Helium Loss or Gain Due to Phase Separation Underground or During Ascent to the Sampling Point Modes of Identification and Correction

Water in confined basins is characterized by high temperature, pressure, and dissolved gas concentrations. With the formation of a vapor or steam phase, helium is preferentially transferred into the gas phase, along with atmospheric Ne, Ar, Kr, and Xe. At the same time, the residual water is depleted in the noble gases. An example from the Great Artesian Basin is given in some detail: The samples were collected by the same field team that provided samples for the work of Torgersen and Clarke (1985), our samples being collected in helium-tight glass tubes with spring-loaded high-vacuum stopcocks at their ends. The noble gas concentrations are seen to vary over a wide range (Fig. 14.2). Percent retention of atmospheric Ne, Ar, Kr, and Xe was calculated compared to 15 °C air-saturated recharge water (Fig. 14.3). Retentions as low as 16% are seen, indicating gas was lost. In contrast, cases with over 100% retention are observed, indicating gas was gained.

These significant changes in gas concentrations are not surprising, as the confined waters of the Great Artesian Basin are rich in CO2 and

Table 14.2 Discordant Groundwater Ages (years) in the East Midlands Triassic Aquifer, U.K.

Well

Tritium

Hydraulic

Far Baulker 2 Ompton 3 Halam 3 Markham 3 Grove 1 Newton 1 Newark 1

220-285 240-320

0-2000

1500-5000

0-4500

3500-7000

7000-11,000

23,000-28,000

3000-8000

5000-14,000

5000-14,000

6000-16,000

20,000-56,000

21,000-60,000

x 10 hydraulic age x 10 hydraulic age

Source: Data from Andrews et al. (1984).

\4022

\4022

He x 10"' Ne x 10"s Ar x 10Kr x 50"e Xc x 10"'

Fig. 14.2 Noble gas concentrations in water samples from the confined part of the Great Artesian Basin, with well numbers (following Mazor and Bosch, 1992b). The concentrations of atmospheric Ne, Ar, Kr, and Xe and of nonatmospheric He vary over a wide range, preserving the relative abundances as in air-saturated water at 15 °C (ASW 15 °C). A distinct enrichment of nonatmospheric (radiogenic) He is seen, correlated to the concentration of the atmospheric noble gases.

He x 10"' Ne x 10"s Ar x 10Kr x 50"e Xc x 10"'

Fig. 14.2 Noble gas concentrations in water samples from the confined part of the Great Artesian Basin, with well numbers (following Mazor and Bosch, 1992b). The concentrations of atmospheric Ne, Ar, Kr, and Xe and of nonatmospheric He vary over a wide range, preserving the relative abundances as in air-saturated water at 15 °C (ASW 15 °C). A distinct enrichment of nonatmospheric (radiogenic) He is seen, correlated to the concentration of the atmospheric noble gases.

Ne Ar Kr Xe

Fig. 14.3 Percent retention of the atmospheric noble gases of the Great Artesian Basin samples included in Fig. 14.2 (compared to air-saturated water at 15 °C at sea level). Significant gas losses are seen. (From Mazor and Bosch, 1992a.)

Ne Ar Kr Xe

Fig. 14.3 Percent retention of the atmospheric noble gases of the Great Artesian Basin samples included in Fig. 14.2 (compared to air-saturated water at 15 °C at sea level). Significant gas losses are seen. (From Mazor and Bosch, 1992a.)

methane and attain temperatures of over 90 °C. Hence, samples collected at the (free-flowing) well head are likely to lose gases during ascent, and gas separation may also occur underground as a result of pressure drops related to the intensive abstraction. The separated gases, forming gas pockets or compartments, are tapped by some wells, producing water enriched by noble gases.

The original helium concentration in the confined water can be reconstructed by dividing the observed concentration by the fraction of gas retained, as calculated from the accompanying atmospheric gases. These corrected helium concentrations, plotted as a function of the distance from the recharge area (Fig. 14.4), reveal a reasonable correlation, which provides confirmation of the reliability of the corrected helium values.

The observation of older groundwaters in the deeper parts of the Great Artesian Basin should not be mistaken as indicating inward flow. The older ages are in good agreement with the evolution of a subsidence basin— the inner parts are buried and disconnected from the groundwater through-flow regime for longer time periods.

Fig. 14.4 Helium concentration (corrected for gas losses) as a function of distance from the recharge area along two transects through the Great Artesian Basin (following Mazor and Bosch, 1992a). The good correlation provides a positive check on the validity of the corrected helium data, as the deeper inland groundwater must have been trapped earlier.

Fig. 14.4 Helium concentration (corrected for gas losses) as a function of distance from the recharge area along two transects through the Great Artesian Basin (following Mazor and Bosch, 1992a). The good correlation provides a positive check on the validity of the corrected helium data, as the deeper inland groundwater must have been trapped earlier.

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