No abstract

Chlorine 36 has many advantages as a dating tool for very old groundwater. These advantages include a suitable half-life (3.01 x l0 s years), simple geochemistry, conservative behavior in groundwater, and a general absence of subsurface sources at levels comparable to the atmospheric input. Recent advances in tandem accelerator mass spectrometry have permitted the analysis of 36C1 at the low abundance expected following residence in the subsurface for 106 years or more. In order to test the suitability of 36C1 for dating very old groundwater, the 36C1/C1 ratios of 26 groundwater samples from the Great Artesian Basin of Australia have been measured. Groundwater ages calculated from the 36C1 data compare favorably with ages computed independently from hydrodynamic simulations. INTRODUCTIONThe age of groundwater can be defined as the length of time the water has been isolated from the atmosphere. The concept of groundwater age is inherently somewhat ambiguous because, due to the effects of diffusion and hydrodynamic dispersion, no two water molecules in a given sample of water can be precisely the same age. Nevertheless, even though all samples are affected to some degree by mixing [Davis and Bentley, 1982], the concept of an "average" groundwater age is still a useful one. In this paper we compare two independent paethods of estimating this average groundwater age: (1) a new method of radiometric calculation dating, 36C1 tracing, and (2) the use of Darcy's law and the continuity equation.Up to the present time, the most successful radiometric method for the measurement of groundwater ages has been •4C dating. Carbon 14 dating of groundwater was first described by Munnich in 1957 and has since seen wide application. However, the inherent limitations of •'•C prevent its application in some circumstances where dating of groundwater is desired. One of these limitations is the age range over which dating is possible. With traditional direct-counting methods of measuring •4C, the maximum water age at which •4C can still be detected is less than 50,000 years. The advent of tandem accelerator mass spectrometry (TAMS) analysis and isotopic enrichment processes may advance this maximum age toward 80,000 years. Although this age limit is satisfactory for many shallow aquifers, deep regional flow systems commonly contain much older water. The need for a radio-nuclide with a longer half-life is particularly acute in hydrogeologic investigations of potential nuclear waste repositories in the deep subsurface. Such sites are intentionally located in low permeability formations containing very old water. Another disadvantage of •4C is the chemical reactivity of itsprincipal chemical form, the bicarbonate ion. The bicarbonate ion interacts with the aquifer matrix by precipitation or solution of carbonate minerals and exchange with carbonates, and is also produced biologically. These interactions complicate both determination of the initial •4C activity and the estimation of transport through the aquifer. A radioisotope with high sol...

The scavenging of uranium from groundwater downgradient of the uranium ore deposit at Koongarra, Australia, has been investigated to provide information about the longterm transport of radionuclides. Rock samples collected from diamond-drill cores were examined mainly using scanning and transmission electron microscopy. Here we focus on the U associated with iron oxides and report on (i) the extent to which U has accumulated in the various types of iron oxides (fissure fillings, clay coatings, and nodules) and (ii) the chemical form of U associated with iron-nodules. The iron nodules have a remarkably large capacity for uranium uptake. The uranium enrichment in the nodules reaches approximately 8 wt %, and their uranium contents are greater than those in the other iron forms, such as fissure fillings and clay coatings. The ability of the iron nodules to enrich uranium (to levels 10 6 times higher than the groundwater) is greater than those of any other natural materials in the system. Although the initial step in uranium uptake appears to be adsorption, the uranium in the nodules has been fixed by precipitation of copper uranyl phosphate microcrystals. This precipitation process leads to the long-term retardation of uranium in the system. This result strongly suggests that an understanding of postadsorption processes is necessary for predicting radionuclide retardation over long time scales.

No abstract