The Arapahoe aquifer encompasses approximately 4,700-square miles and ranges in thickness from zero to 400 feet. Its maximum depth is approximately 1,700 feet. The aquifer consists of an interbedded sequence of conglomerate, sandstone, siltstone, and shale. Municipal wells drilled in this aquifer yield up to 700 gallons per minute. The Arapahoe contains more than 1,000 high-capacity wells with an estimated total annual withdrawal of more than 168,000-acre-feet. Since well yields are much higher, it is widely used as a source for municipal water. Many Douglas County water providers rely on this aquifer for their water supply.
The Dawson covers a surface area of approximately 1,400-square miles in the Denver Basin and is the closest aquifer to the ground’s surface in Douglas County. Its thickness ranges between 400 and 1,200 feet and reaches a maximum depth of approximately 600 feet. The Dawson is the least extensive aquifer of the Denver Basin system but provides some higher volume pumping rates.
The Dawson is an entirely unconfined aquifer and is composed of conglomerates (individual stones that have become cemented together) and coarse-grained sandstones with minor amounts of interbedded clay and clay shale. Due to its relative shallowness, the aquifer is commonly tapped by domestic wells.
The Denver aquifer covers an area of approximately 3,500-square miles and ranges in thickness from 800 to 1,000 feet, a considerably larger footprint than the Dawson. The maximum depth of the aquifer is approximately 1,300 feet. The Denver aquifer consists of both unconfined and confined layers depending on your location within its boundary (see diagram). The aquifer is composed of interbedded shale, claystone, siltstone and sandstone pockets. This can result in a lower well production rate than found in the Dawson aquifer (20-100 gallons per minute).
The Laramie-Fox Hills encompasses approximately 6,700-square miles and ranges in thickness from zero to 350 feet. Its maximum depth is 2,400 feet. The upper layers of the Laramie Formation create a nearly impermeable confining layer, impeding water movement between the Arapahoe and the underlying Laramie-Fox Hills aquifer. The aquifer is composed of two relatively thick sandstone layers of the Fox Hills Sandstone and sandstones from the lower part of the Laramie Formation. Minor coal beds commonly occur in the lower portion of the Laramie Formation. In the deeper portions of the basin, high water temperatures and sulfur content in the coal beds make this waterless desirable for municipal supply. The Laramie-Fox Hills is the deepest of the four aquifers, with typical well yields of 350 GPM.
Nearly a third of Douglas County is referred to as Pike/Rampart area. This area is located in the western and southwestern portions of the County and includes the Pike National Forest. The aquifers of this region are made up of fractures in the granite rock. These fractures or aquifers are recharged only through precipitation and are therefore considered a renewable resource. The availability of water in this portion of the County is limited due to the low permeability of these fractures.
Aquifer Storage and Recovery (ASR) entails the injection of surface water into an aquifer and storing it there for later use. The amount of water injected into an aquifer is highly dependent on surface water yields from rivers and streams and the constraints of the particular hydrologic characteristics of the aquifer being injected into. Please visit the South Metro Water Supply Authority (SMWSA) for more information.
