Data on concentrations of environmental tracers, such as chlorofluorocarbons (CFCs), tritium (3H), and other chemical and isotopic substances in

ground water, can be used to trace the flow of young water (water recharged within the past 50 years) and to determine the time elapsed since

recharge. Information about the age of ground water can be used to define recharge rates, refine hydrologic models of ground-water systems,

predict contamination potential, and estimate the time needed to flush

contaminants from ground-water systems. CFCs also can be used to trace

seepage from rivers into ground-water systems, provide diagnostic tools

for detection and early warning of leakage from landfills and septic tanks,

and to assess susceptibility of water-supply wells to contamination from

near-surface sources.

During the past 50 years, human

activities have released an array

of chemical and isotopic substances to the atmosphere. In the atmosphere, these substances have mixed and

spread worldwide. These atmospheric

substances, such as tritium (3H) in water

vapor from detonation of nuclear bombs

in the 1950s and early 1960s,and chlorofluorocarbons (CFCs) from refrigeration

and other uses from the 1950s through the

1980s, dissolve in precipitation, become

incorporated in the Earth’s hydrologic

cycle, and can be found in ground water

that has been recharged within the past 50

years. The detection of chlorofluorocarbons and tritium in ground water provides

valuable information that can be used for

dating and tracing young ground water—

techniques that help water-resources managers develop management strategies for

shallow ground-water systems.

Young ground water in shallow

ground-water systems

Young ground water is typically found at

depths from 0 to 100 feet in unconsolidated sediments and at depths up to 1000

feet in fractured-rock systems. Shallow

ground-water systems are commonly used

for drinking water sources and they make

up a large part of the baseflow in rivers

and lakes. However, shallow ground-water supplies are generally young (recently

recharged) and, because there has been a

wide variety of man-made pollutants produced in the 20th century, are more susceptible to contamination than deeper

ground water. Information about groundwater age can be used to determine recharge rates and refine hydrologic models of ground-water systems (Reilly and

others, 1994; Szabo and others, 1996)

and thus to predict the contamination potential and estimate the time needed to

flush contaminants through a ground-water system.

The 0- to 50-year time scale is particularly relevant to environmentally sensitive shallow ground-water systems. Prior

to the late 1980s, however, there were no

reliable means of dating ground water recharged during this time scale and, until

recently, none of those methods were considered practical for use in establishing regional patterns. In the early 1990s, USGS

scientists (Busenberg and Plummer, 1992)

developed a method to date ground water

on the basis of chlorofluorocarbon (CFC)

content of the water that is practical, costeffective, and applicable to most shallow

ground-water systems.

Sampling for tracers

The feasibility of using CFCs as tracers of

recent recharge and indicators of groundwater age was first recognized in the 1970s

(see Plummer and Busenberg, 1997 and

references therein). CFCs have been increasingly used in oceanic studies since

the late 1970s as tracers of oceanic circulation, ventilation, and mixing processes. USGS scientists (Busenberg and

Plummer, 1992) adapted analytical procedures developed by the oceanographic

scientific community for ground-water

studies and designed sampling equipment

and procedures for collection and preservation of water samples in the field. Water samples for CFC analysis are now routinely collected from domestic, irrigation,

monitoring, and municipal wells, and

from springs. A closed path is established

USGS Hydrologist analyzes a water sample

for CFCs at the USGS Chlorofluorocarbon

Laboratory, Reston, Virginia.

USGS Hydrologist collects a water sample

for CFC analyses from a domestic well in

south-central Idaho.

Tracing and Dating Young Ground Water

US Department of the Interior

US Geological Survey

Fact Sheet-134-99

September 1999
Chlorofluorocarbons are stable, synthetic organic compounds that

were developed in the early 1930s as safe alternatives to ammonia and

sulfur dioxide in refrigeration and have been used in a wide range of

industrial and refrigerant applications. Production of CFC-12 (dichlorodifluoromethane, CF2Cl2

) began in 1931, followed by CFC-11 (trichlorofluoromethane, CFCl3

) in 1936, and then by many other CFC

compounds, most notably CFC-113 (trichlorotrifluoroethane, C2F3

Cl3).

CFC-11 and CFC-12 were used as coolants in air conditioning and refrigeration, blowing agents in foams, insulation, and packing materials, propellants in aerosol cans, and as solvents. CFC-113 has been

used primarily by the electronics industry in semiconductor chip

manufacturing, in vapor degreasing and cold immersion cleaning of

microelectronic components, and surface cleaning. Probably better

known to the public as FreonTM, CFCs are nontoxic, nonflammable and

noncarcinogenic, but they contribute to ozone depletion. Therefore in

1987, 37 nations signed an agreement to limit release of CFCs and to

halve CFC emissions by 2000. This agreement, the Montreal Protocol

on Substances That Deplete the Ozone Layer, was strengthened in

1990 and again in 1992 when 1996 was established as the cut-off date

for CFC production in industrialized countries. Production of CFCs

ceased in the United States as of January 1, 1996, under the Clean Air

Act. Current estimates of the atmospheric lifetimes of CFC-11, CFC-12,

and CFC-113 are about 45, 87, and 100 years, respectively.

between the well or pump to a valve system that is used to fill glass ampoules

with water, creating a headspace with

CFC-free, ultra-pure nitrogen gas. The

samples are then transported to the U.S.

Nuclear Regulatory Commission-licensed

USGS laboratory for analysis of CFC

content by gas chromatography to a detection limit of about 0.3 picograms per kilogram (0.3 pg/kg) of water, which is

equivalent to 0.3x10-12 grams per kilogram, or 0.3 parts per quadrillion.

Determining ground-water ages

Ground-water dating with CFC-11, CFC12 and CFC-113 is possible because

(1) their amounts in the atmosphere over

the past 50 years have been reconstructed,

(2) their solubilities in water are known,

and (3) concentrations in air and young

water are high enough that they can be

measured. Age is determined from CFCs

by relating their measured concentrations

in ground water back to known historical

atmospheric concentrations and/or to calculated concentrations expected in water

in equilibrium with air. As with any environmental tracer, age applies to the date

of introduction of the chemical substance

into the water, and not to the water itself.

The accuracy of the determined age depends in part on how perfectly the CFCs

are transported with the water. Chemical

processes, such as microbial degradation

and sorption during transit, can also affect the concentration of CFCs and other

compounds used in dating. For this reason, the term “age” is normally qualified

with the word “model” or “apparent,”

that is, “model age” or “apparent age.”

USGS scientists have investigated some

of the most important factors that can affect CFC concentrations in ground water

and the resulting age interpretations

(Busenberg and Plummer, 1992; Plummer

and others, 1993; Dunkle and others,

1993; Ekwurzel and others, 1994; Cook

and others, 1995; Plummer and others

1998a, 1998b; Plummer and Busenberg,

1999). These include (1) uncertainty of

the temperature at the water table during

recharge, (2) entrapment of excess air

during recharge, (3) uncertainty in recharge elevation, (4) thickness of the unsaturated zone, (5) effect of urban air

where CFC values may exceed regional

values, (6) contamination from a specific

local source, (7) microbial degradation

(in anaerobic environments), (8) sorption

onto organic and mineral surfaces, and

(9) mixing of younger and older water in

a well. Because of the effect of these factors on CFC concentration, collection of

additional data is often needed to determine the apparent age of ground water.

For example, measurements of concentrations of dissolved gases, such as dissolved

oxygen, help to define the potential for

microbial degradation. Measurements of

dissolved methane are useful in recognizing environments where all three CFCs

can be degraded. Measurements of dissolved nitrogen and argon can be used to

help determine recharge temperature and

excess air and to recognize environments

undergoing denitrification.

Other chemical dating tools

Tritium (half-life 12.4 years) provides another useful tracer of young ground water. Although it is difficult to evaluate age

Concentrations of CFC-11, CFC-12, and CFC113 in water recharged in equilibrium with

air between the years 1940 and 2000 at temperatures of 5 to 25°C. The CFC analysis determined that concentrations of CFC-11,

CFC-12 and CFC-113 were 493, 203, and 28

pg/kg in the water sample to be dated. The

recharge temperature of 10°C was determined from analysis of dissolved nitrogen

and argon in the water sample.

CFCs and sulfur hexafluoride (SF6) concentrations (mixing ratios) for air over North

America during the last 50 years.

600

Parts

per trillion,

(pptv)

(pptv)

CFC-12

SF CFC-11 6

CFC-113

400

200

0

4

2

0

1940 1960 1980

CFCs have started

to decline due to 

a global ban

SF6 is accumulating rapidly

and is a promising alternative tracer to CFC

1200

Picograms per

kilogram (pg/kg)

800

400

0

400

200

0

1940 1960 1980

150

100

50

0

5° C

10° C

15° C

20° C

25° C

5° C

10° C 15° C

20° C

25° C

5° C

10° C

15° C

20° C

25° C

CFC-11

CFC-12

CFC-113
information from tritium data alone, age

commonly can be reliably determined

from data on tritium (3H) and its decay

product, helium-3 (3He). The 3H/3He age

is based on a calculation that determines

the amount of 3He derived from radioactive decay of 3

H in the water. Several conditions are necessary to solve the calculation and interpret the age: (1) The sample

must contain detectable 3H (greater than

approximately 0.5 tritium unit, or TU,

which is defined as one 3H atom in 1018

hydrogen atoms) and (2) if the sample

contains terrigenic helium from the Earth’s

crust and mantle sources, the relative

abundances of helium-3 and helium-4

isotopes in the terrigenic helium must be

known, and data on dissolved neon concentrations in the sample are needed to

help determine how much helium-3 is derived from tritium decay. Water samples

for 3H/3He age determination can be collected more easily than those for CFC determination, but are more difficult and

costly to analyze than CFC samples.

Krypton-85 (85Kr) has also been used to

date ground water. The source for atmospheric input of 85Kr is reprocessing of

fuel rods from nuclear reactors. Because

of difficulties in collection and analysis,

85Kr is not yet a practical dating tool for

ground-water studies. In CFC-contaminated environments, or environments

where methane is forming from microbial

degradation of organic matter, noble-gas

dating techniques such as those based on

3

H/3He and 85Kr measurements may be the

only techniques for dating. However, because of their low detection limit and low

sensitivity to hydrodynamic dispersion,

CFCs can be more reliable than 3H/3He

and 85Kr dating for waters recharged in

the late 1940s and 1950s.

Seeking alternatives to CFC dating

Alternatives to using CFCs in dating

ground water will clearly be needed as atmospheric CFC concentrations continue

to fall. The use of sulfur hexafluoride

(SF6) appears to be a promising technique

and is under investigation by USGS scientists (Busenberg and Plummer, 1997;

Plummer and Busenberg, 1999). Industrial production of SF6 began in 1953 with

the introduction of gas-filled high-voltage

electrical switches. SF6 is extremely

stable and is accumulating rapidly in the

atmosphere. The historical atmospheric

mixing ratio of SF6 is being reconstructed

from production records, archived air

samples, and atmospheric measurements,

and retrieved from concentrations measured in seawater and in previously-dated

ground water. A preliminary reconstruction has been made of northern-hemisphere SF6 mixing ratios (figure far left).

As atmospheric CFC concentrations fall,

an even more sensitive dating tool will be

the ratio of SF6 to, for example, CFC-12.

Although SF6 is almost entirely of human

origin, there is likely a natural, igneous

source of SF6 that will complicate dating

in some environments. USGS scientists

have successfully used SF6 to date shallow

ground water on the Delmarva Peninsula,

Maryland, and water from springs in the

Blue Ridge Mountains of Virginia with

SF6.

Additional comments and

case studies

For best results, the apparent age should

be determined using multiple dating techniques because each dating technique has

limitations. CFC dating is best suited for

ground water in relatively rural environments where localized contamination,

such as that from septic systems, sewage

effluent, landfills, or urban runoff, can

contain CFC concentrations in excess of

that of atmospheric sources. The dating

method appears to work well in shallow,

aerobic, sand aquifers that are low in particulate organic matter and the results can

be accurate within 2–3 years in this type

of environment. Even where there are problems with CFC dating of ground water,

the presence of CFCs indicates that the

water sample contains at least some post1940s water, making CFCs useful as tracers of recent recharge. Where CFC and

3

H/3He ages agree, or where all three CFCs

indicate similar ages, considerable confidence can be placed in the apparent age.

Delmarva Peninsula Results from the

determination of CFCs in some agricultural areas on the Delmarva Peninsula of

Maryland and Virginia indicate that water

recharged since the early 1970s exceeds

the U.S. Environmental Protection Agency

drinking water Maximum Contaminant

Level (MCL) for nitrate of 10 mg/L (as

N), while water recharged prior to the

early 1970s, before the heavy use of nitrogen fertilizers, does not exceed the MCL

(Dunkle and others, 1992; Ekwurzel and

others, 1994; Böhlke and Denver, 1995).

Nitrate concentrations in ground water under woodlands were low, whereas ground

water recharged under agricultural fields

had nitrate concentrations that exceeded

the MCL. CFC concentrations indicate

that ground water now discharging to

streams that drain agricultural areas of the

Delmarva and then flow into the Chesapeake Bay or the Atlantic Ocean, was recharged in nearby fields in the 1960s and

1970s (Böhlke and Denver, 1995; Focazio

and others, 1998). Thus, even if the application of nitrogen fertilizers to the fields

stopped today, streams, rivers, and estuaries can be expected to receive increasing

amounts of nitrate from ground-water discharge until the contaminated water is

flushed through the system (Modica and

others, 1998); up to 30 years may be

needed to flush the high-nitrate water

present in several small agricultural watersheds.

Central Oklahoma Water discharged

from deep (400 to 800 feet) municipal

supply wells in the central Oklahoma

aquifer contains CFCs (Busenberg and

CFCs and nitrate concentrations were measured between

June 1989 and January 1990 on a section of the Delmarva Peninsula, in the Fairmount watershed. Ground-water dating

reveals a pattern of high nitrate concentrations moving

slowly toward the estuary.

Delmarva

peninsula Viriginia

Maryland

Delaware

Woods

Water

movement

Water table

Approximate year of recharge to aquifer

50

(feet)

50

100

Sea

level

Agriculture Lowlands

Wells

Estuary

0.9

21

Below limit

Above limit

21

0.4

0.1

20

8

5.2

27

32

13

14 12

27

21 0.9

21

13

9.4

1985

1980

1970

1960

W E

Numbers within circles 

show nitrate concentrations, in milligrams per liter 

(mg/L, as N). U.S. Environmental Protection Agency 

Maximum Contaminant 

Level (MCL) is 10 mg/L.
Plummer, 1992). This unexpected finding

indicated that shallow ground water was

being mixed with deep, older water during pumping. In this case, CFCs were a

valuable tracer for testing hydraulic seals

of municipal water wells and their susceptibility to contamination. River water

downstream from outfalls of sewage

treatment plants in central Oklahoma

contained very high concentrations of

CFCs, which were also found in shallow

ground water in the alluvium and terrace

system near rivers.

Idaho At the Idaho National Engineering

and Environmental Laboratory (INEEL),

the ground-water CFC concentrations

were found to be in equilibrium with the

shallow soil air, rather than air from the

deeper unsaturated zone (Busenberg and

others, 1993). This indicates that these

waters passed rapidly through the unsaturated zone to the water table, rather than

recharging by slow percolation. Temperatures derived from gas solubilities in

ground water suggest that the aquifer is

recharged locally. Ground-water velocities on the order of 6 to 24 feet per day

were found in the Snake River Plain aquifer beneath the INEEL. USGS scientists

(Busenberg and others, 1998) have also

used CFCs in water and unsaturated zone

air to trace the movement of organic

waste plumes at INEEL.

Nevada Measurements of CFCs in unsaturated-zone air along the crest of

Yucca Mountain, Nevada, a potential site

for a high-level radioactive waste repository, show that the residence time of

shallow advecting gas is less than 5 years

(Thorstenson and others, 1998). Previous

estimates based on carbon-14 measurements of unsaturated-zone carbon dioxide indicated a residence time of less

than 40 years. In addition to decreasing

the estimated travel time by an order of

magnitude, the CFC data allowed gas circulation patterns in the mountain to be

identified and quantified. These results

are highly relevant to issues pertaining to

isolation of possible repository-generated

radioactive 14CO2.

Georgia CFCs were used to trace and

date leakage of river water through sinkholes into the Upper Floridan aquifer

near Valdosta, Georgia (Plummer and

others, 1998a, 1998b). The ages of the

young-water fractions in the Upper

Floridan aquifer were mapped throughout the Valdosta area. The dating indicates that few domestic and municipal

supply wells produce river-water fractions that are younger than 5 years, and

most river water sampled was likely recharged through the sinkholes during the

past 20 to 30 years. Ground-water velocities ranged from 1 to 27 feet per day.

For additional information,

see http://water.usgs.gov/lab/cfc/

or contact:

Niel Plummer (email: nplummer@usgs.gov)

U.S. Geological Survey

12201 Sunrise Valley Drive, MS 432

Reston, VA 20192

REFERENCES

Böhlke, J.K., and Denver, J.M., 1995, Combined use of

groundwater dating, chemical, and isotopic analyses to resolve the history and fate of nitrate contamination in two

agricultural watersheds, Atlantic coastal plain, Maryland:

Water Resources Research, v. 31, p. 2319–2339.

Busenberg, Eurybiades, and Plummer, L.N., 1992, Use of

chlorofluorocarbons (CCl3F and CCl2F2) as hydrologic tracers and age-dating tools: Example—The alluvium and terrace system of central Oklahoma: Water Resources

Research, v. 28, 2257–2284.

Busenberg, Eurybiades, and Plummer, L.N., 1997, Use of

sulfur hexafluoride as a dating tool and as a tracer of igneous and volcanic fluids in ground water (abs.): Geological

Society of America, Salt Lake City, 1997, Abstracts and

Programs, v. 29(6), p. A-78.

Busenberg, Eurybiades, Plummer, L.N., Bartholomay, R.C.,

and Wayland, J.E., 1999, Chlorofluorocarbons, sulfur

hexafluoride, and dissolved permanent gases in ground water from selected sites in and near the Idaho National Engineering and Environmental Laboratory, Idaho, 1994–97:

U.S. Geological Survey Open-File Report 98-274, 72 p.

Busenberg, Eurybiades, Weeks, E.P., Plummer, L.N., and

Bartholomay, R.C., 1993, Age dating ground water by use

of chlorofluorocarbons (CCl3F and CCl2F2), and distribution of chlorofluorocarbons in the unsaturated zone, Snake

River Plain aquifer, Idaho National Engineering Laboratory,

Idaho: U.S. Geological Survey Water-Resources Investigations Report 93-4054, 47 p.

Cook, P.G., Solomon, D.K., Plummer, L.N., Busenberg,

Eurybiades, and Schiff, S.L.,1995, Chlorofluorocarbons as

tracers of groundwater transport processes in a shallow, silty

sand aquifer: Water Resources Research, v. 31, p. 425–434.

Dunkle, S.A., Plummer, L.N., Busenberg, Eurybiades,

Phillips, P.J., Denver, J.M., Hamilton, P.A., Michel, R.L.,

and Coplen, T.B., 1993, Chlorofluorocarbons (CCl3F and

CCl2F2) as dating tools and hydrologic tracers in shallow

ground water of the Delmarva Peninsula, Atlantic Coastal

Plain, United States: Water Resources Research, v. 29, p.

3837–3860.

Ekwurzel, Brenda, Schlosser, Peter, Smethie, W., Jr.,

Plummer, L.N., Busenberg, Eurybiades, Michel, R.L.,

Weppernig, Ralf, and, Stute, M., 1994, Dating of shallow

groundwater: Comparison of the transient tracers 3

H/3He,

chlorofluorocarbons and 85Kr: Water Resources Research,

v. 30, p. 1693–1708.

Focazio, M.J., Plummer, L.N., Bohlke, J.K., Busenberg,

Eurybiades, Bachman, L.J., and Powers, D.S., 1998, Preliminary estimates of residence times and apparent ages of

ground water in the Chesapeake Bay watershed and waterquality data from a survey of springs: U.S. Geological Survey Water-Resources Investigations Report 97-4225, 75 p.

Modica, Edward, Buxton, H.T., and Plummer, L.N., 1998,

Evaluating the source and residence times of ground-water

seepage to headwaters streams, New Jersey Coastal Plain:

Water Resources Res., v. 34, p. 2797–2810.

Plummer, L.N. and Busenberg Eurybiades, 1999, Chlorofluorocarbons in P. Cook and A. Herczeg, eds., Environmental Tracers in Subsurface Hydrology: Kluwer

Academic Press, in press.

Plummer, L.N., McConnell, J.B., Busenberg, Eurybiades,

Drenkard, Stefan, Schlosser, Peter, and Michel, R.L.,

1998a, Flow of river water into a karstic limestone aquifer,

1. Tracing the young fraction in groundwater mixtures in

the Upper Floridan aquifer near Valdosta, Georgia: Applied

Geochemistry, v. 13, p. 995–1015.

Plummer, L.N., Busenberg, Eurybiades, Drenkard, Stefan,

Schlosser, Peter, McConnell, J.B., Michel, R.L., Ekwurzel,

Brenda, and Weppernig, Ralf, 1998b, Flow of river water

into a karstic limestone aquifer, 2. Dating the young fraction in groundwater mixtures in the Upper Floridan aquifer

near Valdosta, Georgia: Applied Geochemistry, v. 13, p.

1017–1043.

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

P.D., 1993, Environmental tracers for age-dating young

ground water: Alley, W.M., ed., Regional Ground-water

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

Reilly, T.E., Plummer, L.N., Phillips, P.J., and Busenberg,

Eurybiades 1994, Estimation and corroboration of shallow

ground-water flow paths and travel times by environmental

tracer and hydraulic analyses—A case study near Locust

Grove, Maryland: Water Resources Research, v. 30, p.

421–433.

Szabo, Z., Rice, D.E., Plummer, L.N., Busenberg,

Eurybiades, Drenkard, Stefan, and Schlosser, Peter, 1996,

Age-dating of shallow groundwater with chlorofluorocarbons, tritium/helium 3, and flow path analysis, southern

New Jersey coastal plain: Water Resources Research, v. 32,

p. 1023–1038.

Thorstenson, D.C., Weeks, E.P., Haas, H., Busenberg,

Eurybiades, Plummer, L.N., and Peters, C.A., 1998, Chemistry of unsaturated zone gases sampled in open boreholes

at the crest of Yucca Mountain, Nevada: Data and basic

concepts of chemical and physical processes in the mountain: Water Resources Research, v. 34, p.1507–1529.

CFCs were used to trace and date water in

the karst aquifer near Valdosta, Georgia.

0

0 5 Kilometers 

5 Miles

Withlacoochee

Rive r

Valdosta water

supply well field

Well

Sinkhole area 

Valdosta

GEORGIA

0—10

10—20

20—30

30—40

40—50

Greater than 50

Ages (years) of

young-water 

fractions

Map

area

Ground-water dating at Valdosta helps

define the susceptibility to contamination

of the ground-water resources.

—L. Niel Plummer and Linda C. Friedman