Cartwright, Ian, Weaver, Tamie R., Cendón, Dioni I., Fifield, L. Keith, Tweed, Sarah, Petrides, Ben, and Swane, Ian (2012) Constraining groundwater flow, residence times, inter-aquifer mixing, and aquifer properties using environmental isotopes in the southeast Murray Basin, Australia. Applied Geochemistry, 27 (9). pp. 1698-1709.

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Abstract

Environmental isotopes (particularly δ¹⁸O, δ²H, and δ¹³C values, ⁸⁷Sr/⁸⁶Sr ratios, and α¹⁴C) constrain geo-chemical processes, recharge distribution and rates, and inter-aquifer mixing in the Riverine Province of the southern Murray Basin. Due to methanogenesis and the variable δ¹³C values of matrix calcite, δ¹³C values are highly variable and it is difficult to correct ¹⁴C ages using δ¹³C values alone. In catchments where δ¹³C values, ⁸⁷Sr/⁸⁶Sr ratios, and major ion geochemistry yield similar α¹⁴C corrections, similar to 15% of the C is derived from the aquifer matrix in the silicate-dominated aquifers, and this value may be used to correct ages in other catchments. Most groundwater has α¹⁴C above background (similar to 2 pMC) implying that residence times are <30 ka. Catchments containing saline groundwater generally record older ¹⁴C ages compared to catchments that contain lower salinity groundwater, which is consistent with evapo-transpiration being the major hydrogeochemical process. However, some low salinity groundwater in the west of the Riverine Province has residence times of >30 ka probably resulting from episodic recharge during infrequent high rainfall episodes. Mixing between shallower and deeper groundwater results in ¹⁴C ages being poorly correlated with distance from the basin margins in many catchments; however, groundwater flow in palaeovalleys where the deeper Calivil-Renmark Formation is coarser grained and has high hydraulic conductivities is considerably more simple with little inter-aquifer mixing. Despite the range of ages, δ¹⁸O and δ²H values of groundwater in the Riverine Province do not preserve a record of changing climate; this is probably due to the absence of extreme climatic variations, such as glaciations, and the fact that the area is not significantly impacted by monsoonal systems.

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