Hydrogen has two stable isotopes: protium (1H) and deuterium
(2H, D). Deuterium comprises 0.0184-0.0082% of all hydrogen
(IUPAC); ratios of deuterium to protium are reported relative to the VSMOW
standard reference water. A radioactive isotope, tritium (T or 3H)
is discussed at the end of the section.
Craig (1961a) observed that the d18O
and d2H (dD)
values of precipitation that has not been evaporated are linearly related
This equation, known as the "Global Meteoric Water Line" (GMWL), is based
on precipitation data from locations around the globe, and has an r2
value of better than 0.95. This high correlation coefficient reflects the
fact that the oxygen and hydrogen stable isotopes in water molecules are
intimately associated; consequently, the isotopic ratios and fractionations
of the two elements are usually discussed together. The slope and intercept
of any "Local Meteoric Water Line" (LMWL), which is the line derived from
precipitation collected from a single site or set of "local" sites, can
be significantly different from the GMWL. In general, most of these local
lines have slopes of 8 +/- 0.5, but slopes in the range of 5 and 9 are
Natural processes can cause waters to plot off the GMWL. Water that
has evaporated or has mixed with evaporated water typically plots below
the meteoric water line along lines that intersect the MWL at the location
of the original un-evaporated composition of the water; evaporation lines
with slopes in the range of 2 to 5 are common. Low temperature diagenetic
reactions involving silicate hydrolysis can sometimes cause increases in
the d18O and dD
values of waters.
The two main factors that control the isotopic signature of precipitation
at a given location are 1) the temperature of condensation of the precipitation
and 2) the degree of rainout of the air mass (the ratio of water vapor
that has already condensed into precipitation to the initial amount of
water vapor in the air mass). Most water vapor in the atmosphere is derived
from evaporation of low-latitude oceans. Precipitation derived from this
vapor is always enriched in D and 18O relative to the vapor,
with the fractionation between the rain and vapor a function of condensation
temperature. Therefore, progressive rain-out as clouds move across the
continent causes successive rain storms to become increasingly isotopically
lighter (more negative d values).
Precipitation is the ultimate source of ground water in virtually all
systems. Hence, knowledge of the factors that control the isotopic compositions
of precipitation before and after recharge allows the use of oxygen and
hydrogen isotopes as tracers of water sources and processes. On a regional
scale, the distribution of isotopic compositions are controlled by several
At a given location, the seasonal variations in d18O
and d2H values of precipitation and
the weighted average annual d18O
and d2H values of precipitation remain
fairly constant from year to year. This happens because the annual range
and sequence of climatic conditions (temperatures, vapor source, direction
of air mass movement, etc.) remain fairly constant from year to year. In
general, rain in the summer is isotopically heavier (more positive d
values) than rain in the winter. This change in average isotopic composition
is principally caused by seasonal temperature differences but is also affected
by seasonal changes in moisture sources and storm tracks. Shallow ground-water
d18O and d2H
values reflect the local average precipitation values but are modified
to some extent by selective recharge and fractionation processes that may
alter the d18O and d2H
values of the precipitation before the water reaches the saturated zone
(Gat and Tzur, 1967). Some of these processes include: evaporation of rain
during infiltration, selective recharge, interception of precipitation
by the tree canopy, and exchange of infiltrating water with atmospheric
vapor. In the case of snow, various post-depositional processes, such as
melting and subsequent infiltration of surface layers and evaporation,
may alter the isotopic content of the snowpack, often leading to meltwater
d values that become progressively enriched
Altitude effect: On the windward side of a mountain, the d18O
and dD values of precipitation decrease with
increasing altitude. Typical gradients are -0.15 to -0.5 ‰ per 100m for
18O, and -1.5 to -4 ‰ per 100m for D. This pattern is often
not observed in interior mountains, for snow, or on the leeward side of
Latitude effect: The d18O
and dD values decrease with increasing latitude
because of the increasing degree of "rain-out".
Continental effect: Isotopic ratios decrease inland from the coast.
Amount effect: The greater the amount of rainfall, the lower the
d18O and dD
values of the rainfall; this effect is not seen in snow.
Once the rain or snowmelt passes into the saturated zone, the d
values of the subsurface water change only by mixing with waters that have
different isotopic contents. The homogenizing effects of recharge and dispersive
processes produce ground water with d values
that approach uniformity in time and space, and approximate a damped reflection
of the precipitation over a period of years. It is important to remember
that although an individual storm may be large and isotopically very different
from the "old" water in the catchment, the amount of precipitation that
infiltrates will likely be small compared to the amount of "old" water
in storage. Although there may be significant storm-to-storm and seasonal
isotopic variations in precipitation, baseflow d-
values remain relatively uniform in most streams in humid, temperate areas.
In small catchments, temporal and spatial variability in ground-water
and baseflow d- values may reflect seasonal
variability in precipitation. However, these variations are less extreme
and usually delayed relative to the temporal variations that occur in precipitation
(Turner et al., 1987). These variations can be utilized for water residence
time calculations. Superimposed on the seasonal cycles in precipitation
are storm-to-storm and intra-storm variations in the d
18O and d D values of precipitation.
These variations may be as large as the seasonal variations. It is this
potential difference in d values between the
relatively uniform "old" water and variable "new" water that permits isotope
hydrologists to determine the contributions of old and new water to a stream
during periods of high runoff (Sklash et al., 1976).
Hydrogen isotopic ratios can be determined for many H-bearing compounds,
including organics, silicates, and gases. The d
D values of all these compounds are reported relative to VSMOW. For a brief
review of the compositions of these materials and the processes that affect
their compositions, see Hoefs (1987). A "dual" or "multi-isotope" approach
to determining sources or geochemical processes takes advantage of the
fact that compounds consist of several elements, many of which can be easily
analyzed for their isotopic ratios. For example, organic matter can be
analyzed for the major elements C, H, N, O, S -- and for trace elements.
Tritium (3H, T) is a radiogenic and radioactive isotope
of hydrogen with a half-life of 12.43 years (IAEA, 1981) which decays to
3He. It is an excellent tracer for determining time scales for
the mixing and flow of waters, and is ideally suited for studying processes
that occur on a time-scale of less than 100 years. Tritium content is expressed
in tritium units (TU) where I TU equals 1 3H in 1018
atoms of hydrogen. Prior to the advent of atmospheric testing of thermonuclear
devices in 1952, the tritium content of precipitation was probably in the
range of 2-8 TU (Thatcher, 1962); this background concentration is produced
by cosmic ray spallation. Since 1952, tritium produced by thermonuclear
testing ("bomb tritium") has been the dominant source of tritium in precipitation.
A peak concentration of several thousand TU was recorded in precipitation
in the northern hemisphere in 1963, the year that the Atmospheric Test
Ban Treaty was signed. After 1963, the tritium levels in precipitation
began to decline gradually because of radioactive decay, mixing into the
ocean, and the cessation of atmospheric testing.
Tritium concentrations in precipitation are affected by latitude, distance
from the ocean and season. Water vapor over the ocean has a low tritium
concentration due to exchange with oceanic surface water, which has a low
tritium content. With progressive movement of air masses across the continent,
tritium concentrations in precipitation increase due to additions of tritium
from the stratosphere and evapotranspiration. The seasonal effect is primarily
caused by the breakup of the tropopause between 30 and 60° N latitude
each spring, causing a leakage of stratospheric water vapor with a higher
tritium content into the troposphere. This leakage results in a north-south
gradient in tritium concentrations in precipitation, with higher concentrations
in the north and spring-summer high concentrating that are typically 2.5-6
times greater than the winter lows (Gat, 1980).
The simplest use of tritium is to check whether detectable concentrations
are present in the water. For waters with higher tritium contents, some
fraction of the water must have been derived after 1953; thus, the tritium
concentration can be a useful marker for recharge since the advent of nuclear
testing. There are a number of models in use for this application. The
simplest is the piston-flow method, which assumes that 3H moves
like a slug through the aquifer. To use the piston-flow approach, it is
necessary to know the direction of the flow and to collect samples along
the flow path; consequently, the main problem with this approach is the
assumption that no dispersion or mixing occurs as the water moves through
the aquifer. The opposite of this approach is the reservoir or box model,
which assumes that the system is the same everywhere, and that the mixing
of the reservoir occurs on a short time scale compared to the input (this
rarely holds true for ground-water systems). Compartment models strike
a balance between these two extremes. The aquifer is envisioned as a series
of compartments, with water coming in from other compartments or the boundaries,
becoming mixed internally, and flowing out to other compartments or discharge
points (see Przewlocki and Yurtsever, 1974).
For storm and snowmelt runoff separations, distinct "old" and "new"
water 3H values are required for storm and snowmelt runoff hydrograph
separation. In some larger catchments with residence times (the average
time it takes for precipitation to enter the ground and travel to the stream)
longer than a few months, old and new waters may be distinctive as a result
of the gradual decline in precipitation 3H values since 1963
and the even more gradual decline in ground-water 3H values.
Tritium measurements are frequently used to calculate recharge rates,
rates or directions of subsurface flow and residence times. For these purposes,
the seasonal, yearly and spatial variations in the tritium content of precipitation
must be accurately assessed (see Michel, 1989) and Plummer et al. (1993).
Further information can be found in Clark and Fritz (1997), Environmental
Isotopes in Hydrology (CRC Press):
Source of text: This review was assembled by Carol Kendall, Dan
Snyder and Eric Caldwell, primarily from Kendall et al. (1995) and Plummer
et al. (1993), and also the citations below.
||Coplen, T.B.(1993). "Uses of Environmental Isotopes", in Regional
Ground-Water Quality (Ed. W.M. Alley), Van Nostrand Reinhold, New York,
||Craig, H., (1961). "Isotopic variations in meteoric waters". Science,
||Fontes, J.-Ch.(1983). "Dating of groundwater", in Guidebook on Nuclear
Techniques in Hydrology, IAEA Technical Report Series 91, IAEA, Vienna.
||Gat, J.R. (1980). "The isotopes of hydrogen and oxygen in precipitation",
in: P. Fritz and J. Ch. Fontes (Eds.), Handbook of Environmental Isotope
Geochemistry, Elsevier, Amsterdam. pp. 21-47.
||Gat, J.R., and Tzur, Y., (1967). "Modification of the isotopic composition
of rainwater by processes which occur before groundwater recharge". Isotope
Hydrology, Proc. Symp. Vienna 1966, IAEA, Vienna, pp. 49-60.
||Hoefs, J. (1987). Stable Isotope Geochemistry, third edition,
Springer-Verlag, Berlin. 241 pp.
||International Atomic Energy Agency (IAEA) (1981). Statistical treatment
of environmental isotope data in precipitation, IAEA Technical Report
Series No. 206, IAEA, Vienna.
||International Atomic Energy Agency (1981). Stable Isotope Hydrology,
Deuterium and Oxygen-18 in the Water Cycle, (Eds. J. R. Gat and R.
Gonfiantini) Technical Reports Series No. 210., IAEA, Vienna.
||International Union of Pure and Applied Chemistry (IUPAC) (1991). "Isotopic
Compositions of the Elements". Pure & Appl. Chem., 63,
||Kendall, C., Sklash, M. G., Bullen, T. D. (1995). "Isotope Tracers of
Water and Solute Sources in Catchments", In: Solute Modelling in Catchment
Systems, J. Wiley & Sons, New York, pp. 261- 303.
||Michel, R. L. (1989). "Tritium deposition over the continental United
States, 1953-1983", In: Atmospheric Deposition, International Association
of Hydrological Sciences, Oxford, UK. pp.105-115.
||Payne, B. R. (1983). "Ground water salinisation", In: Guidebook on
Nuclear Techniques in Hydrology, 1983 Edition, IAEA, Vienna. pp. 351-57.
||Plummer, L. N., Michel, R. L., Thurman, E. M. and Glynn, P. D. (1993).
"Environmental tracers for age dating young ground water", In: W.M. Alley
(Ed.), Regional Ground-Water Quarterly, V. N. Reinhold, New York.
||Sklash, M.G., Farvolden, R.N., and Fritz, P., (1976). "A conceptual
model of watershed response to rainfall, developed through the use of oxygen-18
as a natural tracer". Can. J. Earth Sci., 13, 271-283.
||Stichler, W. (1987). "Snowcover and snowmelt process studies by means
of environmental isotopes", In: H.G. Jones and W.J. Orville-Thomas (Eds.),
Seasonal Snowcovers: Physics, Chemistry, Hydrology D. Reidel Publishers,
||Thatcher, L. L. (1962). "The distribution of tritium fallout in precipitation
over North America". International Association, Hydrological Sciences,
Publication No. 7, Louvain, Belgium. pp. 48-58.
||Turner, J.V., Macpherson, D.K., and Stokes, R.A., (1987). "The mechanisms
of catchment flow processes using natural variations in deuterium and oxygen-18."
In: "Hydrology and Salinity in the Collie River Basin, Western Australia."
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