Stratigraphic interpretations of the Denver Basin lead to the conclusion that there is limited vertical hydraulic connection between the Denver Basin aquifers. However, little data are available to quantify vertical flow between the aquifers. Two independent sets of data collected in Douglas County support the conclusion of limited vertical hydraulic connection between the aquifers. These data demonstrate that more information is needed to evaluate the degree of vertical connection throughout the basin.

Approximately 100 feet of whole core were recovered from shale-dominant confining beds in the Denver and Arapahoe Formations. From the core, ten plugs were analyzed for vertical hydraulic conductivity using both water and air. The analyses using water resulted in saturated vertical hydraulic conductivity values ranging from 1.31 x 10-9 to 1.72 x 10-11 centimeters per second (cm/sec), and averaging about 2 x 10-10 cm/sec. In contrast, the analyses using air resulted in vertical hydraulic conductivity values almost five orders of magnitude higher. Previous hydraulic conductivity values of low permeability material reported in the literature for the Denver Basin were obtained using air. The values obtained using water may better represent the confining layers, implying that there is less flow between aquifers than previously thought.

Two aquifer tests were conducted at production wells completed in the Lower Dawson and Arapahoe aquifers at rates of 350 gallons per minute (gpm) and 1,500 gpm. During the tests, water levels were monitored at adjacent wells completed in overlying aquifers using sensitive transducers. In each case, water levels in the observation wells rose between 0.5 to 1.0 feet. The observed water-level rises provide a direct indication of the lack of vertical hydraulic connection between the aquifers.

Although these data represent local conditions from a small area within the Denver Basin, they suggest that there is less vertical hydraulic connection within the Denver Basin than previously believed. Accordingly, estimates of long term production rates from the aquifers may need to be re-evaluated.