Behavior of rare earth elements in groundwater during flow and mixing in fractured rock aquifers: An example from the Dandenong Ranges, southeast Australia

A previously-defined flow system is used to investigate REE behavior at different stages along groundwater flow paths in fractured basalt and sedimentary aquifers. Results from this study indicate that REE concentrations and patterns predominantly reflect different types and rates of reactions taking place to those controlling major ion chemistry. In areas where recharge is through sediments, REE concentrations are high (Nd: 8.62-31.61 ng/L), and localised processes, such as source heterogeneity, control REE patterns and concentrations. Increased concentrations of REEs in these groundwaters, where pH < 6.1, is a result of early-stage REE mobilisation in the flow system. In contrast, where groundwater recharges through basalt REE concentrations are low (Nd: 2.40-4.75 ng/L), and REE concentrations are relatively constant along groundwater flow paths, indicating steady state conditions. Groundwater from both the sediment and basalt aquifer systems flows into the local discharge area. In contrast to TDS contents and major ion ratios, groundwater from the discharge area has REE concentrations (Nd: 1.84-4.18 ng/L) and patterns that reflect a larger component of flow from the basalt aquifer. Lower REE concentrations of groundwater from the regional sedimentary aquifer may result from progressive REE sorption or co-precipitation, as indicated by an increase in (Y/Ho)_Cl-CN ratios and HREE-enrichment for groundwater with pH ∼ 7.1 due to greater abundances of HREE vs. LREE dicarbonato complexes.