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, COLORADO
12-
10
8
U
0
d
Y 6-
ca
a) 4 - - - -- - -- --
F-
2-
0
0 5 10 15 20 25 30 35
Time [ka]
Figure 2. Recharge temperature of groundwater samples (solid
squares) and uncertainty (error bars), Denver and Arapahoe
aquifers, Denver Basin, Colorado, and glacial and Holocene tem-
perature estimates (dashed lines) reconstructed from mean annual
modern temperature (solid diamond) and glacial temperature
depression of 5.5 °C.
Samples from wells farther toward the center of the basin,
regardless of aquifer, have apparent residence times of
22,000- 24,000 years. The sample from The Pinery (PIN -02)
showed an estimated residence time of greater than 30,000
years, but there were some problems associated with the
analysis of this sample. In a qualitative sense, the apparent
residence times are what would be expected, with the
younger water toward the outcrops (recharge area) and
the older waters in the middle of the basin.
Estimated Paleotemperatures
The estimated temperatures at the time of recharge are
shown in Table 4. They range from 2.5 to 9.8 °C. The x2
values of all samples were below or very near 3.84, the
level of 95% probability for the model. These results sug-
gest that the mean elevations used are reasonable, that the
excess -air component of the samples is not fractionated,
and the estimated recharge temperatures are reasonable.
The uncertainty in derived recharge temperature resulting
from the level of instrument precision was calculated as
0.9 °C (Manning and Solomon, 2003). Recalculating
recharge temperature with minimum and maximum eleva-
tions was used to estimate uncertainty in recharge temper-
ature resulting from the assumption of recharge elevation
(Table 2). The sum of these was used to approximate the
total uncertainty in recharge temperature in this study.
Recharge temperatures and uncertainties were plotted
with estimates of modern and glacial temperatures (Fig. 2).
The sample from Donala (DON -04) is very close to the
modern temperature in the recharge area as would be pre-
dicted by a residence time that is younger than the end of
the last glacial period. The recharge temperatures of the
other samples are all lower than the modern temperature.
The residence times and lower recharge temperatures of
these samples suggest these were recharged during the last
glacial period. Samples PARK -01 and AUR -07 have
recharge temperatures that are significantly greater than
the estimated glacial temperature. This could reflect signifi-
cant temperature fluctuations within the glacial period,
mixing of Holocene groundwater and /or the dilution of
14C activity that makes the groundwater appear `old.' The
latter explanation suggests that residence times could be as
young as the end of the last glacial period, approximately
12 ka in Colorado. While confined conditions are observed
Table 3.
Laboratory analysis results and field measurements of groundwater conditions; 14C reported in percent modern carbon (pmQ;
relative 13C/12C reported as delta 13- carbon per mil VPDB (813C %o); total dissolved gas pressure (PTDG).
165 The Rocky Mountain Association of Geologists
Relative Gas Concentrations
DIC
Field
Measurements
28N2
3202
40Ar
84Kr
20Ne
3He
4He
14C
813C
T
PTDG
Sample
Aquifer
100
10-4
10.2
10-7
10-5
10-11
10-4
[pmC]
[ %o]
1,C]
[atm]
DON -04
Arapahoe
0.95
5.38
1.05
6.98
2.05
0.89
0.16
33.3
-13.6
15.5
0.908
PIN -02
Arapahoe
0.99
0.55
1.15
8.49
1.52
2.92
1.53
<2.27
-12.7
22.4
1.119
PARK -01
Arapahoe
0.97
192.01
1.08
8.00
1.72
3.19
1.11
4.59
-13.0
28.7
1.155
AUR -08
Arapahoe
1.00
3.55
1.14
8.44
0.97
5.13
2.33
5.60
-10.8
30.2
1.122
PARK -03
Denver
0.99
0.27
1.07
7.69
1.57
1.65
5.13
4.57
-13.2
29.7
1.050
AUR -07
Denver
0.86
3.46
0.95
6.67
0.86
0.83
2.71
5.00
-9.7
17.9
1.012
165 The Rocky Mountain Association of Geologists