STRATIGRAPHY OF THE ARAPAHOE AQUIFER IN THE DENVER BASIN: THE ROCKS RULE THE WATER

A sandstone-dominated distributary fan system, covering an area of over 1000 square miles, formed between 68 and 66.7 mybp, during the Laramide orogeny, on the western margin of the subsiding Denver Basin in the area presently occupied by Douglas County, Colorado. This system, named the Wildcat Mountain fan after proximal outcrops of the fossil distributary fan, is exposed as small hills on the west side of the Basin. Outcrops show multi-storied sandstone beds deposited by high-energy river systems flowing from a now vanished mountain-front canyon. The water-saturated sandstone in this fluvial distributary fan is in a complex wedge-shaped feature with aggregate sandstone thicknesses ranging from over 400 to less than 100 feet. In the study area, the sandstone-rich stratigraphic unit comprising the Wildcat Mountain fan is largely coincident with the State-defined Arapahoe aquifer.

The Arapahoe aquifer is the most prolific aquifer in the rapidly urbanizing region south of Denver. Municipal water wells are commonly drilled to depths of over 1500 feet to reach into sandstone layers of the fan system, and individual wells can yield in excess of 800 gallons per minute of potable water.

Data published by the Colorado State Engineer’s Office indicate that the potentiometric surface in the Arapahoe aquifer is falling at the rate of 1 inch per day. If this rate of decline continues, large areas on the western side of the Denver Basin will face severely impaired water well recoveries from the Arapahoe aquifer in the next few decades. Increasing costs of obtaining groundwater will force development of alternative water sources. Local subsurface stratigraphic studies, together with the use of modern analogues, will allow greater precision in efforts to numerically model and quantitatively predict the depletion and demise of this major water resource. These results will contribute to realistic planning of groundwater utilization.