CONVERSION FROM CONFINED TO UNCONFINED CONDITIONS IN THE CRITICAL ARAPAHOE AQUIFER OF THE DENVER BASIN, COLORADO

The Laramide Denver Basin is filled with sediments derived from the erosion of the uplifting Rocky Mountains. About 68 MYA, abrupt tectonic uplift in the area from Colorado Springs to Boulder created a high relief fault-bounded mountain front, characterized by incised valleys cutting deeply into the crystalline core of the nascent mountains. One of the largest fluvial distributary fans constructed from the outwash of granitic debris is the Wildcat Mountain fan, sourced from the Jarre Canyon area west of Sedalia and spreading 20 miles east towards Castle Rock and Parker.

This sand-rich lobe of arkosic sandstone forms the Arapahoe Aquifer, the primary and often sole source for municipal water for communities between Denver and Colorado Springs.

Water level data reported by the office of the Colorado State Engineer allowed potentiometric surface maps to be made for the Arapahoe Aquifer in 2004. Observed rates of annual fall, often exceeding 30 feet per year, together with structure maps on the top of the aquifer, allowed estimates to be made of the timing of the transition from confined to unconfined conditions. It was predicted that the transition would take place between 2004 and 2014 in the Castle Rock area. Data reported in 2012 indicates the prediction to have been optimistic with many wells having transitioned in 2005-2006. A diminished rate of fall (from 10’s of feet per year to single digit feet per year) and diminished well yields characterize the transition. While the water resource remains large, many communities are proactively seeking alternative water supplies so as to reduce or eliminate ground water mining.