Vertical Scaling of Transmissivity and Storativity by Comparing Pumping Tests at the ISR Wellfield Operation Scale to Traditional Whole Aquifer Scale Tests

Two pumping tests in the Chilson Member of the lower Lakota Formation were completed at nearly the same location in 1979 and 2008. The 1979 test consisted of a pumping well and several surrounding observation wells penetrating the full 55-m thickness of the aquifer. The 2008 test consisted of a pumping well and three surrounding observation wells screened over 10 m in one of the known uranium ore horizons in the lower portion of the Lakota penetrated only about 3 meters of the more than 55-m thick lower Lakota aquifer. The average transmissivity and storativity values determined from the 1979 tests were 17.6 m2/day and 1.8 x 10-4, respectively. Comparing median transmissivity of 13.9 m2/day and storativity of 1.2 x 10-4 determined in the 2008 test to the 1979 test, the new aquifer parameters for the lower Lakota are respectively about 80 and 70 percent of the 1979 results. Because transmissivity and storativity depend on aquifer thickness, comparing the results suggests that there may be some scaling effect between the tests due to the differing lengths of screened intervals. However, the effect cannot be predicted by calculations such as relative screen lengths or open intervals to the formation, or sandstone versus shale thicknesses on geophysical logs. The scaling effect is attributed to partial disconnection between the lower and upper portions of the Lakota Formation aquifer by locally continuous shale interbeds. Suggestions to resolve this scaling issue are presented.