U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2005–3099

January 2006

Prepared in cooperation with the Oklahoma Water Resources Board

Age-Dating Ground Water Beneath Tinker Air Force Base, 

Midwest City, Oklahoma, 2003-04

Printed on recycled paper

By Scott Christenson, David Parkhurst, Andrew G. Hunt, and 

Denae Athay

Introduction

Since the 1940s Tinker Air Force Base, near Midwest City, 

Oklahoma (fig. 1), has been a base for strategic bombers and 

early-warning radar planes and a major aircraft repair facility. 

Activities at the Base contaminated the underlying Garber-Wel1

Science Applications International Corporation

lington aquifer (fig. 2) with volatile organic compounds, metals, 

and other compounds to depths exceeding 200 feet below land 

surface. U.S. Air Force environmental staff and civilian contractors have characterized the extent of contamination in the upper 

part of the aquifer (IT Corporation, 1999). 

The aquifer at the Base is divided into layers in the first 200 

feet below land surface by numerous discontinuous confining 

layers (fig. 2). A 30-foot thick layer of shales and siltstones is 

found at about 200 feet below land surface (shown as the LSZPZ confining layer in figure 2), which forms the bottom of the 

shallow part of the aquifer. This layer may form a continuous 

confining layer beneath most of or the entire Base. PotentiometFigure 1. Location of wells sampled for this investigation.

97°25’ W 97°22’ W 97°19’ W

35°26’ N

35°25’ N

35°24’ N 0 0.25 0.50 MILES

0 0.25 0.50 KILOMETERS

2-137A

2-137D

1-73A

1-74D

2-139D

2-139C

1-71D

1-71C

1-71A

1-50CR

1-50DR

21D

2-21D

2-21C

2-103C

40

INTERSTATE

40

INTERSTATE

40

INTERSTATE

OKLAHOMA

Tinker Air

Force Base

40

INTERSTATE 40

INTERSTATE

44

INTERSTATE

35

INTERSTATE

35

INTERSTATE

44

INTERSTATE

Midwest City

Wells

Base border

Runways

EXPLANATION
ric heads decrease by about 40 feet from land surface to the base 

of the shallow part of the aquifer (S. Bowen, U.S. Air Force, 

written commun., 2005). 

Public-supply wells at the Base and surrounding areas are 

completed in a layer of the Garber-Wellington aquifer known 

as the “deep production zone,” which lies beneath the 30-footthick LSZ-PZ confining layer. Potentiometric heads in the deep 

production zone are about 50 feet below those in the shallow 

part of the aquifer above the LSZ-PZ confining layer, which 

indicates a substantial potential for downward seepage of contaminated ground water from the shallow part of the aquifer to 

the deep production zone (S. Bowen, U.S. Air Force, written 

commun., 2005). 

In 2003-04, the U.S. Geological Survey (USGS), in cooperation with the Oklahoma Water Resources Board and the U.S. 

Air Force, conducted an investigation to determine the age of 

ground water in the shallow, contaminated part of the aquifer 

and in the deep production zone beneath the Base. These ages 

are useful for assessing the vulnerability of the deep production 

zone to contaminants in ground water in the shallow part of the 

aquifer. 

Age-Dating Methods

Chemicals and isotopes that are dissolved in ground water 

can be used to determine the apparent age of ground water. 

These chemicals and isotopes are referred to as “age-dating 

tracers” in this report. The apparent age of ground water is 

considered to be the amount of time determined from an agedating tracer that has elapsed since the water was last in contact 

with the atmosphere. Modern ground water generally is more 

susceptible to contamination than old ground water because of 

the many man-made contaminants introduced during the 20th 

century (Plummer and Friedman, 1999). 

Three age-dating tracers were used at the Base to determine 

the age of ground-water samples from wells completed beneath 

the Base:

Tritium: Tritium is a short-lived radioactive isotope of 

hydrogen with a half-life of 12.32 years. Tritium forms naturally 

as cosmic radiation interacts with the upper atmosphere, and 

all precipitation that falls to Earth has small amounts of tritium. 

During the 1950s and early 1960s, global atmospheric testing 

of nuclear weapons raised the atmospheric concentrations of 

tritium hundreds of times above the normal background concentration (Plummer and others, 1993). After the early 1960s, 

when the Nuclear Test Ban Treaty was signed and atmospheric 

testing of nuclear weapons ceased, tritium concentrations in 

the atmosphere have decreased and are approaching natural 

levels. Tritium concentration alone generally cannot be used to 

quantitatively date ground water, but can be used to qualitatively 

determine if ground water is modern (less than about 50 years 

in age) or pre-modern (older than about 50 years in age) (Clark 

and Fritz, 1997). 

Ground-water samples were collected for tritium analysis 

from monitoring wells at the Base on September 9 and 10, 

2003. Sampled wells were completed in both the shallow part 

of the aquifer and the deep production zone. Samples were analyzed by electrolytic enrichment and gas counting at the USGS 

Tritium Laboratory in Menlo Park, California. 

Helium-3/Tritium: Tritium, as part of a water molecule, 

is transformed by radioactive decay to helium-3. By measuring both the tritium and helium-3 content from a ground-water 

sample, an apparent age can be determined because the rate that 

Figure 2. Diagramatic section through Tinker Air Force Base.

LLSZ 

Sands

Intra-USZ

Clays

Intra-LSZ Clays 

USZ 

Sands

LSZ Sands

LSZ-PZ Confining Layer

PZ Sands D 

B 

Example Well

Cluster A, B, C, D 

USZ = Upper Saturated Zone

LSZ = Lower Saturated Zone

LLSZ = Lower Lower Saturated Zone

PZ = Deep Production Zone

Hennessey 

Group

A 

C 

USZ-LSZ Confining Layer 

1,300

1,200

1,100

1,000

0.5 1 MILE

0 0.5 1KILOMETER

0

FEET

Upper part

of aquifer

VERTICAL SCALE GREATLY EXAGGERATED

DATUM IS NAVD 1988

Zones in the Garber-Wellington aquifer
tritium decays to helium-3 is known (Lucas and Unterwager, 

2000). 

Ground-water samples were collected for helium-3/tritium 

analysis from four monitoring wells at the Base on February 12, 

2004. Samples were collected from wells that were thought to 

be producing modern ground water on the basis of the results of 

the tritium analyses. Helium isotopes were analyzed on a magnetic sector mass spectrometer (Bayer and others, 1989) and 

tritium was measured by the helium in-growth method (Clark 

and others, 1976 and Bayer and others, 1989) at the USGS 

Noble Gas Laboratory in Denver Colorado. 

Carbon-14: Carbon-14 is the radioactive isotope of carbon 

with a half-life of 5,730 years. Carbon-14 is widely used and 

is well-known to the public as a tool for dating archeological 

sites, but carbon-14 also can be used to date ground water. Like 

tritium, carbon-14 is produced in the upper atmosphere and 

thus occurs naturally. Atmospheric carbon-14 concentrations 

were elevated by as much as 20 percent by the testing of nuclear 

weapons in the 1950s and 1960s. Radioactive carbon-14 in 

ground water is diluted by reactions that introduce non-radioactive carbon-12 into ground water, which results in apparent carbon-14 ground-water ages that are too old. Geochemical modeling is used to correct for the effects of dilution to obtain better 

estimates of ground-water age (Plummer and others, 1994).

Ground-water samples were collected for carbon-14 analysis from 11 monitoring wells at the Base on March 15 and 16, 

2004. Samples were collected from wells that were thought 

to be producing pre-modern ground water on the basis of the 

results of the tritium analyses, and from some additional wells 

on the basis of the site hydrogeology and well completion 

reports. Samples were analyzed by accelerator mass spectrometry at the Environmental Isotope Laboratory at the University 

of Waterloo, Waterloo, Ontario, Canada. 

All three methods are subject to measurement errors and 

age dates cannot be considered to be absolute. Ground water 

collected from a well may be a mixture of water from several 

sources of different ages. The apparent age determined by 

one tracer may not be equal to an apparent age determined by 

another tracer because of the methods by which the apparent 

ages are determined. The water samples were collected from 

some wells at the Base for analysis using different age-dating 

tracers to provide a basis for comparison of methods. 

Age-Dating Results

Samples analyzed for tritium alone ranged in concentration 

from <0.3 to 9.6 picocuries per liter (pCi/L). Tritium concentrations below 1 pCi/L were considered to indicate that ground 

water was at least 50 years old (pre-modern) and tritium conTable 1. Results of age-dating analyses at Tinker Air Force Base

[pCi/L, picocuries per liter; --, not analyzed; ND, not determined; color denotes well clusters]

Site

Well

depth

(feet)

Tritium

concentration

(pCi/L)

Helium-3/ 

tritium

age

Tritium age 

category

Carbon-14

modeled

age (years

before 2004)

1-50CR 120 7.2 March 1982 Modern 2,600

1-50DR 250 0.3 -- Pre-modern 3,100

1-71A 125 6.8 May 1968 Modern --

1-71C 187 <0.3 -- Pre-modern 1,100

1-71D 243 1.3 -- Modern 2,900

1-73A 81 9.6 May 1987 Modern --

1-74D 198 -- -- ND 690

2-103C 177 -- -- ND 1,700

2-137A 78 5.4 -- Modern --

2-137D 250 <0.3 -- Pre-modern 1,100

2-139C 136 0.3 -- Pre-modern 1,600

2-139D 237 <0.3 -- Pre-modern --

2-21C 151 1.0 -- Modern Modern

2-21D 245 6.5 September 1990 Modern 560

21D 212 -- -- ND 460
centrations equal to or greater than 1 pCi/L were considered to 

indicate modern ground water (table 1).

For the samples analyzed for the helium-3/tritium age, all 

four samples contained measurable amounts of tritium and 

tritiogenic helium (helium derived from tritium decay). Sample 

apparent ages ranged from 13.4 to 35.7 (+/- 0.7) years in age 

from the time of sampling, giving recharge dates that range 

from 1990 to 1968. These dates are consistent with modern 

recharge; however, a comparison of tritium plus helium (original tritium recharged) to historical tritium measurements in 

precipitation show that there was some deviation from the 

measured results. Samples that fall below the tritium input 

curve were interpreted as mixed ground waters that have been 

diluted with older, pre-modern waters. Because the helium-3/

tritium age is based on the daughter to parent ratio measured in 

the sample, the calculated apparent age represents the younger 

(modern) component and older (pre-modern) component is considered tritium dead; no tritium or tritiogenic helium is added to 

the mixing system, preserving the modern apparent age.

The measured carbon-14 concentrations range from 52 to 

93 percent modern carbon (pmc). The carbon-14 concentration 

of water that recharged the aquifer prior to atmospheric nuclear 

testing is assumed to be 100 percent modern, or less if chemical 

reactions diluted the dissolved carbon pool with low carbon-14 

carbon. For radioactive decay to decrease the carbon-14 value 

from 100 to 50 percent modern requires one half life, or about 

5,700 years. Thus, without any corrections for chemical reactions, the maximum carbon-14 age of any ground water sampled 

is less than about 5,700 years. Chemical reactions may have 

occurred in the water after it fell as precipitation and entered 

the Garber-Wellington aquifer. In all cases, accounting for the 

effects of chemical reactions tends to decrease the calculated 

age of the ground water. The program NETPATH (Plummer and 

others, 1994) was used to generate inverse models that account 

for the evolution of pure water (approximation of precipitation) 

to each of the sample compositions. A reaction-corrected carbon-14 concentration was calculated by using NETPATH. The 

difference between the reaction-corrected concentration and the 

observed concentration is assumed to be caused by radioactive 

decay, and the adjusted age is calculated as the number of years 

necessary to reduce the reaction-corrected concentration to the 

observed concentration, given the half-life of carbon-14 (5,730 

years).

The adjusted ages based on the carbon-14 data and NETPATH model for these samples are between modern and 3,000 

years. There are uncertainties in these adjusted ages caused by 

analytical errors and by the assumptions in the calculation.

The results from the three age-dating tracers are not completely consistent (table 1). For example, water samples from 

Well 1-50CR had a tritium concentration of 7.2 pCi/L, which 

implies that some fraction of the water in this sample is modern, and the calculated helium-3/tritium age is March 1982. 

However, the modeled carbon-14 age for the well is 2,600 

years before present, indicating that some fraction of this water 

is pre-modern. This indicates that water from this well probably is a mixture of modern and pre-modern water. Detectable 

tritium implies a fraction of the water must have recharged 

in the last 50 years. Some of the water appears to be modern, 

either because of the presence of tritium or by the carbon-14 age 

estimate. Although some samples have carbon-14 ages that are 

estimated to be 500 to 3,000 years, these ages are at the youngest end of the spectrum of ages that can be measured by carbon14. Thus, it is possible that all the water samples collected from 

the Garber-Wellington aquifer at the Base contain some fraction 

of water that is modern, even water samples that have ages 

greater than 50 years by tritium and carbon-14 estimates. The 

mixing of modern and pre-modern ground water could occur by 

natural ground-water gradients or gradients induced by pumping 

of supply wells. 

References 

Bayer, R., Schlosser, P., Bonisch, G., Rupp, H., Zaucker, F., and 

Zimmek, G., 1989, Performance and blank components of a 

mass spectrometric system routine measurement of helium 

isotopes and tritium by 3He ingrowth method: Sitzungsberichte der Heidelberger Akademie der Wissenschaften. Mathematisch-naturwissenschaftliche Klasse: Heidelberg, Springer 

Verlang, p. 241-279.

Clark, I.D., and Fritz, Peter, 1997, Environmental isotopes in 

hydrogeology: New York, Lewis Publishers, 328 p.

Clark, W.B., Jenkins, W.J., and Top, Z., 1976, Determination of 

tritium by mass spectrometric measurements: International 

Journal of Applied Radioactive Isotopes, v. 27, p. 515-522.

IT Corporation, 1999, Final basewide non-NPL groundwater 

phase II RCRA facility investigation report for Appendix I 

and II, SWMUS Addendum 1, Tinker Air Force Base, Oklahoma.

Lucas, L.L., and Unterweger, M.P., 2000, Comprehensive 

review and critical evaluation of the half-life of tritium: 

Journal of Research of the National Institute of Standards and 

Technology, Vol. 105.

Plummer, L.N. and Friedman, L.C., 1999, Tracing and dating 

young ground water: U.S. Geological Survey Fact Sheet 134-

99, 4 p.

Plummer, L.N., Michel, R.L., Thurman, E.M., and Glynn, P.D., 

1993, Environmental tracers for age dating young ground 

water, in Alley, W.M., 1993, ed., Regional Ground-Water 

Quality: Van Nostrand Reinhold, New York, p. 255–294.

Plummer, L.N., Prestemon, E.C., and Parkhurst, D.L., 1994, An 

interactive code (NETPATH) for modeling NET geochemical 

reactions along a flow PATH–version 2.0: U.S. Geological 

Survey Water-Resources Investigations Report 94-4169, 130 p.

For additional information contact:

U.S. Geological Survey

Oklahoma Water Science Center

202 NW 66 St., Bldg. 7

Oklahoma City, OK 73116

(405) 810-4400

Oklahoma Water Science Center home page:

http://ok.water.usgs.gov