ASSESSING THE EFFECTS OF THE PROPOSED RUETER-HESS RESERVOIR TO THE DENVER BASIN BEDROCK AQUIFERS UNDER INSTITUTIONAL AND HYDROGEOLOGIC UNCERTAINTY

As part of an EIS evaluation, the Denver Basin bedrock aquifer system was modeled to determine what effects the construction of Rueter-Hess reservoir near Parker, Colorado would have on (1) stream systems that cross the Denver Basin and (2) other bedrock aquifer users near the proposed project.

Various uncertainties constrained how the Denver Basin model was constructed, calibrated, and analyzed. Hydrogeologic uncertainty is a result of an incomplete or incorrect understanding of the hydrogeological system that leads to sweeping assumptions and generalizations with respect to model conceptualization. It affects model components such as parameterization and the number of model layers. The water-yielding section consists of five formations of Late Cretaceous to Eocene age with a maximum total thickness of over 3300 ft. The five formations are legislatively divided into four aquifers separated by confining units. The model representation is quasi-3D, with six explicitly defined aquifer layers interbedded with implicitly defined aquitards. The actual stratigraphy of the water-yielding horizons is far more complex in detail.

Institutional uncertainty arises when engineers are unable or unwilling to collect accurate and complete hydrogeologic records such as wells, stream flows, climate, land use, heads, and others that affect the model’s initial and boundary conditions. For example, the number of wells, their locations, screened intervals, pumping rates, and completion dates are largely unknown.

It is unrealistic to expect a model with these uncertainties to be capable of robust predictions. Therefore, a “differential-impact” approach was justified for this project. Two models are built, one with the reservoir and one without. The only difference in the two models is the pumping scenarios imposed by the projected water demands by the presence or absence of the reservoir. The impacts to the Denver Basin are evaluated by comparing the difference in flows to and from streams and the difference in heads in the model layers between models with and without the reservoir. Although institutional and hydrogeologic uncertainty is present, it is present in both models and balances out when the differences are calculated. Results indicate that constructing the reservoir will slow head loss and infiltration from streams.