Further Checks on Atmospheric Noble Gas Retention Warm Groundwater

The closed system conditions seen in cold groundwater systems may be further checked in warm springs, that is, in waters that are trapped at a depth of 1-2 km and get heated by local (normal) heat gradients. The first study of this type was done on warm springs and wells (up to 60 °C) of the Jordan Rift Valley (Mazor, 1972). The isotopic compositions of the encountered dissolved Ne, Ar, Kr, and Xe were found to be atmospheric, that is, very close (in the range of the analytical accuracy) to the values given in Table 13.2.

The atmospheric noble gases are presented in Fig. 13.5 in a fingerprint diagram. An overall similarity of the line patterns to those of air-saturated water at 15-25 °C is clear. Thus the relative abundances of the four noble

Fig. 13.5 Noble gases dissolved in thermal waters in the Jordan Rift Valley. (Data from Mazor, 1972.) The nonsaline waters lie between the values of air-saturated water (ASW) at 15 and 25 °C, demonstrating the meteoric (atmospheric) origin and the closed-system conditions that prevailed in the ground. The saline Tiberias hot water is suggested to have originated by saline water entrapment, a hypothesis supported by the lower noble gas content (Mazor, 1972).

Fig. 13.5 Noble gases dissolved in thermal waters in the Jordan Rift Valley. (Data from Mazor, 1972.) The nonsaline waters lie between the values of air-saturated water (ASW) at 15 and 25 °C, demonstrating the meteoric (atmospheric) origin and the closed-system conditions that prevailed in the ground. The saline Tiberias hot water is suggested to have originated by saline water entrapment, a hypothesis supported by the lower noble gas content (Mazor, 1972).

gases Ne, Ar, Kr, and Xe indicate they originated from air dissolved in the recharge water. The fingerprint lines in Fig. 13.5 are parallel to each other, but vary in their height on the concentration axis. What might be the explanation for this? The answer is differences in the recharge areas and, hence, in the initial noble gas concentrations due to differences in altitude (temperature and pressure). In one case, the Tiberias Hot Springs, the initial salinity played a significant role.

Let's have a closer look at the Tiberias Hot Springs case. It is lowest in Fig. 13.5, revealing the lowest noble gas concentrations. However, it has a salt concentration almost equal to seawater. So the initial noble gas concentration was as in seawater—30% less than in fresh water under similar conditions. Hence, to normalize the Tiberias Hot Springs line in Fig. 13.5 and compare it with fresh water samples, the values have to be increased by 30%. In doing so, the Tiberias Hot Springs line falls right in the middle of the other Rift Valley warm waters, close to ASW 15 °C, agreeing with closed system conditions. The noble gases thus provide independent support of the origin of the Tiberias Hot Springs from saline water.

As noble gas concentrations are obtained, the hydrochemist is always confronted with the need to establish whether the data reflect the indigenous noble gas concentrations, or whether reequilibration with the atmosphere occurred prior to sampling. The problem is acute in springs: the samples are collected in the spring as deep as possible below the water surface, but did the water reequilibrate while flowing below gravel, for example? Or did the water circulate in the spring's outlet? An answer may be obtained by comparing the observed noble gas concentration with the concentration calculated for equilibration with air at the emergence temperature of the spring.

Example: A spring emerges at 34 °C and is found to contain 3.8 x 10~4 cc STP Ar/cc water. Is it oversaturated, that is, does it contain more gas than expected from air equilibration at the emergence temperature? The air-water solubility at 34 °C is seen in Fig. 13.1 to be 2.5 x 10~4 cc STP Ar/cc water. Hence, the observed value of 3.8 x 10~4 cc STP Ar/cc water is oversaturated by

of the equilibration value at the temperature of emergence. (The observed value of Ar concentration indicates a recharge temperature of 10 °C. Is this right?)

The observed krypton and xenon concentrations in each water source in the Rift Valley study were converted in this way to percent air saturation in Fig. 13.6. It is seen that the samples contain more than 100% air saturation—they are oversaturated, indicating reequilibration subsequent to heating did not occur. This diagram shows the importance of duplicate or triplicate sampling. The one low value of sample 3 is clearly wrong due to gas loss during sampling, the higher sample 3 values being more representative (and not the average).

Warm springs are a rigorous check of closed-system conditions for the atmospheric noble gases, as their waters circulate deep and their ages are relatively high. Figures 13.7 and 13.8 show results for another case, in Swaziland.

Fig. 13.6 Percentage of air saturation of krypton and xenon for the Jordan Rift Valley waters. The values were obtained by dividing the measured amounts by those expected for water equilibrated with air at the temperature at which each sample was collected. All samples, except one, were found to be air supersaturated, indicating that the rare gases were retained under closed system conditions. Differences in duplicate samples are attributed to gas losses to the atmosphere prior to sampling (Mazor, 1975).

Fig. 13.6 Percentage of air saturation of krypton and xenon for the Jordan Rift Valley waters. The values were obtained by dividing the measured amounts by those expected for water equilibrated with air at the temperature at which each sample was collected. All samples, except one, were found to be air supersaturated, indicating that the rare gases were retained under closed system conditions. Differences in duplicate samples are attributed to gas losses to the atmosphere prior to sampling (Mazor, 1975).

Fig. 13.7 Noble gas pattern of atmospheric noble gases in warm springs in Swaziland. The isotopic compositions were found to be atmospheric, and the abundance patterns, similar to ASW 25 °C, confirm this. The variations in concentrations are caused by different recharge altitudes (Mazor et al., 1974).

Fig. 13.7 Noble gas pattern of atmospheric noble gases in warm springs in Swaziland. The isotopic compositions were found to be atmospheric, and the abundance patterns, similar to ASW 25 °C, confirm this. The variations in concentrations are caused by different recharge altitudes (Mazor et al., 1974).

Fig. 13.8 Percent air saturation of krypton and xenon for warm springs in Swaziland (same spring numbers as in Fig. 13.7). All samples were oversaturated at the temperature of emergence (35-52 °C), indicating the systems were closed (Mazor et al., 1974).
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