Using groundwater geochemistry and environmental isotopes to assess the correction of 14C ages in a silicate-dominated aquifer system

SummaryThe use of 14C to constrain groundwater residence times requires careful correction of 14C activities (a14C) and an assessment of the degree of inter-aquifer mixing. In groundwater of the Campaspe Valley in the southern Murray Basin, Australia, δ13C values of dissolved inorganic carbon (DIC) range from −18‰ to +2‰. Using these δ13C values to correct 14C 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, 87Sr/86Sr 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 δ13C values are most probably due to methanogenesis via reduction of CO2 in the groundwater that results in the residual DIC being enriched in 13C. As methanogenesis by this mechanism has only a minor impact on a14C, uncorrected 14C 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 20–25 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 14C ages, especially in silicate-dominated aquifers where dissolution of carbonates may not be the major process controlling δ13C values.