A comparison of groundwater dating with 81Kr, 36Cl and 4He in four wells of the Great Artesian Basin, Australia

Lehmann, B.E.; Love, A.; Purtschert, R.; Collon, P.; Loosli, H.H.; Kutschera, W.; Beyerle, U.; Aeschbach-Hertig, W.; Kipfer, R.; Frape, S.K.; Herczeg, A.; Moran, J.; Tolstikhin, I.N.; Gröning, M. (2003). A comparison of groundwater dating with 81Kr, 36Cl and 4He in four wells of the Great Artesian Basin, Australia. Earth and planetary science letters, 211(3-4), pp. 237-250. Elsevier 10.1016/S0012-821X(03)00206-1

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The isotopic ratios 81Kr/Kr and 36Cl/Cl and the 4He concentrations measured in groundwater from four artesian wells in the western part of the Great Artesian Basin (GAB) in Australia are discussed. Based on radioactive decay along a water flow path the 81Kr/Kr ratios are directly converted to groundwater residence times. Results are in a range of 225–400 kyr with error bars in the order of 15% primarily due to counting statistics in the cyclotron accelerator mass spectrometer measurement. Additional uncertainties from subsurface production and/or exchange with stagnant porewaters in the confining shales appear to be of the same order of magnitude. These 81Kr ages are then used to calibrate the 36Cl and the 4He dating methods. Based on elemental analyses of rock samples from the sandstone aquifer as well as from the confining Bulldog shale the in situ flux of thermal neutrons and the corresponding 3He/4He and 36Cl/Cl ratios are calculated. From a comparison of: (i) the 3He/4He ratios measured in the groundwater samples with the calculated in situ ratios in rocks and (ii) the measured δ37Cl ratios with the 4He concentrations measured in groundwater it is concluded that both helium and chloride are most likely added to the aquifer from sources in the stagnant porewaters of the confining shale by diffusion and/or mixing. Based on this ‘working hypothesis’ the 36Cl transport equation in groundwater is solved taking into account: (i) radioactive decay, (ii) subsurface production in the sandstone aquifer (with an in situ 36Cl/Cl ratio of 6×10−15) and (iii) addition of chloride from a source in the confining shale (with a 36Cl/Cl ratio of 13×10−15). Lacking better information it is assumed that the chloride concentration increased linearly with time from an (unknown) initial value Ci to its measured present value C=Ci+Ca, where Ca represents the (unknown) amount of chloride added from subsurface sources. Using the 81Kr ages of the four groundwater samples and a reasonable initial 36Cl/Cl ratio of 125×10−15, which is consistent with other studies in this part of the GAB, it is then possible to determine (Ci,Ca) parameter sets for all four samples and consequently to simulate the Cl and the 36Cl evolution with time. Strong evidence that the whole procedure is adequate comes from: (i) a comparison of Ci with the calculated noble gas recharge temperatures (NGRT) indicating that a higher NGRT is related to higher input chloride concentrations Ci (because of higher evapotranspiration) and (ii) a comparison of Ca with the measured 4He concentration confirming the idea that both chloride and helium are added to the groundwater in parallel. It turns out that the four samples fall into two groups: (i) for two of the samples (Raspberry Creek and Oodnadatta) initial 36Cl concentrations are high and 36Cl dating based on radioactive decay is possible. The 4He accumulation rate for these two samples is low (0.2×10−10 cm3 STP 4He/(cm3 water yr)); (ii) for the other two samples (Duck Hole and Watson Creek) the initial 36Cl concentration is low and therefore subsurface processes dominate resulting in almost constant 36Cl concentrations with time; 36Cl groundwater dating is not possible. The 4He accumulation rate for these two samples is about 10 times higher (1.9×10−10 cm3 STP 4He/(cm3 water yr)). 129I concentrations are interpreted as a simple mixing between an atmospheric and a subsurface source.

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