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Abstract
Environmental isotopes can be divided into stable and unstable (radioactive) species. In nature, the number of stable isotopes may exceed 300, while over 1700 radionuclide have been identified (Clark and Fritz, 1997). Isotopes are atoms of the same element (the same number of electrons and protons) that have different numbers of neutrons.
Environmental isotopes of hydrogen (H), oxygen (O), carbon (C) nitrogen (N) and sulphur (S) are of great importance to study the geological, hydrogeologic and biological systems( Fritz and Fontes, 1980, Hoefs, 1987 and Clark and Fritz,1997).Because of their relatively conservative nature, oxygen-18, deuterium (D or 2H) and tritium( 3H ) are considered good tracers for reconnaissance of water movement, mechanisms of recharge (origin, time and rate of recharge), geochemical processes such as groundwater salinization and aquifers interconnection as well as the relation between surface water and groundwater bodies (IAEA,1981). The information provided by isotope tools is valuable to assess and manage the groundwater resources, especially in areas where long term series of observation data are missing.
2.11.2- Isotopes fractionation processes
The partitioning of isotopes between two substances with different isotopes ratios is called isotope fractionation. Fractionation arises from the differences in the diffusive velocities between isotopes (Hoefz, 1987 and Clark and Fritz, 1997).
Two different types of processes, equilibrium and kinetic isotopes effects, cause isotope fractionation (Kendall et al.(1995).Equilibrium isotope-exchange reactions are characterized by redistribution of isotopes of an element between different chemical substances or between different phases. At equilibrium, the forward and back ward reaction rates of any particular isotope are identical. This does not mean that the isotopic compositions in the two phases are identical, but only that the ratios of the different isotopes in each phase at equilibrium are identical.
A natural example is the fractionation between oxygen (16O & 18O) and hydrogen (1H & 2H) isotopes in the water vapor of a cloud and the raindrops released from that cloud. As water vapor condenses (an equilibrium process), the heavier water isotopes (18O and 2H) become enriched in liquid phase while the lighter isotopes (16Oand 1H) tend towards the vapor phase. Under equilibrium condition, when the relative humidity is about 100 %, the isotopic compositions of vapor and liquid phases is as fallows (Olive, 1995 ):
δ 18O (vapor) = δ 18O (liquid) – 10 ‰
and
δ 2H (vapor) = δ 2H (liquid) - 80 ‰
where -10 ‰ and – 80 ‰ are the enrichment factors ( ∑ ).
This means that the liquid phase will be more enriched in δ 18O and δ D than vapor phase. It is important here to mention that the slope of the straight meteoric water line ( Craige,1961), which equals 8, expresses the condensation process of water vapor in clouds and its falling as rains under equilibrium conditions. This could be deduced from the following relation:
∑ D 80
-------- = = 8 - - - - - - - - live 1995
∑ 18O 10
Kinetic (non equilibrium) isotope fractionations occur in systems where the forward and backward reaction rates are not identical (Kendall, et al, .1995), i.e. the reaction is unidirectional and depends on atomic masses. Thus, the lighter isotopes react more and become concentrated in the products, and the residual reactants become enriched in the heavy isotopes. Biological processes are considered unidirectional and are good examples of kinetic isotope reactions, and so are evaporation processes when the relative humidity is less than 100 %.
Two consecutive isotopic enrichments, or fractionations, are evident during evaporation, one is caused by the liquid-vapor phase change (Known as the equilibrium fractionation effect) and the second takes place during vapor transport (Kinetic fractionation effect).
These processes have been described by Francey and Tans(1987) and Walker et al.(1989). The intercept of
the meteoric water line with deuterium axe, in Craig’s equation, represents the deuterium excess ”d”:
Where:
d = δ D – 8 δ 18O ‰ - - - - - - - - - - - -Dansgaard (1964)
Kinetic fractionation occurring during the evaporation process (kinetic evaporation) of ocean water causes the deuterium excess. Jozel et al. (1982) proved that this value equals + 10 ‰, where the average relative humidity in atmosphere above ocean equals 80 %. In case of evaporation of water of closed sea, the d-excess value is found to be + 22 ‰ (Gat and Carami, 1970), however, the slope of meteoric water line is still constant (i.e. = 8).
2.11.3– δ- Notation for expressing stable isotope ratio
variations
The Greek letter is generally used to express the isotopic composition of a water sample. It expresses the relative difference in the ratio of heavy isotope to the more abundant light isotope of the sample with respect to a reference called Standard Mean Ocean Water (SMOW) as fallows:
R (sample) – R (standard)
δ in ‰ = ------------------------------ × 1000
R (standard)
Where:
R: represents the isotope ratio of the heavy isotope to the
more abundant
Light isotope of the water sample R = 2H / 1H for
Deuterium or
18O / 16O for oxygen -18
‰ : means parts per thousand.
2.11.4- Stable isotopes in precipitation and d-excess
The collected precipitation data from different stations around the globe were analyzed for oxygen-18 and deuterium isotopes. Craig (1961) has observed that δ18O and δ D values are linearly related by: the following equation:
δ D = 8 δ 18O + 10 ‰. This relationship is represented by the regression line shown in Fig. (31) and is described as” Meteoric Water line”. The numerical coefficient 8 and the constant 10 (d -excess) depend on the evaporation, vapor transport and precipitation conditions. Based on values of d-excess, divided the rain waters in Negev desert into three groups. The two extreme groups, are has d-excess values above 22 ‰ and the other has values less than 10 ‰, have quite different origins. The intermediate group represents the combination of the two extreme groups. The study of the deuterium excess, introduced by Dansgard (1964) may help in understanding the prevailed climatic conditions. The d-excess value is primarily governed by the relative humidity of the air mass at its origin (”VanDer Straoten and Mook, 1983). Partial evaporation of raindrops during their fall, with a non equilibrium fractionation, may cause enrichment of heavy isotopes in rainfall and a decrease in the ”d” values. Datta et al. (1991) stated that the d-values may be not only governed by the composition of air mass from which the rainfall derives, but also by the intensity and distribution of rainfall.
The enrichment of 18O and 2H in return flow water will follow a slope of about 2.2 to 5.5 depending on the relative humidity. (Clark, 1997) .Water that has evaporated or mixed with evaporated water plots below MWL on a line with aslope less than 8 (Coplen, 1993). During extensive evaporation from
Soil samples represent different locations that are actually irrigated with
different water types in the area. They were taken at different depth intervals depending on the depth of root zone. Then, they were air-dried and crushed to pass through a 2 mm screen.