USING DISSOLVED NOBLE GASES TO DATE THE GROUNDWATERS IN THE BEDROCK AQUIFERS OF THE DENVER BASIN

The Colorado Front Range area is one of the fastest growing regions in the United States. Denver and surrounding communities have relied upon surface water for their main resource. Since surface waters are fully appropriated, newer developments have been using deep groundwater to meet their needs. The source for the groundwater is from the four bedrock aquifers of the Denver Basin: Laramie/Fox Hills; Arapahoe; Denver; and Dawson Formations. Some of the communities are drilling water supply wells to depths on the order of 1000 meters in the Denver Basin. These wells are being drilled despite the fact that little research has been done on determining the stratigraphic architecture of the aquifers, recharge rates and residence times of water in the aquifers, information necessary for the development of accurate groundwater management models. Only crude estimates of hydraulic parameters, such as hydraulic conductivity, recharge rates, and porosity, exist. These parameters are closely tied to the stratigraphy of the aquifers.

Estimates made of residence times in the aquifers based on mass balance and hydraulic calculations using these parameters range from 4000 - >100,000 years. To improve upon these crude estimates, a study is under way to date the groundwater in the Denver Basin aquifers using dissolved noble gases ( 4He). Preliminary results suggest that the ages fall within the ranges given above but additional samples are being collected at several other locations throughout the Denver Basin to confirm these results. The age data will be incorporated with new stratigraphic information in order to begin to improve upon conceptual and numerical models used for future water resource planning and management.