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 Thm AND RECHARGE TEMPERATURE ESTIMATES OF THE DENVER AND ARAPAHOE AQULFms, DENVER BASIN, COLORADO
Recharge Temperature
The temperature of recharging groundwater, an approxi-
mation of mean - annual air temperature, can be estimated
from concentrations of atmospheric noble gases dissolved
in groundwater using the general temperature -gas solubil-
ity relationship, Henry's Law (e.g., Stute et al., 1992;
Aeschbach -Hertig et al., 1999). The concentrations of inert
gases in groundwater result from the temperature- and
pressure (elevation)- dependent equilibration of water with
the atmosphere, plus the addition of excess air arising
from dissolution of entrained air bubbles below the water
table (Stute and Schlosser, 2000). The excess air may be
fractionated due to partial dissolution or re- equilibration of
the gas (Aeschbach -Hertig et al., 1999).
Variation in paleotemperatures estimated from concen-
trations of noble gases suggests a temperature suppression
ranging from 5 -9 °C during the last glacial period
(Aeschbach - Hertig et al., 2000). Noble gas temperatures in
the south - central and southwestern U.S. during the last
glacial period were approximately 5 °C cooler than
Holocene temperatures (Stute et al., 1992; Clark et al.,
1998), consistent with other estimates of continental tem-
perature change and local glacial - climate reconstructions
(Muhs et al., 1999).
METHODS
Sampling
Six samples were collected from locations that lie along
a transect parallel to the presumed modern groundwater
flow path beginning near the southern outcrop of the
aquifers (Fig. 1). The samples were collected from ground-
water of the Arapahoe and Denver aquifers near Parker,
Colorado during the spring of 2002 and near Aurora and
Donala (near Colorado Springs), Colorado during the
spring of 2003 (Table 1). One -liter water samples were
analyzed for dissolved inorganic carbon (DIC) isotopes by
atomic mass - spectrometry at Beta Analytical Labs. Results
of DIC analysis are reported in percent modern carbon
(pmQ for 14C and delta 13- carbon per mil VPDB (813C %o)
for relative 13C /12C. Samples for dissolved gas analyses
were collected following the protocol outlined by Manning
et al. (2003) and were analyzed by the Noble Gas Labora-
tory at the University of Utah for 28N2 3202, 20Ne, 40Ar,
84Kr, 3He and 4He. Results of the dissolved gas analysis are
reported in molar- fraction (unit -less) and precision of mea-
surements is a few percent. Additional field measurements
of temperature (T) and total dissolved gas pressure (PTDG)
of the groundwater were made. A detailed description of
the sampling procedure and analysis may be found in
Novotny (2004).
A 0 5 10 20 3(�
files
Denve
UR -07
CAAUR -08
PARK-01
�P
PIN-02
LEGEND`
O Sample Location
Aquifer Outcrop -20N-04
Urban Area
Increments f 1 Surface:
Increments of 100 ft from
high (dark) to low (light)
surface contour Colorado Springs
Figure 1. General locations of groundwater samples with 1978
potentiometric surface of Arapahoe aquifer (Robson, 1987) for
reference.
Carbon -14 Activity Dilution and Determination of
Residence Time
Additional sources of carbon exist in most aquifers and
can cause dilution of the 14C activity. Therefore, a correc-
tion for the interaction of these sources with groundwater
was applied prior to the calculation of residence time. Cor-
rection methods involving mass - balance of 13C/12C (e.g.,
Pearson and Swarzenki, 1974) were not used; ratios of
these isotopes in the potential sources of carbon in the
aquifers and recharge area are not known and the infer-
ence of these from other information is unreliable.
Resources were unavailable for isotopic- exchange or reac-
tion path models, and so these methods were not used. A
non - site - specific, empirically- derived dilution quotient
equal to 0.85 of the initial groundwater sample activity was
used to correct the 14C activities (Vogel, 1970). While this
correction does not physically represent 14C dilution in the
aquifers, other research indicates that differences between
this simple, empirical correction and more sophisticated
approaches are not significant in the lower Tertiary
aquifers of the San Juan Basin (Phillips et al., 1989), a
hydrogeologic and hydrochemical setting similar to the
Arapahoe and Denver aquifers.
163 The Rocky Mountain Association of Geologists