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advantage of the Jacob modification to the Theis equation is that it allows for site specific <br />information to be used. <br />The Jacob modification to the Theis equation is given as: <br />T=Qx 2.3 xl0 2.25xTxtime) <br />s 4xTC g~ ~'~ xS <br />Where: <br />Q =Well discharge rate (ft3/day) <br />s = drawdown in well (ft) <br />T =transmissivity (ft2/day) <br />time =duration of pumping (day) <br />S =storage coefficient (unitless) <br />r =hydraulic radius <br />This method requires estimates or assumptions for the storage coefficient, well efficiency <br />and effective hydraulic radius of the well. In the analysis, a storage coefficient value of <br />0.0001 was used for all bedrock tests. This value is in the range expected for confined <br />bedrock aquifers in the Denver Basin based on estimates by Robson (1983) and <br />determined by CDM (2005). All wells were assumed to be 100 percent efficient and to <br />have an effective hydraulic radius equal to the radius of the casing reported for the well. <br />With these assumptions, the equation was solved iteratively for transmissivity. Due to <br />differences in specific capacity test duration, aquifer thickness, and well efficiency, use <br />of the theoretical approach provides more flexibility for the bedrock aquifers than <br />applying a single empirical equation. <br />Validation of this method used 30 aquifer tests where both a specific capacity and an <br />aquifer test-derived transmissivity were available in the database. The transmissivity <br />values estimated from specific capacity using the Theis equation provided above were <br />compared to the aquifer test derived values. Figure 4 shows this comparison, along with <br />the regression line relating the transmissivity estimates from the two methods. The R- <br />squared value relating the two sets of data is 0.76, a reasonably good correlation. As can <br />be seen, the specific capacity based estimates are biased high by about 10 percent <br />compared to the aquifer test-based estimates. This is likely due to the assumption that <br />wells are 100 percent efficient in the specific capacity analysis. <br />1.2.4 Conversion of Transmissivity Data to Hydraulic Conductivity Values <br />Many sources of data present only aquifer transmissivity (T) results. Aquifer T is the <br />parameter obtained from analytical equations used for aquifer pumping test analysis <br />and also obtained from specific capacity data. Transmissivity is the product of two <br />physical characteristics of an aquifer, hydraulic conductivity (K) and aquifer saturated <br />thickness (b). Aquifer K characterizes the potential for flow through a unit volume of <br />the aquifer material under a unit hydraulic gradient. Hydraulic conductivity is also <br />used as a direct input to the groundwater flow model being developed, so it was <br />necessary to convert all of the T data into K by dividing the T value by the aquifer's <br />SPDSS Phase 2 Task 43.2 TM -Final 12 <br />2/ 13/ 2006 <br />