PUB00070

• Year: 1998
• Title: Ground Water and Surface Water A Single Resource U.S. Geological Survey Circular 1139
• CWCB Section: Interstate & Federal
• Author: T.C. Winter, J.W. Harvey, O.L. Franke, W.M. Alley
• Description: Overview of current understandimg of interaction of ground water and surface water in terms of quality and quantity
• Publications - Doc Type: Historical

G

Use of Environmental Tracers to Determine the
Interaction of Ground Water and Surface Water

Environmental tracers are naturally occurring dissolved
constituents, isotopes, or physical properties of water that
are used to track the movement of water through watersheds,
Useful environmental tracers include (1) common dissolved
constituents, such as major cations and anions; (2) stable
isotopes of oxygen esO) and hydrogen eH) in water
molecules; (3) radioactive isotopes such as tritium eH) and
radon (222Rn); and (4) water temperature, When used in
simple hydrologic transport calculations, environmental
tracels can be used to (1) determine source areas of water
and dissolved chemicals in drainage basins, (2) calculate
hydrologic and chemical fluxes between ground water and
surface water, (3) calculate water ages that indicate the length
of time water and dissolved chemicals have been present in
the drainage basin (residence times), and (4) determine
average rates of chemical reactions that take place during
transport. Some examples are described below,

Juday Creek, Indiana

Walker Branch, Tennessee

Major cations and anions have been used as
tracers in studies of the hydrology of small watersheds
to determine the sources of water to streamflow during
storms (see Figure G-1),ln addition, stable isotopes of
oxygen and hydrogen, which are part of water molecules,
are useful for determining lhe mixing of waters from different
source areas because of such factors as (1) differences
in the isotopic composition of precipitation among lecharge
areas, (2) changes in the isotopic composition of shallow
subsurface waler caused by evaporation, and (3) temporal
variability in the isotopic composition of precipitation
relative to ground water.
Radioactive isotopes are useful indicators of the
time that water has spent in the ground-water system, For
example, tritium eH) is a well-known radioactive isotope of
hydrogen that had peak concentrations in precipitation in the
mid-1960s as a result of above-ground nuciear-bomb testing
conducted at that time, Chlorofluorocarbons (CFCs), which

are industrial chemicals that are present in ground water less
than 50 years old, also can be used to calculate ground-water
age in different parts of a drainage basin,
222Radon is a chemically inert, radioactive gas that has
a half-life of only 3,83 days, It is produced naturally in ground
water as a product of the radioactive decay of 226radium in
uranium~bearing rocks and sediment. Several studies have
documented that radon can be used to identify locations of

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MARCH,1991

Figure G-1, The relative contributions of different
subsurface water sources to streamflow in a
stream in Tennessee were determined by
analyzing the relative concentrations of calcium
and sulfate, Note that increases in bedrock zone
(ground water) flow appear to contribute more to
the stormflow response at the downstream site
than to the storm flow response at the upstream
site in this small watershed, (Modified from
Mulholland, P,J., 1993, Hydrometric and stream
chemistry evidence of three storm ffowpaths in
Walker Branch Watershed: Journal of Hydrology,
v, 151, p, 291-316,) (Reprinted with permission
from Elsevier Science-NL, Amsterdam, The
Netherlands,)

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