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Kansas Geological Survey, Open-file Report 88-39
Great Plains and Cedar Hills Aquifers--Page 15 of 25

6.3.3 Isotope Studies

The use of isotopes in hydrologic studies is based on the premise that the relative mass abundance of light to heavy isotopes is large and that the occurrence of some natural process modifies the relative abundance of an element's isotopes in a system (Turan, 1982). Measurement of absolute isotope abundance is difficult; therefore, relative isotopic ratios are measured. The common method is to compare the abundance of the natural isotope with a standard. For example, 18O/16O of a sample is compared to the 18O/16O of SMOW (Standard Mean Ocean Water as established by the International Atomic Energy Agency). The relative abundance of is represented by the formula:

delta 18O = (Rsample/Rstandard - 1) x 1000

where R = 18O/16O. The symbol (delta) designates relative difference in concentration with units of permil (0/00). A value of delta 18O = -10 0/00 means that the sample contains 10 parts per thousand less 18O than the standard. Isotope values with a negative sign are considered isotopically depleted with respect to a standard; those samples with a plus sign are considered isotopically heavier with respect to a standard. The variation of sign for isotopes is useful in interpreting the type of environment the water has come from or migrated into or the type of water/rock interaction that has occurred along the flow path.

Each isotope such as deuterium (2H or D), and sulfur-34 (34S) is defined similarly. The only difference is the standard to which each is compared. Oxygen-18 and deuterium are compared to SMOW. Sulfur-34 is compared to the Canyon Diablo meteorite (Nielson,1979).

Fractionation by chemical and physical reactions is the process by which isotopes are separated and concentrated. Physical fractionation processes include rates of diffusion, evaporation, condensation, freezing, or melting. A phase change such as the condensation of rain separates isotopes by physical fractionation (Faure, 1979; Turan, 1982). Chemical fractionation occurs due to the different bond energies of light and heavy isotopes. Heavier isotopes concentrate in chemical compounds with the strongest chemical bonds. The strength of the chemical bonds is a function of vibrational frequency and relative velocity of the molecules. Lighter molecules have higher vibrational frequencies and therefore the bonds between molecules are easier to break. The heavier isotopes have lower vibrational frequencies (i.e. stronger bonding tendencies) and therefore concentrate in resulting products (Bigeleison, 1965; Turan, 1982).

Twenty-six samples were collected for determination of 18O, D, and 34S. The isotope values and designation letters are listed in Table 5. These isotopes were chosen for study because of their usefulness in determining the mixing of waters, cross-formational flow, and invasion of salt water into other formations.

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Kansas Geological Survey, Dakota Project
Original document dated December, 1988
Electronic version placed online April 1996
Scientific comments to P. Allen Macfarlane
Web comments to webadmin@kgs.ku.edu