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Peter E. Barkmann critical to characterizing basic groundwater flow of the entire aquifer system, but it is also critical to understanding well performance in multi - layered aquifers. There is uncertainty about how the presence of layering within each aquifer will affect pumping capacities in individual wells as regional water levels drop below the top of the designated aquifers. It is believed that pumping capacity will be reduced more sig- nificantly than theory would predict (Black and Veatch, et al., 2004). This has profound economic ramifications on the overall performance of water supply wells, since rapidly diminishing well yields will necessitate installing more wells and expanding the supporting infrastructure in order to pro- duce an equivalent volume of water; hence, a much higher unit cost for water. Recent simulations performed as part of the South Metro Water Supply Study (Black and Veatch, et al., 2004) included a sensitivity analysis wherein different vertical hydraulic con- ductivity values of the shale layers in a multi - layered aquifer were used in mathematical simulations used to predict how well performance may decline over time as regional water levels decline (Palumbo, 2004). Changes of one or two orders of magnitude in vertical hydraulic conductivity resulted in very significant differences in predicted well per- formance as shown in Figure 5. This relationship under- scores the need for understanding and quantifying vertical hydraulic conductivity throughout the aquifer system. VERTICAL HYDRAULIC CONDUCTIVITY MEASUREMENTS The hydrologic properties of each of the aquifers have been fairly well documented (Robson, 1983, 1987). However, the characterization of aquifer parameters focuses on the ability of an aquifer to yield water to wells, or to transport water through the aquifer. Direct measure- ments on the scale of an aquifer typically involve pumping tests wherein analytical methods allow interpretation of drawdown data to arrive at estimates of transmissivity, hydraulic conductivity, and storage capacity (Moore, 2002). It is also possible to estimate leakage values for overlying confining units (Lohman, 1972). However, the parameters arrived at this way typically are average values for an entire aquifer, or at least that part of the aquifer that is open to the well in which the test is completed. By nature of the way the wells are completed in the multi - layered aquifers of the Denver Basin, with many different water - bearing sandstone layers open to the well, it is not possi- ble to characterize individual layers. Characterizing individual layers requires more robust testing methods such as obtaining core samples, perform- ing straddle - packer tests over isolated layers (Robson and Banta, 1993), or other technologies including spinner flow surveys performed in conjunction with pumping tests and integrated borehole logging methods (Paillet and Pedler, 1996). While these methods may provide esti- mates of hydraulic conductivity for individual water -bear- ing layers, they still will not provide direct information about the surrounding shale intervals. Furthermore, the results would provide estimates of horizontal hydraulic conductivity, not vertical. Direct in -situ measurements would provide the best rep- resentative data. However, performing in -situ vertical per- meability measurements on individual shale layers in a basin where depths can reach 3000 ft and hydrostatic heads above --------------- 500 - ----- _----- -_ _ _ _- .._..._ _ -. High Kv (cm/sec) 5.6 x 101 j \, 5.6 x 10 5 ... 400 `;. 5.6x10 m-.--5.6 5.6 x 10-7 x 10-8 2.8 x 10-7 300 I Low Kv (cm/sec) _..,..,._ . 200 I 5.6 x 10-9 1010 100 0' 0 20 40 60 80 100 Time In Years HRS Water Consultants, Inc. The Rocky Mountain Association of Geologists 176 Figure 5. Mathematical model simulations used to predict well performance in an individual well as water levels in a multi - layered aquifer decline using different values for vertical hydraulic con- ductivity (Kv) show that very low Kv values result in rapidly declining well yields because water can- not move downward through the entire aquifer. From Palumbo, 2004.