Isotopic Fractionation During Evaporation and Some Hydrological Applications

Evaporation is a physical process in which energy-loaded water molecules move from the water phase into the vapor phase. Isotopically light water molecules evaporate more efficiently than the heavy ones. As a result, an isotopic fractionation occurs at partial evaporation of water: the vapor is enriched in light water molecules, reflected in relatively negative dD and d18O values. In contrast, the residual water phase becomes relatively enriched in the heavy isotopes, reflected in more positive dD and d18O values. The isotopic separation, or fractionation, is more efficient if the produced vapor is constantly removed, as for example by wind blowing away vapors produced above an evaporating pond. Figure 9.1 presents the composition of water samples successively collected at a pond in Qatar (Yurtsever and Payne, 1979). The composition of the samples is plotted on a dD-d18O diagram. A progressive enrichment in the heavy isotopes is noticed, indicating that the residual water in the pond was progressively enriched in dD and d18O as the isotopically light vapor was removed. The original water had a composition of dD = 0% and d18O = — 1.4%, and the last sample reached values of dD = 33% and d18O = 5.6%. The line connecting the sample points is called the evaporation line, and its slope is determined primarily by the prevailing temperature and air humidity.

Fig. 9.1 Isotopic composition of water samples successively collected at an evaporating pond at Qatar. The water in the pond became progressively heavier in its composition. (From Yurtsever and Payne, 1979.)

In surface water bodies that evaporate to a very great extent, the isotopic enrichment ceases or is even reversed. However, except in very rare cases, natural water bodies, such as lakes or rivers, are only partially evaporated in the range in which isotopic enrichment of the residual water occurs.

Isotopic fractionation during evaporation causes fractionation during cloud formation: the vapor in the clouds has a lighter isotopic composition than the ocean that supplied the water. Upon condensation from the cloud, during rainformation, the reverse is true: the heavy water molecules condense more efficiently, leaving the cloud residual vapor depleted of deuterium and 18O.

Residual evaporation water is thus tagged by high dD and dD18O values, an observation used to trace mixing of evaporation brines with local fresh water. An example of two mineral springs located on the Dead Sea shore is given in Fig. 9.2. The Hamei Zohar and Hamei Yesha springs are seen to lie on a fresh water-Dead Sea mixing line, indicating that these springs contain recycled Dead Sea water brought to the surface with the emerging fresh water recharged at the Judean Mountains (Gat et al., 1969). From the information included in Fig. 9.2, one can calculate the percentage of Dead Sea brine intermixed in the Hamei Yesha and the Hamei Zohar springs.

At Orapa, northern Botswana, a new well field of confined water was developed and two hypotheses were proposed to explain the origin of

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—r 1 1 r 1 1


Dead Sea ~


___ J» -

fresh woters ^^

/ Homei

• Zohar •

Hamei Yesha

1 1 1 1 1 1 l._L

Fig. 9.2 Stable hydrogen and oxygen isotopic composition of the Hamei Zohar and Hamei Yesha mineral springs, Dead Sea shores. The linear correlation indicates that the springs' water is formed by intermixing of Dead Sea brine with local fresh water. (From Gat et al., 1969.)

recharge: either underground flow of recharge water from lakes and rivers 45 km distant, or local rain. The consultants of the local diamond mine ruled out local recharge and favored replenishment from the lakes. Results of an isotopic composition survey, depicted in Fig. 9.3, show the dD and dD18O values of the groundwater in the wells, the average annual rain composition in neighboring meteorological stations, and the lakes and rivers mentioned.

Fig. 9.3 Isotopic data of confined groundwater at Orapa, Botswana (O), average annual rain in neighboring meteorological stations (letters), and 45-km-distant lakes (A) and rivers (x). (From Mazor et al., 1977.)

The dD and d18O values of the groundwater in the wells were observed to be significantly lighter (more negative) than in the lake and river waters, which were enriched in the heavy isotopes due to intensive evaporation losses. Thus the hypothesis of recharge from the lakes could be ruled out, and recharge by local rain was supported. However, a slight but analytically significant difference can be seen between the composition of average annual rain and local groundwater. This was explained by an observation that only intensive rains are effectively recharging the groundwater, and these were observed by Vogel and Van Urk (1975) to have an isotopic composition that is lighter than average annual rain composition (the amount effect, section 9.6).

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