What is an Aquifer?

An aquifer is an underground geologic unit that is saturated with water that has sufficient permeability to yield that water to wells.  A misconception exists that an aquifer is simply a large underground lake sandwiched between rock at both its top and bottom.  This only exists in limestone caverns.  The diagram below represents two different types of geologic materials, with different aquifer properties.  As you can imagine, the well-sorted material allows water to move quickly through it (larger pore space), while the poorly sorted material requires a bit more energy and time for water to travel through it (see diagram below).


An aquifer can then be anywhere between a nicely developed assemblage of the same size material, or one where grains of varying shapes, sizes and material coexist. Therefore, the physical properties of aquifers will differ greatly depending on the areas unique geology, the manner in which water moves through the geologic units, their unique water storing capacities, and how they react to the withdrawal from well pumping.

An important fact to remember is that multiple aquifers can exist at different elevations underneath ones feet, each with its unique water producing characteristics.  In the non-mountainous areas of Douglas County, four aquifers exist that have confined and unconfined properties depending on your location within that aquifer. More specific aquifer characteristics are introduced below in the Denver Basin aquifer section.

What is a Confined Aquifer?

Confined aquifers are those that are overlain by an impermeable material, such as clay or shale, resulting in the water being under pressure.  The impermeable nature of this overlying material restricts the amount of water that can move freely through it.  For the most part, the water located within these confining layers does not intermingle with the waters of other aquifers or surface water systems, and thus do not have meaningful natural recharge rates.  In the diagram below, the confined aquifer can only gain meaningful recharge from a small area at the outcrop location (or where it becomes unconfined), such as in the western portion of our county.  The amount of recharge is limited to 10% or less of precipitation, and often cannot move quickly to the lower portions of the confined aquifer.  When ground water withdrawal rates exceed the natural recharge rates (more gallons pulled out than are naturally recharged), the aquifer is being mined or depleted.  A majority of the aquifers of the Denver Basin are confined and practically non-renewable.  The lower portion of the diagram below demonstrates the qualities of a confined aquifer.

Denver Basin Aquifers Source:  Colorado Geological Society
Denver Basin Aquifers
Source: Colorado Geological Society


What is an Unconfined Aquifer?

Unconfined aquifers have a water table at atmospheric pressure and are often recharged from surface water percolation.  The level and speed of recharge can differ drastically depending on the geology, surface topography, surface vegetation, precipitation and climate.  In general, if natural recharge to the unconfined aquifer equals the pumping withdrawal rate, the system is considered a sustainable source of water. The diagram above illustrates the lack of any overlying impervious layer, or confining layer located between the land surface and the water table, allowing this type of aquifer to interact with surface water.


Water production properties can differ greatly from aquifer to aquifer, not to mention the variabilities that exist within that same aquifer system.  The science involved in analyzing the complexities of such ground water characteristics and conditions is referred to as ground water hydrology.  For more detailed presentation of Colorado’s ground water resources, please refer to the Ground Water Atlas of Colorado on the Colorado Geological Survey.