Trace element geochemistry of the Milk River aquifer groundwater, Alberta, Canada
Date
1998
Authors
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Type
Degree Level
Masters
Abstract
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.
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Degree
Master of Science (M.Sc.)
Department
Geological Sciences
Program
Geological Sciences