Oxygen has three stable isotopes: 16O
(99.63%), 17O (0.0375%) and 18O
(0.1995). Ratios of 18O to 16O
in waters, rocks, and most solutes are reported in ‰ (permil) relative
to Vienna Standard Mean Ocean Water (VSMOW).
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
> 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 not uncommon.
Several processes 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; slopes in the range
of 2 to 5 are common. Geothermal exchange also increases the 18O
content of waters and decreases the 18O
content of rocks as the waters and rocks attempt to reach a new state of
isotopic equilibrium at the elevated temperature. This causes a shift in
the d18O values,
but not the dD values of geothermal waters.
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 character of precipitation
at a given location are the temperature of condensation of the precipitation
and 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
lighter. For example, non-equilibrium evaporation from the ocean with a
d18O = 0‰ produces
vapor of -12‰. Later equilibrium condensation of rain from this vapor results
in water with a d18O
= -3‰ and residual vapor with a d18O
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 dD values of precipitation and the weighted
average annual d18O
and dD 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 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 dD 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 dD 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 (Stichler, 1987).
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 mountains.
Latitude effect: The d18O
and dD values decrease with increasing latitude
because of the increasing degree of "rain-out".
Continental effect: The 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 that 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
variations in precipitation d18O
and dD values, 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 d values. 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 d values are
storm-to-storm and intra-storm variations in the d18O
and dD 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).
Oxygen isotope ratios can be determined for many O-bearing compounds,
including organics, silicates, carbonates,
phosphates, nitrates, sulfates, and gases. The d18O
values of all these compounds are reported relative to VSMOW with the exception
of carbonates, which are often reported relative to Vienna Pee Dee Belemnite(VPDB).
Oxygen isotopic ratios can be determined for many O-bearing compounds,
including organics, silicates, carbonates, phosphates, nitrates, sulfates,
and gases. 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.
Nitrate-18O: The recent development
of methods to analyze nitrate for d18O
should expand the use of isotope techniques to trace nitrate sources and
transformations. Nitrate d18O
values show a wide range of values, ranging from about -10 to +80 ‰. Many
nitrate sources have distinctive d18O
and d15N values;
in particular, nitrate derived from atmospheric or nitrate fertilizer sources
is easily distinguished from nitrified ammonium and urea. A figure
showing the approximate ranges of compositions of many compounds is
available. A useful application of the "dual isotope approach" is the determination
of the relative contributions of atmospheric and microbial-derived sources
of nitrate in shallow ground water (Durka et al, 1994) and stream water
(Kendall et al., 1995). Amberger and Schmidt (1987) showed that denitrification
results in enrichment in 18O of the residual
nitrate, as well as enrichment in 15N.
Therefore, analysis of both d15N
and d18O of
nitrate should allow denitrification effects to be distinguished from mixing
of sources. This dual isotope approach takes advantage of the observation
that the ratio of the enrichment in 15N
to the enrichment in 18O in residual nitrate
during denitrification appears to be about 2:1 (Amberger and Schmidt, 1987).
For more information about nitrate-d18O,
Sulfate-18O: Use of a dual isotope
approach to tracing sources of sulfur (i.e. measurement of d18O
and d34S of
sulfate) will often provide better separation of potential sources of sulfur
and, under favorable conditions, provide information on the processes responsible
for sulfur cycling in the ecosystem. The rate of oxygen isotope exchange
between sulfate and water is very slow at normal pH levels. Even in acidic
waters, the "half-life" of exchange is on the order of 1000 years. Depending
on the reaction responsible for sulfate formation, between 12.5 to 100%
of the oxygen in sulfate is derived from the oxygen in the environmental
water; the remaining oxygen comes from O2
(Taylor et al., 1984). Reviews of applications of d18O
of sulfate include Holt and Kumar (1991) and Pearson and Rightmire (1980).
Source of text: This review was assembled by Carol Kendall, Eric
Caldwell and Dan Snyder, primarily from Kendall et al. (1995) but also
from the references below.
||Amberger, A., and Schmidt, H.-L. (1987). "Naturliche Isotopengehalte
von Nitrat als Indikatoren fur dessen Herkunft" (Natural isotope abundance
of nitrate as an indicator of its origin). Geochim. et Cosmochim. Acta,
||Bottcher, J., Strebel, O., Voerkelius, S. and Schmidt, H.L. (1990).
"Using isotope fractionation of nitrate-nitrogen and nitrate-oxygen for
evaluation of microbial denitrification in a sandy aquifer", J. Hydrol.,
||Coplen, T.B. (1993) "Uses of Environmental Isotopes", In: W.M. Alley
(Ed.), Regional Ground-Water Quality , Van Nostrand Reinhold, New
York, pp. 227-254.
||Craig, H., (1961). "Isotopic variations in meteoric waters". Science,
||Durka, W., Schulze, E.-D., Gebauer, G., and Voerkelius, S. (1994). "Effects
of forest decline on uptake and leaching of deposited nitrate determined
from 15N and 18O
measurements". Nature, 372: 765-767.
||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, 49-60.
||Hoefs, J. (1987).Stable Isotope Geochemistry, Third Edition,
Springer-Verlag, Berlin, 241 p.
||Holt, B.D. and Kumar, R. (1991). "Oxygen isotope fractionation for understanding
the sulphur cycle." In: H.R. Krouse and V.A. Grinenko (Eds.), Stable
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||Kendall, C., Campbell, D.H., Burns, D.A., Shanley, J.B., Silva, S.R.,
and Chang, C.C.Y. (1995). "Tracing sources of nitrate in snowmelt runoff
using the oxygen and nitrogen isotopic compositions of nitrate: pilot studies
at three catchments", In: K. Tonnessen, M. Williams, and M. Tranter (Eds.),
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of Hydrol. Sci. Pub., July 3-14, 1995, Boulder CO, IAHS Publ. # 228, pp.
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||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
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sulfur compositions of sulphate in acid mine drainage: evidence for oxidation
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