The Helium-Based Groundwater Age Equation

During migration and subsurface storage groundwater comes into contact with crustal rocks that continuously release helium from the decay of uranium and thorium. The basic assumption is made that the water acts as a sink for the helium evolved from the local rocks. The age of groundwater, t, is calculable from the equation given in section 14.2. 

Example In the Milo Holdings 3 well, 1421 m deep, in the Jurassic rock sequence of the Great Artesian Basic, east Australia, the helium concentration of 1000 x 10 x cc/cc water was found (Mazor and Bosch, 1990). The following local parametric values were applied to calculate the water age H = 1 U = 1.7 ppm Th = 6.1 ppm d= 2.6 effective porosity of 0.2 or a rock water ratio of 4. Thus the age of this groundwater was found to be 

The obtained age for the deep water in the artesian Milo Holdings 3 well is semiquantitative because of uncertainties in the parameters applied in the calculation. Yet the order of magnitude—millions of years—is of utmost importance the water resource is not renewed, but it is entirely shielded from the surface and, hence, is immune to anthropogenic pollution. 

The low helium concentrations of rocks of known ages indicate emanation efficiencies of 0.8-1 (Podosek et al., 1980 Zaikowski et al., 1987). The concentrations of uranium and thorium vary by a factor of 10 in common rocks, but knowledge of the lithologies of the main aquifer rocks can narrow the uncertainty to a factor of three or less. The rock water ratio is more difficult to assess, but reasonable values are 4-20. Local information 

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