PUB00006

• Year: 2005
• Title: Colorado's Historic Water Projects
• CWCB Section: Administration
• Author: Applegate Group, Inc
• Description: A calender with pictures and descriptions of historic water projects.
• Publications - Doc Type: Historical

Milos M. Novotny and William E. Sanford

Table 1.
Location and physical characteristics of wells sampled; data in meters above mean sea level (MSL).

Screen Interval
Surface Top Bottom
CO-DWR PLSS Location Datum Depth Depth
Sample Aquifer Permit T.R.S.Q.Q em] em] em]
DON-04 Arapahoe 1 6141-F 11 S.66W.31.NWSW 2,103 243 359
PiN-02 Arapahoe 51782-F 07S.65W18.SE.NW 1,948 538 657
PARK-01 Arapahoe 50563-F 06S.66W15.NE.SW 1,787 396 518
AUR-08 Arapahoe 30326-F 05S.65W21.SE.NE 1,829 378 512
PARK-03 Denver 50562-F 06S.66W15.NE.NW 1,787 235 372
AUR-07 Denver 30323-F 05S.65W21.sWNE 1,829 121 351

Table 2.
Land surface elevation of southern recharge zones (meters above MSL) used in model of atmospheric noble gases.

Standard Coefficient
Aquifer Mean Maximum Minimum Deviation of Variation
Denver 2,084 2,250 1,861 112 0.05
Arapahoe 1,994 2,100 1,814 81 0.04

Model of Recharge Temperature

Aeschbach-Hertig et al. (1999) have created a model
that takes into account the effects of temperature, eleva-
tion, salinity, and excess-air fractionation on atmospheric
noble gas concentrations. Measured values of Ne, Ar, Kr,
and Xe are used in the model to minimize an objective
function, essentially producing a statistical measure of the
"goodness-of-fit" of the data to the model. Novotny (2004)
provides a more detailed discussion of the application of
this model in the Denver Basin.
Salinity of recharging water often cannot be measured
and is assumed equal to zero; other researchers using simi-
lar methodology have assumed that this parameter is negli-
gible in waters with low salinity (e.g., Clark et aI., 1998).
The assumption appears valid in this study area where
modern recharging water is closely tied to precipitation
and no significant sources of salinity exist. Concentrations
of two of the measured inert gases (helium and nitrogen)
were not used because it is suspected that there are signifi-
cant sources or modifications of these in the subsurface.
The mean elevations of the Arapahoe and Denver aquifer
recharge areas were estimated from digital elevation maps
to be approximately 2,000 and 2,075 m above MSL, respec-
tively (Table 2). With three known noble gas concentra-
tions (Ne, Ar, Kr) and two unknown parameters (recharge

The Rocky Mountain Association of Geologists

temperature and excess air), the model equations are over-
determined and may be inverted. A statistical fit of the data
to the model is given by the objective function, "I}. For the
equation with one degree of freedom, and having a x,z
value of 3.84, the probability that the unknown parameters
deviate from their true values because of errors in gas con-
centration analysis is less than 5%.

RESULTS AND DISCUSSION

The results of the noble gas and 14C analyses are pre-
sented in Table 3. As expected, the majority of the gas
content is dissolved nitrogen. The 14C activity is highest in
the well located nearest the recharge areas and is lowest in
samples from the middle of the Basin.

Residence Times

Applying the assumptions and techniques outlined
above, the activity of 14C was used to determine the
apparent residence time of the waters at each location
(Table 3). The shortest residence time of 8,000 years was at
Donala (DON-04) in the Arapahoe aquifer, the location
nearest the outcrops and the highest potentiometric level.

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