##  Virginia Aquifer Susceptibility Study:

# Dating of Ground Water for Source Water Assessment Screening

The U.S. Geological Survey (USGS), in cooperation with the Virginia
Department of Health (VDH), is conducting a study to determine the
intrinsic natural susceptibility of the regional aquifers in Virginia
that serve as public water supplies. The information will be used to
screen public ground-water supplies and identify those that require
detailed source-water assessments. Approximately 150 sites will be
sampled during this 4-year study. This study is part of the State Source
Water Assessment program, which is coordinated by the VDH, Office of
Water Programs.

The fundamental premise of the USGS study is to use age determinations
as a guide for the classification of areas in terms of the
susceptibility of ground water to near-surface contamination. Various
environmental isotopes and tracers will be used to determine the age of
ground water (table 1). Carbon-14 will be used to date ground water that
is older than 1,000 years. Chlorofluorocarbons (Freon) and tritium (a
product of above ground nuclear weapons testing) techniques will be used
to date ground water that is less than 50 years old. Analysis of
dissolved gas samples will estimate the temperature of ground water at
the time of recharge, which is used in the age-determination
techniques.\
\
**Table 1**. List of environmental tracers and isotopes used for age
determinations

+--------------------+-------------+----------------------------------+
| Environmental      | Age\        | Laboratory                       |
| isotope/tracer     | range\      |                                  |
|                    | (years)     |                                  |
+--------------------+-------------+----------------------------------+
| Ch                 | 0 to 50     | USGS, National Research Program  |
| lorofluorocarbons\ |             |                                  |
| (*CFC-11, CFC-12,  |             | Reston Chlorofluorocarbon Lab,   |
| and CFC-113*)      |             | VA                               |
+--------------------+-------------+----------------------------------+
| Tritium            | 0 to 50     | USGS, National Research Program  |
|                    |             |                                  |
|                    |             | Menlo Park, CA                   |
+--------------------+-------------+----------------------------------+
| Tritium/Helium-3   | 0 to 30     | Lamont-Doherty Earth Observatory |
|                    |             |                                  |
|                    |             | Columbia University, NY          |
+--------------------+-------------+----------------------------------+
| Carbon-14          | 1,000 to    | Univ. of Waterloo, Canada        |
|                    |             |                                  |
|                    | 30,000      | Rafter Radiocarbon Laboratory    |
|                    |             |                                  |
|                    |             | Institute of Geological &        |
|                    |             | Nuclear Sciences, Ltd., New      |
|                    |             | Zealand                          |
+--------------------+-------------+----------------------------------+

Ground-water samples will also be analyzed for major dissolved
constituents, nitrate, radon-222, and dissolved organic carbon (table 2)
to aid in the interpretation of age determinations and in the assessment
of natural susceptibility. Field properties (pH, temperature, specific
conductance, and dissolved oxygen) will be monitored during sample
collection. Alkalinity will be determined at the time of sample
collection. The estimated time to collect the sample is between 2 and 3
hours.

**Table 2**. List of dissolved constituents

+-----------------------+--------------+------------------------------+
| Type                  | Constituent  | Laboratory                   |
+-----------------------+--------------+------------------------------+
| Major Ions            | Ca, Mg, Na,  | USGS, National Research      |
|                       | K,\          | Program, Reston, VA          |
|                       | Cl, SO~4~,   |                              |
|                       | SiO~2~,\     |                              |
|                       | Fe,Mn        |                              |
+-----------------------+--------------+------------------------------+
| Nutrients             | NO~2~+NO~3~  | USGS, National Water Quality |
|                       |              | Lab, Denver, CO              |
|                       | (as N)       |                              |
+-----------------------+--------------+------------------------------+
| Organic Carbon        | DOC          | USGS, National Water Quality |
|                       |              | Lab, Denver, CO              |
+-----------------------+--------------+------------------------------+
| Radionuclide          | Radon-222\   | USGS, National Water Quality |
|                       | (^222^Rn)    | Lab, Denver, CO              |
+-----------------------+--------------+------------------------------+

# Chlorofluorocarbons (CFC's)

Chlorofluorocarbons (CFC\'s) are stable synthetic organic compounds that
were first manufactured in the 1930\'s and used as refrigerants, aerosol
propellants, cleaning agents, solvents, and blowing agents in the
production of foam rubber and plastics (Busenberg and Plummer, 1992).
CFC\'s are eventually released to the atmosphere and hydrosphere and
depletion of the Earth\'s ozone layer has been\
\
![](media/image3.wmf)\
\
attributed to the rapid atmospheric accumulation of these compounds
(Molina and Rowland, 1974). The method developed by Busenberg and
Plummer (1992) uses CFC\'s as dating tools\--trichlorofluoromethane
(CCl~3~F, CFC-11, Freon 11), dichlorodifluoromethane (CCl~2~F~2~,
CFC-12, Freon 12), and trichlorotrifluoroethane (C~2~Cl~3~F~3~,
CFC-113). CFC-11 and CFC-12 make up 77 percent of total global
production of CFC\'s (Derra, 1990). Atmospheric partial pressures of
CFC\'s are determined by Henry\'s law from the recharge temperature
estimated from dissolved gases in the sample (N~2~/Ar ratio) and
measured concentrations of CFC\'s in the ground-water samples. These
calculated partial pressures are compared with the atmospheric mixing
ratios of CFC\'s (fig. 1) to determine the CFC-modeled recharge date,
which equates to the time that the water was isolated from air in the
unsaturated zone.

# Tritium and Tritium/Helium-3

Tritium (^3^H) is the radioactive isotope of hydrogen with a half-life
of 12.43 years (IAEA, 1981) and is an excellent indicator of ground
water recharged since 1952 (Clark and Fritz, 1997). Production of ^3^H
in the atmosphere naturally occurs by cosmic ray spallation, but the
principal source was the atmospheric testing of thermonuclear weapons.
The standard unit of measure for ^3^H is a tritium unit (TU) for which
one TU is equivalent to one ^3^H atom per 10^18^ atoms of hydrogen or in
terms of radioactivity 3.2 picocuries per liter (Clark and Fritz, 1997).
Tritium content in precipitation closely mimics world events during the
Cold War years with a maximum concentration occurring in 1963, commonly
referred to as the \"bomb peak\" (fig. 1). Atmospheric concentrations
have gradually declined since 1963 and present-day ground water
typically contains \<1 to 10 TU, seldom exceeding 50 TU (Clark and
Fritz, 1997).

The tritium/helium-3 (^3^H/^3^He) method is based on the radioactive
decay of ^3^H to ^3^He. Both tritium and helium are relatively inert
gases. Multiple sources of ^3^He are present in the environment such as
the Earth\'s mantle and atmosphere, fluid inclusions within rocks, and
excess air entrained in ground water during recharge (Schlosser, 1992).
This method separates ^3^He derived from ^3^H (tritogenic ^3^He) from
^3^He derived from natural sources. Apparent age estimates from the
^3^H/^3^He method can be extremely accurate (within months) for ground
water containing high ^3^H concentrations (waters recharged since 1963).
Unlike the chlorofluorocarbon dating method, ^3^H/^3^He is a valid
technique for sites contaminated with organic compounds.

# Carbon-14 (^14^C)

Carbon-14 (^14^C) is commonly referred to as radiocarbon and is
continually produced in the atmosphere through cosmic ray bombardment of
nitrogen nuclei (Bradley, 1985). ^14^C content in the atmosphere
increased almost twofold during the mid-1960\'s as a result of the
atmospheric testing of thermonuclear weapons (Clark and Fritz, 1997).
^14^C activities are expressed as a percent of modern carbon (pmC). With
a half-life of 5,730 years, ^14^C is useful for dating ground water that
is between 1,000 and 30,000 years old (Coplen, 1993). Diffusion and
geochemical reactions within an aquifer, however, can alter the ^14^C
activity. A geochemical reaction path model (Plummer and others, 1983;
1991; 1994) is used to account for dilution and addition of carbon along
ground-water flowpaths, which improves the accuracy of age
determinations.

# References

Busenberg, Eurybiades, and Plummer, L.N., 1992, Use of
chlorofluorocarbons (CCl3F and CCl2F2) as hydrologic tracers and
age-dating tools: The alluvium and terrace system of central Oklahoma:
Water Resources Research, v.28, no. 9, p. 2257-2283.

Derra, S., 1990, CFCs\--No easy solutions: Research and Development, v.
32, p. 54-66.

Molina, M., and Rowland, F.S., 1974, Stratospheric sink for
chlorofluoromethanes: Chlorine atom catalyzed destruction of ozone:
Nature, no. 249, p. 810-812.

Plummer, L.N., and Busenberg, Eurybiades, 1999, Chlorofluorocarbons, in
Cook, P.G., and Herczeg, Andrew, eds., Environmental tracers in
subsurface hydrology: Klumer Academic Press (in press).

Bradley, R.S., 1985, Quaternary paleoclimatology: Boston, Unwin Hyman,
472 p.

Clark, I.D., and Fritz, Peter, 1997, Environmental isotopes in
hydrogeology: New York, Lewis Publishers, 328 p.

Coplen, T.B., 1993, Uses of environmental isotopes, in W.M. Alley, ed.,
Regional ground-water quality: New York, Van Nostrand Reinhold, p.
227-254.

Plummer, L.N., Parkhurst, D.L., and Thirstiness, D.C., 1983, Development
of reaction models for ground-water systems: Geochimica et Cosmochimica
Acta, v. 47, p. 665-686.

Plummer, L.N., Prestemon, E.C., and Parkhurst, D.L., 1991, An
interactive code (NETPATH) for modeling net geochemical reactions along
a flow path: 91-4078, 227 p.

\_\_\_\_\_\_1994, An interactive code (NETPATH) for modeling net
geochemical reactions along a flow path, version 2.0: 94-4169, 130 p.

Clark, I.D., and Fritz, Peter, 1997, Environmental isotopes in
hydrogeology: New York, Lewis Publishers, 328 p.

International Atomic Energy Agency (IAEA), 1981, Statistical treatment
of environmental isotope data in precipitation: Vienna, IAEA, Technical
Report Series No. 206.

Michel, R.M., 1989, Tritium deposition in the Continental United States,
1953-1983: U.S. Geological Survey Water-Resources Investigations Report
89-4072, 46p.

Schlosser, Peter, 1992, Tritium/3He dating of waters in natural systems,
in Isotopes of noble gases as tracers in environmental studies: Vienna,
IAEA, p. 123-145.

# For more information contact:

David L. Nelms (804) 261-2630\
George E. Harlow, Jr. (804) 261-2631

*\
Web Sites:* Virginia District

[http://va.water.usgs.gov](http://va.water.usgs.gov/)\
\
VAS Project

<http://va.water.usgs.gov/vas>

(Not available at this time)\
\
USGS Reston CFC Laboratory

<http://water.usgs.gov/lab/cfc>