Periodic Table--Helium

There are two stable isotopes of He, ³He (1.37 x 10^-4 %) and ⁴He (>99.99 %). Although ground waters contain ³He from several sources other than ³H decay (see below), determination of both ³H and tritogenic ³He (³He*) can be used as a dating tool (Tolstikhin and Kamensky, 1969; Torgersen et al., 1977; Takaoka and Mizutani, 1987; Schlosser et al., 1988). While decay of ³H to ³He commences in the unsaturated zone, dissolved ³He generated from ³H decay can be lost to the atmosphere. It is not until it enters the ground water that ³He is preserved because gas transport in the unsaturated zone is rapid relative to transport in ground water. Once isolated from the atmosphere below the water table, dissolved ³He concentrations will rise. For this reason, a ³H/³He age of t = 0 should occur at the water table, and ages should increase with depth as ground water becomes older. In the case of a fluctuating water table, t = 0 is set at the seasonal low water table position (Solomon et al., 1993).

The tritogenic ³He component is calculated by subtracting from the measured value that due to atmospheric solubility (a function of recharge temperature), excess air and radiogenic production (by decay of U/Th-series elements). These different sources of ³He are identified by measuring concentrations of ⁴He and other noble gases (Torgersen et al., 1977, 1979; Weise and Moser, 1987; Schlosser et al., 1989).

³H/³He ages accurately reflect hydraulic ages (subject to the uncertainties discussed above) in ground-water systems with piston flow and negligible mixing. However, the concentration ratio of ³H and ³He at any given point will not always accurately reflect the ground-water age because the concentration gradients are usually different and the diffiusion coefficient for ³He > ³H in water. Underestimates of ages of surface water bodies have been shown to be the result of diffusive loss of ³He to the atmosphere (Wuest et al., 1992), and will similarly lead to underestimates of ground-water ages (Schlosser et al., 1989). Analytical uncertainties usually result in errors in age estimates of less than 10% (Solomon et al., 1993). In addition to dating waters, ³He levels have also been shown to be a function of the vertical-flow velocity (recharge rate) and dispersivity (Schlosser et al., 1989).

Radiogenic stable ⁴He has been used to locate areas of deep ground water discharge. High ⁴He concentrations are common in deep ground waters as a result of subsurface a-decay of natural U- and Th-series elements in rocks and sediments. Mass transfer between aquifer solids and ground water occurs, resulting in a relationship between ground-water travel time (or contact time) and the ⁴He concentration in ground water. If the ⁴He solid-to-liquid mass transfer rate is known, it is possible to use ⁴He as a quantitative tracer of ground-water travel times (e.g. Marine, 1979; Andrews and Lee, 1979; Torgersen, 1980; Stue et al., 1992). If the solid-to-liquid mass transfer rate is governed by the radioactive decay of U and Th, significant (i.e. measurable) amounts of radiogenic ⁴He will exist in ground water after a contact time on the order of 1000 years and can theoretically continue to accumulate for millions of years. Thus, ⁴He has generally been considered to be a hydrologic tracer over time scales of 10³ to 10⁶years (e.g. Mazor and Bosch, 1992); however, a recent study by Solomon et al. (1996) has shown that some aquifer solids are currently losing ancient ⁴He (previously trapped within crystal lattices) at rates that are controlled by solid state diffusion. If quantified, such high solid-to-liquid mass transfer rates allow ground-water dating with ⁴He for water as young as 10 years and possibly as old as 10³ years.

Ground water ages using ⁴He are calculated by determining the solid-to- liquid mass transfer rate, and then measuring the ⁴He content of ground-water samples. The solid-to-liquid mass transfer rate can be determined directly in the laboratory or by calibration using ground-water age data obtained using other (i.e. CFCs or ³H/³He) dating methods. Once the solid-to-liquid mass transfer rate has been estimated, ground-water ages are determined by measuring total He and Ne in ground-water samples, and computing the component of radiogenic ⁴He. Because ground water contains ⁴He from the atmosphere as well as that released from solid phases, it is necessary to separate radiogenic from atmospheric ⁴He.

The uncertainty in ground-water ages determined using radiogenic ⁴He results mostly from uncertainty in the solid-to-liquid mass transfer rate. While the age uncertainty using ⁴He is much greater than with CFCs, ³H/³He, or ⁸⁵Kr, the potential for dating waters in the range of 50 to 1000 (where no other dating methods are practical) makes the use of radiogenic ⁴He attractive.

• Groundwater Dating with ³H-⁴He
• ⁴He and Old Ground Water

Source of text: This review was assembled by Eric Caldwell and Dan Snyder, primarily drawing from Solomon et al., 1998.