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aggregate (net) sand thickness at each test location. As described in section 1.2.1, the net <br />sand thickness, as determined by the DWR, includes both sandstone and siltstone <br />lithologies. Therefore, the K values obtained are representative of the properties of the <br />beds of sandstone and siltstone. For the purposes of modeling these aquifers, additional <br />processing will be required to account for the presence throughout the aquifers of shale <br />beds that were not explicitly tested. <br />To obtain the saturated aggregate sand thickness value, the DWR geophysical log <br />database was queried to determine whether an individual well had a corresponding <br />geophysical log providing an aggregate sand thickness. If a geophysical log was <br />available for the well, the log's aggregate sand thickness for the relevant aquifer was <br />used for the K calculation. In the event a geophysical log was not available, the sand <br />thickness was obtained from the net sand thickness map developed in Phase 2 Task 42.2. <br />Many wells in the data set do not fully penetrate the aquifer in which they are <br />completed. To use the full saturated thickness in these cases would lead to under <br />predicting K due to overestimation of the sand thickness. A total of 1,198 wells had <br />specific capacity data but no geophysical log available. For these wells the sand <br />thickness intersected by the well was estimated based upon the proportion of the aquifer <br />thickness penetrated by the well. The total aquifer thickness was interpolated from the <br />bedrock aquifer configuration information presented in the Task 42.2 TM. The sand <br />thickness in the well was then estimated by multiplying the sand/aquifer ratio by the <br />length of well completion in the aquifer and assuming a uniform vertical distribution of <br />sand lenses between the top and bottom of the aquifer. A comparison of estimated sand <br />thickness in a well to actual sand thickness, as reported in the DWR geophysical log <br />database, was performed on 73 bedrock wells for which geophysical logs and detailed <br />well completion information were available. The comparison showed a correlation (R2) <br />of 0.76, which suggests that this is a reasonable method for estimating sand thickness for <br />the specific capacity data set. Sand thicknesses at wells that fully penetrate the aquifer <br />did not require a sand thickness adjustment. <br />Specific capacity-derived K values were compared with those derived from aquifer <br />pumping tests and a good correlation was observed for all aquifers except the Upper <br />Dawson. The values estimated for the Upper Dawson Aquifer were reviewed and felt to <br />be subject to a systemic bias . This bias is attributed to the unconfined nature of the <br />entire Upper Dawson Aquifer. A sampling of driller's logs indicates most wells in the <br />Upper Dawson Aquifer are completed only in the first significant saturated sand body <br />encountered and the wells' perforated interval is a good approximation of the sand <br />thickness. As a result of this additional analysis, for the Upper Dawson Aquifer only, <br />the wells' perforated interval was used to approximate the sand thickness from which <br />specific capacity-derived K was calculated. <br />A good correlation can be made between specific capacity and aquifer transmissivity. <br />However, because of the varying duration of the specific capacity tests, their reporting <br />accuracy, and the assumptions used to calculate transmissivity and K ,this type of data <br />should be regarded as an approximation and should be used as a secondary source of <br />data where aquifer pumping test data are available. It also should be noted the sand <br />SPDSS Phase 2 Task 43.2 TM -Final 13 <br />2/ 13/ 2006 <br />