科研之友
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摘要
The use of C-14 to constrain groundwater residence times requires careful correction of C-14 activities (a C-14) and an assessment of the degree of inter-aquifer mixing In groundwater of the Campaspe Valley in the Southern Murray Basin, Australia, delta C-13 values of dissolved inorganic carbon (DIC) range from -18 parts per thousand, to +2 parts per thousand, Using these delta C-13 values to correct C-14 ages assuming that dissolution Of or exchange with matrix calcite had occurred implies that locally >90% of the DIC is derived from carbonates In turn, this Suggests that low-salinity groundwater in the deeper confined Calivil-Renmark Formation up to 50-60 km from the basin margins in the Campaspe Valley has a component of modern recharge, which would require that widespread leakage from the overlying Shepparton Formation had occurred. However, Sr-87/Sr-86 ratios and major Ion geochemistry of groundwater imply that negligible inter-aquifer mixing has occurred and that calcite dissolution is a very minor process The variable delta C-13 values are most probably due to methanogenesis via reduction of CO2 in the groundwater that results in the residual DIC being enriched in C-13 As methanogenesis by this mechanism has only a minor impact on a(14)C, uncorrected C-14 ages are a better estimate of groundwater residence times Groundwater in the north of the Campaspe Valley has a residence time of 9-13 ka. which agrees with ages calculated from head gradients and hydraulic conductivities. Calivil-Renmark groundwater from the adjacent Pyramid Hill region, in an area where the aquifers contain lower hydraulic conductivity sediments, IS Substantially older (up to 2025 ka) The age Of groundwater from the Shepparton Formation increases irregularly with depth, with ages of in excess of 25 ka recorded at its base This study Illustrates the need to fully understand hydrogeological processes in order to correct C-14 ages, especially in silicate-dominated aquifers where dissolution of carbonates may not be the major process controlling delta C-13 values.
- 出版日期2010-3-1
