capability of the aquifer to move water and the lower the drawdown in the well. It equals the aquifer’s hydraulic conductivity (K) times the aquifer thickness (b). Transmissivity is the rate water is transmitted through an aquifer under a unit width and a unit hydraulic gradient. The initial Specific Capacity value can also be used to estimate the transmissivity (T) of the aquifer. While this is considered an estimate and more accurate calculations can be developed with a field test, it is a useful approximation for understanding the pumping limits of the well. If the well has an available drawdown of 50 feet, the maximum pumping rate would be 5 times 50 feet, or 250 gpm. For example, with a well that has 40 feet of drawdown and is pumping 200 gpm has a Specific Capacity of 5. The maximum pumping rate is calculated as the Specific Capacity times the maximum available drawdown. The maximum pumping rate of a well can be estimated using the initial Specific Capacity. Rehabilitation work should be scheduled when a well’s Specific Capacity drops by 15% or more. Specific Capacity testing should be performed at least semi-annually and water levels (static and pumping) should be collected monthly to provide early detection of potential well problems. As time goes by, the Specific Capacity will decline as plugging of the well’s perforations or filter pack occurs or as static water levels change. The Specific Capacity obtained just after a well is drilled and properly developed is typically the highest value that will be produced and is the baseline for comparison for all future values. Ideally, to provide the best comparison of Specific Capacity measurements over time and a comparison to the original test results, the same continuous pumping time frame should be used for each subsequent test. Typically, a well should run continuously for at least 24 hours at a constant yield before recording the drawdown to allow drawdown to stabilize (Driscoll, 1986). If the well was pumping at 200 gpm, the Specific Capacity would be 200 gpm divided by 40 feet of drawdown to give a Specific Capacity of 5. In the figure at the left, the well has a drawdown of 40 feet. Note: SC = Specific Capacity (gpm/ft) Q = discharge (gpm) s = drawdown (ft) The following equation is used to calculate Specific Capacity: Specific Capacity can also be used to provide the design pumping rate or maximum yield for the well and to estimate the transmissivity of the surrounding formations penetrated by the well screens. The Specific Capacity of a well is the pumping rate (gpm) (Q) divided by the drawdown in feet (s).
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