A comparison of groundwater dating with 81Kr, 36Cl and 4He in four wells of the Great Artesian Basin, Australia
Introduction
The concentrations of the two naturally occurring radionuclides 81Kr (half-life 229 kyr) and 36Cl (half-life 301 kyr), produced by cosmic rays in the atmosphere and present in surface waters, are expected to decrease in groundwater along a water flow line due to radioactive decay. In parallel, 4He concentrations generally increase due to the accumulation of helium generated in the natural decay series of uranium and thorium in rocks. Groundwater dating is possible if the input values at recharge (81Kr, 36Cl) and/or the accumulation rate (4He) in the subsurface are known. However, other processes that could also change isotope concentrations in groundwater (e.g. subsurface production, diffusive exchange with porewaters or mixing processes) also need to be quantified.
Among the three nuclides, 81Kr lends itself to the most straightforward age interpretation because fewer assumptions are required. At the same time it is by far the most difficult to analyse due to the very low concentrations of <1000 atoms/l of water. Converting measured 81Kr/Kr ratios into a groundwater age is comparatively simple because: (i) the ratio in atmospheric air is known and was most likely constant over the past several hundred thousand years because the atmosphere is the only important reservoir for Kr and possible short-term fluctuations in the cosmic ray production rate will therefore be averaged out on such time scales, (ii) this input ratio into an aquifer does in particular not depend on the climatic conditions at the time of recharge and (iii) subsurface production in rocks or groundwater is small. In contrast, hydrogeologists working with 36Cl always have to address the ‘initial value problem’ [1] and in many cases, in particular in deeper and more saline groundwaters, disentangling the atmospheric 36Cl signal is difficult due to subsurface production of 36Cl by the 35Cl(n,γ)36Cl reaction and transport from adjacent aquifers or porewater reservoirs.
For helium it has been demonstrated that concentrations often increase with water residence times (for a recent review see [2]), however, accumulation rates in different aquifers vary or are not known. In particular, a very small admixture of ancient groundwater with an extremely high concentration of 4He can drastically change the 4He concentration along a water flow path. Consequently, 4He groundwater dating is generally considered to be a ‘semi-quantitative’ method only.
In this work we present for the first time a complete set of data for all three isotopes from groundwaters sampled from four wells in the Great Artesian Basin (GAB) in Australia. This enables a comparison of possibilities and limitations of the different nuclides in dating old groundwaters.
Section snippets
Field work
A 81Kr sampling campaign was organised in January 1998 by researchers from the Department of Water, Land and Biodiversity Conservation of South Australia (DWLBC) in Adelaide as part of a Coordinated Research Program (‘Isotope Techniques for the Assessment of Slow Moving Deep Groundwaters’) of the International Atomic Energy Agency (IAEA) in Vienna, Austria. Water degassing equipment of the University of Bern, Switzerland, was used to extract gases from water samples of 16 000 litres each at
Data
After the field campaign samples were shipped to the various laboratories in the different countries. Table 2 summarises all available isotope data of the four groundwater samples and Table 3 those of nine rock samples from the sandstone of the aquifer and from the confining Bulldog shale. The analyses were made in the following laboratories (number in parentheses refers to the parameter number in Table 2):
- 1.
Long-lived radionuclides
81Kr/Kr (#18) at the National Superconducting Cyclotron
Subsurface production of radionuclides
Based on the available U, Th and K concentrations in rock it is easy to calculate the in situ 4He and 40Ar production rates. Using the concentrations of all the other elements (based on rock analyses performed at Activation Laboratories Ltd in Canada) one can also estimate the flux Φn of thermal neutrons in the respective rock formations and consequently the in situ 36Cl/Cl and 3He/4He ratios [7]. Subsurface production of 81Kr by spontaneous fission of 238U is expected to be small based on
81Kr ages
The 81Kr isotope with a half-life of 229 000 years has several unique advantages for dating old groundwaters as already listed in Section 1. After the first measurement in atmospheric air by low level decay counting [8] efforts to develop a routine analytical tool include both laser-based optical techniques [9], [10], [11], [12], [13], [14] as well as accelerator mass spectrometry [15], [16]. Most of these efforts are still in progress and a routine analytical facility is currently not
4He dating
As observed in various studies before [2] 4He concentrations in an aquifer often increase with time; however, rates are variable and depend on the importance of exchange and mixing processes. For our four samples for the first time a calibration is possible using 81Kr. In Fig. 2 the 4He concentrations are plotted vs the 81Kr ages. Obviously, different accumulation rates result of about 1.9×10−10 cc STP/(cm3 water yr) (for Duck Hole and Watson Creek) and 0.2×10−10 cc STP/(cm3 water yr) (for
36Cl dating
For some 20 years 36Cl has been used to study groundwater movement over time scales of several hundred thousand years. A number of researchers have addressed the potential and possible limitations of a 36Cl dating technique [19], [20], [21], [22], [23], [24]; most recently in [25]. In many situations it is possible to disentangle all the various interfering processes. However, evolutionary scenarios which cannot unambiguously be reduced to one consistent picture sometimes result and 36Cl dating
Discussion
Strong evidence that the Cl and 36Cl evolution derived from 81Kr decay ages are valid comes from Fig. 7, Fig. 8.
In Fig. 7 the input value Ci is plotted vs the calculated NGRT (Table 3). Higher recharge temperatures reflect higher rates of evapotranspiration yielding higher Cl input concentrations. In Fig. 8 the amount Ca of chloride added from subsurface sources is plotted vs the 4He concentration of the four samples. The correlation is in agreement with Fig. 3, Fig. 4 clearly pointing to a
Iodine-129
The isotope 129I has a half-life of 15.7 million years. In the atmosphere it is mainly produced by cosmic ray-induced spallation of xenon atoms [20]. The initial pre-bomb 129I/I ratio is in a range of Ri=(600–1000)×10−15 based on theoretical considerations which is supported by several field studies. Values in a range of (1200–1500)×10−15 have been measured in pre-bomb marine sediments [28]. In the underground, subsurface production by spontaneous fission of 238U yields in situ equilibrium
Summary
Measured 81Kr/Kr ratios are converted to water residence times based on a decrease of 81Kr by radioactive decay only. These ages are then used to simulate the 36Cl and the Cl evolution. Based on 3He/4He data of rocks and groundwaters and on δ37Cl data of groundwaters it is concluded that Cl and He both are transported from the porewaters of the shale (by diffusion and/or advection) and mixed into the flowing groundwaters. As a result, the measured chloride concentration Cm of each sample can be
Acknowledgements
The authors thank Sergei Tarakanov for numerical estimates with a simple diffusive–advective transport model to quantify the uncertainties of the 81Kr ages (Fig. 1) and Beat Ihly for analytical solutions of the 36Cl transport equation in groundwater (Fig. 5). The Isotope Hydrology Section of the IAEA substantially supported the fieldwork of this study. The Russian collaborators were supported by a grant from SCOPES, the Scientific Cooperation between Eastern Europe and Switzerland. A very
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