The Mountain Geologist

• Year: 2004
• Title: The Mountain Geologist
• Author: Rocky Mountain Association of Geologists
• Description: October 2004, Volume 41, Number 4
• Publications - Doc Type: Other

Groundwater Residence Time and Recharge Temperature Estimates of the
Denver and Arapahoe Aquifers, Denver Basin, Coloradol
MILOS M. NOVOTNY2
WILLIAM E. SANFORD3
1. Manuscript received August 2; Accepted August 12, 2004.
2. S.S. Papadopulos & Associates, 1877 Broadway S703, Boulder, CO 80302
3. Department of Geosciences, Colorado State University, Fort Collins, CO 80523
ABSTRACT
Groundwater samples were collected from six groundwater production wells in the Denver and Ara-
pahoe aquifers of the Denver Basin along a transect from near the suspected recharge area in the south
to urban centers in the central basin. Residence times were estimated with 14C using an empirical factor
to correct for dilution. Recharge temperature was estimated with a model that relates concentrations of
dissolved atmospheric noble gases (Ne, Ar, and Kr) in the groundwater samples to temperature, eleva-
tion and an excess -air component. Residence time estimates range from 8,000 years near the outcrops of
the aquifers to greater than 30,000 years near the center of the basin. The recharge temperature estimate
of the youngest water is similar to modern mean - annual temperature while waters from the central basin
have significantly cooler recharge temperatures. The relationship of recharge temperature and residence
time to general climate records suggests possible mixing of younger Holocene waters in two of the cen-
tral basin samples.
INTRODUCTION .........................161
Background . ............................162
Groundwater Flow and Environmental Tracers ...162
Carbon -14 Residence Time .................162
Recharge Temperature .....................163
METHODS ... ............................163
Sampling ... ............................163
Carbon -14 Activity Dilution and Determination
of Residence Time .......................163
INTRODUCTION
The population of Colorado's Front Range has grown
tremendously over the last 30 years and growth is predicted
to continue at a similar rate over the next 20 years; however,
the volume of usable water will remain relatively fixed. Sur-
face water is already fully allocated, requiring new demand
to be partially satisfied by exploiting the bedrock aquifers of
the Denver Basin. Overexploitation of deep groundwater in
similar basins has led to many deleterious effects including
lowering of the potentiometric surface, land subsidence and
degradation of water quality (Custodio, 2002).
Model of Recharge Temperature ...........164
RESULTS AND DISCUSSION ...............164
Residence Times .......................164
Estimated Paleotemperatures ..............165
SUMMARY ............................166
ACKNOWLEDGMENTS ...................166
REFERENCES ..........................166
Numerical groundwater models of the Denver Basin
aquifers have been and are currently being created and mod-
ified. These models can be used to predict the effects of
increased withdrawals and to estimate the total amount of
water available. Many basin -scale models are calibrated to
present -day potentiometric surfaces, and recharge to the
aquifers is merely estimated. Data collected providing insight
to the residence time of water at various locations can greatly
decrease uncertainties in the groundwater models.
In this paper, apparent residence times of water sam-
pled from several locations within the Arapahoe and Den-
ver aquifers of the Denver Basin are presented along with
The Mountain Geologist, Vol. 41, No. 4 (October 2004), p 161 -167 161 The Rocky Mountain Association of Geologists