Oxygen 18 (18O)
The stable isotopes of water, O-18 and deuterium, have been commonly used in hydrologic studies (we are using
18O only due to the problems with alalyzing deuterium by conventional methods due to the samples extremely high ion concentrations).
They are used by hydrologists because they are naturally occurring, do not decay over time, and are conservative within the watershed.
18O and deuterium isotopic concentrations vary seasonally, and are observed precipitation. Isotopes undergo fractionations according to mass
differences during phase changes. Isotopically heavier precipitation occurs as rain (typically ~ -3‰ - 0‰ for O-18), while snow is
dramatically lighter (~ -20‰, O-18). By recognizing the isotopic variations of source waters, they can be distinguished from each other
using analytical tools such as hydrograph separation. Stable isotopes can be used within a mine system by giving information about
precipitation inputs into the subsurface mine workings. A well mixed, groundwater dominated system will not vary much isotopically,
while a system (or source) receiving seasonal input (event water) will vary considerably more. The significance in determining
this when approaching a mine from a remediation standpoint is the applicability of remediation. If the mine, particularly the source of
contamination, is dominated by groundwater, the remediation approach would be considerably different than a system which has a large
component of event water.
Note that the values:
1.Get lighter with increasing latitude;
2.Get lighter toward the continental interior;
3.Demonstrate sharp changes in the mountain areas, notably the Sierra Nevada of California. Apart from temperature/elevation effects,
the windward side of a range receives precipitation enriched in 18O and 2H because of the rainout effect.
Background
Mass differences cause variations in physical and chemical properties of an element. These mass differences can cause mass dependent
fractionation to occur for elements of low atomic numbers. Fractionation is caused by two processes:
kinetic fractionation and equilibrium fractionation. The strength of the chemical bonds of isotopes drives their
fractionation. Fractionations can be described by differences between their zero point energies (ZPE). There is
about a 2kcal/mole difference in ZPE associated with the breaking of a H-H bond compared with a D-D (deuterium)
bond. These quantum effects are present at low temperatures, but disappear at high temperatures.
O-18 values vary spatially and temporally, as shown by the above figure. The driving force in these variations is
temperature, which affects precipitation amount and type. Isotopic distribution factors include:
- Altitude/Latitude/Continental/Amount effect: On the windward side of a mountain, the δ18O and δD 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.
In any region with even minor relief, orographic precipitation will occur as a vapour mass rises over the landscape.
Rainout proceeds as the air mass cools, imparting a decrease on precipitation. At higher elevation air cools
adiabatically (by expansion), thus driving rainout. At higher altitudes where the average temperatures are lower,
precipitation will be isotopically depleted. For 18O, the depletion varies between about –0.15 and –0.5‰ per 100-m
rise in elevation.
As the cloud moves inland, isotopically heavier water is preferentially precipitated, and the resulting precipitation
will be depleted as well, as there is less and less "heavy" isotope, O-18. The continental and amount effect is a result of this.
Meteoric Water Line
The "meteoric water line" (GMWL) as calculated by Craig (1961). Warm regions are characterized by more enriched
(positive) values of hydrogen and oxygen isotopes and cooler regions characterized by more depleted (negative)
values. Isotope ratios are reported with respect to Standard Mean Ocean Water (smow). Oxygen and hydrogen, being the
constituients of water, are natural isotopes of hydrolgical inquiry. The two fractionate similarly, and the relationship
is provided by the GMWL. Samples which fall below the GMWL often result from evaporation.
Isotope Distribution
An example on the distribution of O-18 globally, from the IAEA.
The distribution pattern remains much the same throughout the year, though the values change in the mountains as precipitation
type changes. Snow is the isotopically lightest with regards to O-18, as the heavier isotope generally preferentially accumulates
in the species with the higher oxidation state, often in the more dense of material (i.e. solid>liquid>gas, for water). So snow
will have a dramatically different value from the same location than rain will (snow=~-20‰, rain=~0‰).
O-18 in Research
O-18 is used as a tracer in many hydrologic studies. It is most often used as a component in a mixing-model and
hydrograph separation, as O-18 acts conservatively and is applied naturally and uniformly over broad areas. O-18
can be used when different sources (old water/new water) have different isotopic values.
The model is essentially a mass-balance model using concentrations of inflow and outflow to establish the contributions
of each inflow to the final outflow.
Qstream = Qold + Qnew
Cstream Qstream =Cold Qold +Cnew Qnew
The mixing model can ascertain the importance of individual sources during different stages of runoff, highlighting sources that may not have
been seen as critical or diminishing the importance of others. Other tracers than O-18 can be used as well, as long as they are
conservative at ambient temperatures within the system.
