Methods

The tritium/helium-3 method of dating ground water is well established (for example, Schlosser and others, 1989; Solomon and Cook, 2000). Thus, only a brief description of the method is given here.

Tritium (³H) is a radioactive isotope of hydrogen. Prior to 1953, tritium concentrations in western Oregon precipitation were about 3–5 tritium units (TU) (Thatcher, 1962); this range represented natural background (“pre-bomb”) concentrations prior to widespread atmospheric testing of thermonuclear weapons. Thermonuclear testing injected large quantities of tritium into the atmosphere, and tritium concentrations in precipitation increased markedly in the 1950s. Tritium concentrations peaked in the northern hemisphere in 1963, and have been steadily decreasing since that time. The half-life of tritium is 12.43 years. The decay product of tritium is helium-3 (³He).

Helium has two stable isotopes, helium-3 and helium-4 (⁴He). Most helium in the natural environment is helium-4; helium-3 concentrations are orders of magnitude smaller than helium-4 concentrations. Helium concentrations in ground water originate from several sources; these sources must be accounted for in order to estimate tritium/helium-3 apparent ages.

Water in contact with the atmosphere contains gases, including helium, as a result of equilibrium (Henry’s law) partitioning between atmosphere and water. In ground water, final solubility equilibration between water and air occurs as water reaches the water table. This source of gases to water is referred to as “air-water solubility.” Air-water solubility is a function of recharge temperature and recharge altitude. When recharge temperature and recharge altitude are well characterized, air-water solubility is well constrained.

Another source of gases to ground water is known as “excess air” (Heaton and Vogel, 1981; Busenberg and Plummer, 2000; Stute and Schlosser, 2000). Excess air is thought to result from a fluctuating water table entraining air bubbles or from recharge water trapping air bubbles near the water table. These air bubbles dissolve into ground water as ground water migrates below the water table and hydrostatic pressure increases. Excess air is nearly ubiquitous in ground water (Busenberg and Plummer, 2000). Gases, including helium and neon, in excess air commonly are assumed to be added to ground water in the proportions in which they exist in the atmosphere (that is, unfractionated) (Heaton and Vogel, 1981; Schlosser and others, 1989; Klump and others, 2008). Unlike helium, neon does not have significant subsurface sources, so neon concentrations in ground water are used, along with estimates of recharge temperature and altitude, to determine the air-water solubility and excess air components of helium in ground water.

Helium also is added to ground water from subsurface radiogenic sources. Radiogenic helium primarily is derived from uranium- and thorium-series decay reactions in the Earth’s crust. The helium-3/helium-4 ratio of radiogenic helium is about 2×10^-8 (Mamyrin and Tolstikhin, 1984).

Mantle helium also may be present in ground water, but probably is rare in young ground water (Solomon and Cook, 2000). The helium-3/helium-4 ratio of mantle helium is on the order of 3×10^-5 (Mamyrin and Tolstikhin, 1984).

Finally, helium-3 is added to ground water by decay of tritium to helium-3. For determination of apparent ground-water age by the tritium/helium-3 method, tritiogenic helium-3 is considered to be the helium-3 not accounted for by air-water solubility, excess air, and radiogenic sources. Helium-3 and helium-4 from air-water solubility are determined from recharge temperature and altitude, and the known helium-3 and helium-4 content of air. Helium-3 and helium-4 from excess air are proportional to the neon from excess air. Neon contributed from excess air is the neon in excess of air-water solubility. Any helium-4 not accounted for by air-water solubility and excess air is assumed to represent helium from radiogenic sources, and the helium-3/helium-4 ratio of radiogenic helium is used to determine the helium-3 contributed from radiogenic sources. Any remaining helium-3 is assumed to represent tritiogenic helium-3. Once the tritiogenic helium-3 is known, the reconstructed (that is, original, or beginning-of-the-flowpath) tritium concentration can be calculated; it is the sum of the measured tritium and the tritiogenic helium-3. Determination of the ground-water apparent age is then simply a matter of calculating the time required for the original tritium concentration to decay to the measured tritium concentration. When recharge temperature and altitude are known, these various sources of helium may be well defined, and reliable estimates of ground-water apparent ages may be obtained. The reliability of the tritium/ helium-3 method has been demonstrated in studies comparing tritium/helium-3 apparent ages with estimates of traveltimes obtained by other means such as chlorofluorocarbon dating (for example, Solomon and others, 1993; Ekwurzel and others, 1994; Szabo and others, 1996).

Based on argon and nitrogen concentrations in ground water, Hinkle and Snyder (1997) calculated a mean recharge temperature of 8°C for Portland basin ground water. This recharge temperature was used in calculating tritium/helium-3 apparent ages. Recharge altitudes were assumed to be the altitude of the water table at the time of sampling. Tritium/helium-3 apparent ages in this report would vary by about 0.6 year for a difference of 2°C in the recharge temperature, and by about 0.1 year for a 100-m change in recharge altitude.

Water samples were collected in-line, in 8-mm inside-diameter, 0.9-m length copper tubes and sealed at each end with cold-weld clamps. Samples were analyzed at Lamont-Doherty Earth Observatory of Columbia University. Helium-4 concentrations and helium-3/helium-4 ratios were measured in a dedicated helium isotope mass spectrometer. Neon was measured in a quadrupole mass spectrometer. Tritium was determined using the helium-3 ingrowth method. Details of methods used to collect and analyze samples are given in Koterba and others (1995) and Plummer and Mullin (1997a, 1997b), and references therein.