PROJECTING LOCAL RESPONSES TO PUMPING STRESSES IN THE DENVER BASIN AQUIFERS USING A REGIONAL FLOW MODEL COUPLED WITH LOCAL WELL-FIELD APPROXIMATIONS

To assess long-term well drawdown responses to pumping stresses in the Denver South Metro area, we applied the Colorado State Engineer’s Office (SEO) SB-74 Denver Basin regional groundwater model in conjunction with local approximation models. The SEO regional model represents each of the six major aquifers (Upper Dawson, Lower Dawson, Denver, Upper Arapahoe, Lower Arapahoe, and Laramie-Fox Hills) as areally heterogeneous units based on synthesis of hydrogeologic data and information gathered from across the basin over the past several decades. However, in the cross-sectional view, the aquifers were treated as vertically homogeneous units, despite the fact that geophysical and well-drillers’ logs indicate a substantial amount of layering and vertical heterogeneity within each of the aquifers.

A key issue to be addressed is the local (well and well-field) effects of the pumping. Hydraulic heads predicted by the regional groundwater model were used to develop boundary conditions for the well and well-field analysis tool. The well-field analysis modeling approach was validated via comparison to observed well drawdowns as well as simulating field pumping tests.

The likely impacts of observed fine-grained interbeds within the aquifer units were investigated using a comparative modeling analysis that focused on the South Metro portion of the Denver Basin. The results from the comparative modeling study were used to guide the treatment of intra-aquifer heterogeneity in the regional model and local approximation models.