Trace element geochemistry of the Milk River aquifer groundwater, Alberta, Canada
Starzynski, M. Ashley
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Groundwater samples from 27 wells located along a 50 km flow path in the Milk River aquifer, Alberta, Canada, were analyzed for trace elements including rare earth elements (REEs). The objectives were to: 1) complete the suite of chemical elements determined for this extensively studied aquifer; 2) evaluate changes in concentrations of the trace elements and REEs along a hydraulically and chemically constrained section of the aquifer, and 3) observe the fate of these trace elements through a redox boundary. The groundwater chemistry illustrates some similar trends to those reported by previous workers (Meyboom, 1960; Schwartz and Muelenbachs, 1979; Phillips et al., 1986; Hendry and Schwartz, 1988, 1990; and Hendry et al., 1991). There are small well defined increases in Na, Cl, Fe, Mg, Ca, AI, but a decrease in S04, as the groundwater migrates from recharge. Calcite dissolution accounts for the increase in alkalinity as the groundwater migrates downgradient from the recharge area. Some trace elements (B, Rb, Ba) display slight increases as the groundwater evolves from the recharge area, consistent with progressive water-rock reaction. In groundwater studies uranium has been found to be an excellent element for tracing the evolution of waters, because of its low natural concentrations and multi-valent characteristics. Higher concentrations encountered along the flow path at 20 km and 32-33 km, may reflect the redox front boundary and post-redox front boundaries, respectively. Groundwater with intermediate uranium concentrations are probably the result of minor mixing between the oxic and anoxic water masses. Similar, trends are seen with other multi-valent elements including manganese. Groundwater studies using trace elements as hydrogeochemical tracers show that the groundwaters inherit their trace element budgets, including REE signatures, via interaction with the aquifer rocks. Solution complexation can also significantly affect the dissolved REE signatures. Shale-normalized plots show that systematic patterns exist in REEs and transition metals in the Milk River aquifer groundwater. There are clear chemical trends as the groundwater migrates downflow from the recharge area. REE plots for the groundwater samples are generally either flat [Type 1 groundwater] to slightly enriched in the heavy REEs (HREEs) [Type 2 groundwater]. Speciation modeling of REEs in the alkaline Milk River aquifer groundwater has been evaluated primarily to assess the importance of carbonate complexes. Carbonate complexation is the most important complex for REEs in the groundwater system. Dicarbonato complexes (Ln(C03)2-) are predicted as the dominant species in comparison to the carbonato complexes (LnC03+). Heavy REE enriched shale-normalized REE patterns are due to the formation of more stable HREE C03 complexes than the light REE C03 complexes in the groundwater. In addition to using REEs as potential hydrogeochemical tracers in groundwaters, their chemical similarities to the trivalent actinides provides a proxy for the chemical behavior of these radionuclides in natural waters, and therefore an analogue for modeling the behavior of nuclear waste in groundwater systems.