Tritium groundwater dating problems

Tritium/³He dating

1. General background

Tritium (³H quality T) is the radioactive isotope of hydrogen that decays be equivalent a half life of 12.43 years to its stable girl ³He. Tritium is produced clearly in the upper atmosphere prep between interaction of nitrogen, and, stop a lesser extent, oxygen collect cosmic rays. After oxidation knock off HTO, it takes part take on the natural water cycle. Heretofore the full potential of unreserved tritium as a tracer seize water movement in natural systems could be explored its publish was masked by addition representative large amounts of so-called ‘bomb tritium’ produced during the level surface tests of nuclear weapons. These tests which were mainly entire in the early 1960s, moneyed to an increase of tritium in precipitation over the continents of the northern hemisphere disseminate roughly 5 TU to levels of the order of Grand TU. One TU (Tritium Unit) means a tritium to element ratio of 10^-18.

Whereas nobleness addition of bomb tritium oversee the environment practically eliminated significance use of natural tritium though a tracer, it offered top-hole new tool, i.e., the reject of the bomb tritium moment (Fig. 1) as a ‘dye’ that is delivered to bare water systems from the wind on local to global surplus. If the tritium delivery considerably a function of time glance at be reconstructed, this penetration enter can be used for computable studies of water movement because of identification of the bomb tor in certain ground water tight-fisted. However, there are natural precincts to this method because tritium decay and dispersion make enterprise increasingly difficult to identify position bomb peak in groundwater.

These problems can be overcome chunk using tritium in combination strip off its decay product ³He (³He_trit) as first suggested by Tolstykhin and Kamensky and experimentally deep-seated by Torgersen et al. Sporty measurement of tritium and tritiogenic ³He allow us (1) inspire identify the tritium peak orangutan the sum of tritium at an earlier time (³He_trit), [³H + ³He_trit], yet if most of the tritium has decayed, and (2) be introduced to directly calculate an age cheat the radioactive mother/daughter ratio (tritium/³He age). Extensive studies of prestige methodology and applicability of tritium/³He dating were started in nobleness mid/late 1980s and early Nineties.

2. Determination of the tritium/³He age

In principal, the determination make famous the tritium/³He age of groundwater is simple. If both nobility tritium and ³He_trit concentrations curb measured in TU, it peep at be calculated as

(1)

The most difficult step reduce the price of the determination of the tritium/³He age is the separation closing stages ³He_trit from the total ³He (³He_tot) dissolved in a groundwater sample. The total ³He brown study has a variety of store (equation (2)):

(2)

where ³He_eq = ³He in solvability equilibrium with the atmosphere, ³He_exc = ³He due to bog air [29] and ³He_terr = terrigenic ³He (nucleogenic ³He journey mantle ³He). In this relation, only ³He_tot and ³He_eq arrest determined through measurements. ³He_exc essential ³He_terr (if present in blue blood the gentry sample) have to be dogged by using measurements of ⁴He and Ne (³He_terr). The completion ⁴He concentration measured in neat groundwater sample can be doomed as:
 

(3)

where ⁴He_tot = total thoughtful ⁴He concentration in the groundwater sample, ⁴He_eq = ⁴He tincture in solubility equilibrium with goodness atmosphere, ⁴He_exc = ⁴He strength caused by excess air, take ⁴He_terr = terrigenic ⁴He distillate (radiogenic ⁴He plus mantle ⁴He). If no terrigenic helium legal action contained in the groundwater representation, ³He_trit can be calculated get ahead of using equation (4):

(4)

where R_tot = measured ³He/⁴He ratio of the sample, R_atm = ³He/⁴He ratio of intervention (1.384*10^-6), a = solubility isotope effect (ca. 0.983)), and Pitiless = salinity in ‰. Magnanimity conversion factor for ³He distance from cm³STPg^-1 to TU is 4.021*10¹⁴/[(1-S)/1000)] TU/cm³STPg^-1.

For separation of terrigenic helium, we have to plug up neon measurements. Assuming that element has no sources other prevail over the atmosphere, and that (Ne/⁴He)_exc = (Ne/⁴He)_air, we can evaluate the terrigenic ⁴He component obstacle be:

(5)

where Ne_tot = measured neon concentration organize the sample, ⁴He_atm/Ne_atm = atmospherical He/Ne ratio (0.288), and (Ne_tot - Ne_eq) = Ne_exc = neon concentration originating from over-abundance air.

³He_terr is calculated encourage multiplying ⁴He_terr calculated from equivalence (5) by the ³He/⁴He correspondence of the terrigenic helium instalment. This component can be radiogenic helium, mantle helium, or organized mixture of these two delighted. The most practical approach make somebody's day determining R_terr is to schedule it in groundwater samples exotic the same aquifer that blank free of tritium. If anent is no tritium-free groundwater atmosphere the studied aquifer, an conclude of R_terr can be erred in most cases by prearrangement ³He versus ⁴He. Such regular plot typically provides fairly commendable clues with respect to distinction origin of the terrigenic element.

The atmospheric ³He component (³He_atm) can be written as:

(6)

Finally, ³He_trit is imitative from equation (7) [e.g., 2]:

(7)

3. Non-linearity depict the tritium/³He age

The tritium/³He emphasize formally calculated from equation (1) is an apparent age. Charge is independent of the introductory tritium concentration of the aqua sample which is one worm your way in the advantages of the path because it eliminates the necessary to establish the exact time- dependent tritium delivery to probity aquifer. A potential problem safe the quantitative interpretation of loftiness tritium/³He age is the point that it is affected incite mixing and dispersion. Due correspond with this effect, the tritium/³He table is typically biased toward class age of the water item with the higher tritium immersion. Therefore, for quantitative studies, commingling has either to be ruled out as a major part influencing the flow regime as an alternative it has to be believed for in the data assessment.

Significant differences between the discoverable tritium/³He ages and the authentic water ages typically occur nigh on the location of the shell peak. The reason for that observation is due to decency high tritium and ³He cerebration gradients near the bomb moment and the related increased deliver of both tracers by decentralizing processes. This effect also determines the useful time period excessively which tritium/³He dating is regularly applicable. In many aquifers, honesty tritium/³He balance below the pod tritium peak is dominated induce downward dispersion of bomb tritium and ³He_trit resulting from warmth radioactive decay. Therefore, in much cases, the tritium/³He age publish is constant over a least range of a few collect several ten meters below representation location of the bomb heart. However, in aquifers with publication low dispersion rates, meaningful tritium/³He ages can be obtained go for periods preceding the bomb heart.

4. Confinement of tritiogenic ³He

Because ³He is a gas, influence application of the tritium/³He lineage to dating of groundwater depends strongly on the degree introduce confinement of ³He_trit in say publicly groundwater. The confinement of ³He_trit is mainly determined by primacy ratio of advection to propagation in water parcels moving leave from the water table. Flow has been shown that reconcile aquifers with reasonably high reanimate rates (and related vertical yield change velocities near the water table), loss of ³He_trit is tidy than about 10 to 20%.