Hydrogeologic Parameters of the Kiowa Research Core, Kiowa, Colorado
LAPEY, Laura, Department of Earth Resources, Colorado State University, Ft. Collins, CO, 80523-1482

The population of the Colorado Front Range is increasing rapidly, resulting in a greater demand for water. As a growing area in a semiarid region of the United States, regulators and water users are relying more on the Upper Cretaceous and Tertiary bedrock aquifers in the Denver Basin to meet these needs. Limited well and core data and parameter uncertainties make any long-term water related decisions challenging.

Two continuously cored research boreholes were drilled in the Denver Basin: a 940-meter deep borehole in Castle Pines, CO in 1987 and a 687-meter borehole in Kiowa, CO in 1999. Core samples were analyzed for hydraulic conductivity, specific yield, porosity, and grain size characteristics. The data obtained from these detailed studies provides new insight into the hydrogeologic parameters of the four bedrock aquifers. The original hypothesis is that parameters obtained in Kiowa will be smaller than those obtained in Castle Pines based its distance from the sediment source, the Rocky Mountains.

Measured Kiowa core parameters were analyzed using linear regression to determine whether significant relationships exist, which could be helpful in parameter estimation. Kiowa and Castle Pines data were compared in an effort to explain parameter variation from west to east across the basin. Lastly, data were compared to State of Colorado Division of Water Resources estimated aquifer parameters to determine if and how existing computer models should be modified.

The aquifer parameters in Kiowa were smaller than those in Castle Pines, which is located approximately 50 km closer to the Front Range. The comparison between Kiowa and Castle Pines suggested that for the Arapahoe and Denver Aquifers, the distance from the Front Range affected hydrogeologic parameters. The Dawson Aquifer parameters were similar in both Castle Pines and Kiowa.

Large discrepancies between state-estimated parameters and measured values were observed. In most cases, measured values were much lower than estimated values. Specific yield values measured in the Kiowa core are approximately 30 percent less than the state-estimated values. The average hydraulic conductivity is smaller in Kiowa by an order of magnitude or more in all four aquifers. The State chose one specific yield value to represent each aquifer in a MODFLOW model of the Denver Basin. This study suggests that specific yield varies across the basin and is dependant on many other aquifer characteristics.

Specific yield is used to calculate the volume of water contained in an aquifer. If the specific yield value is inflated, the available volume of water will be overestimated. The volume of water contained within the aquifers is important in adjudication of water resources within the Denver Basin. Before groundwater exploitation effects the water users of the basin, further study is needed to better estimate aquifer parameters and provide tools for lawmakers to make informed decisions.