^(234)U-^(238)U-^(230)Th-^(232)Th systematics in saline groundwaters from central Missouri

Abstract

Saline groundwaters with 4.7 to 26‰ total dissolved solids were sampled from springs and artesian wells in Mississippian and Ordovician carbonates and sandstones in central Missouri. U—Th isotopic variations provide a means of evaluating processes of water-rock interaction and fluid mixing and estimating the time scales of element transport. Recently developed mass spectrometric techniques are used to make isotopic measurements on small-volume groundwater samples (0.1–4 l) with high precision (e.g., < ±5% for ^(234)U/^(238)U activity ratios). The groundwaters have a wide range of ^(238)U concentrations, 50 × 10^(−12) to 200 × 10^(−12) g/g; ^(234)U/^(234)U activity ratios, 2.15–16.0; ^(232)Th concentrations, 0.10 × 10^(−12) to 33 × 10^(−12) g/g; and ^(230)Th concentrations, 0.91 × 10^(−17) to 26 × 10^(−17) g/g. Unfiltered and filtered (0.4 μm, 0.1 μm) aliquots of a saline sample have the same isotopic composition and concentration of U, indicating that ^(234)U and ^(238)U occur almost entirely as dissolved species. The concentration of ^(232)Th is up to seven times lower in filtered vs. unfiltered aliquots, indicating that ^(232)Th is predominantly associated with particulates in the groundwaters. In contrast, most of the ^(230)Th is in solution. Previous geochemical studies indicate that: (1) the saline waters originated as meteoric recharge and evolved through halite dissolution, reactions with silicates and saline-dilute mixing processes during a long-distance flow history; and (2) interaction with limestone and dolomite aquifer rocks in central Missouri has been limited. A consistent relationship between U/Ca and ^(234)U/^(238)U activity ratio is observed in the groundwaters and provides constraints on the U/Ca ratios and ^(234)U/^(238)U activity ratios of end-member reservoirs and on the processes of isotopic exchange in this water-rock system. Model calculations that simulate (1) saline-dilute groundwater mixing and (2) limited extents of dissolution of carbonate aquifer minerals by the groundwaters can account for the variations in U/Ca, ^(234)U/^(238)U and ^(18)O/^(16)O in the suite of water samples. The model calculations demonstrate that dissolved U isotopic compositions can be a sensitive indicator of water-rock interaction, which in turn limits the usefulness of ^(234)U—^(238)U disequilibria for groundwater age determinations. The concentration of dissolved ^(230)Th in the groundwaters is (1) two to three orders of magnitude below ^(230)Th—^(234)U equilibrium activity levels, and (2) significantly in excess of concentrations estimated for the supply of Th to solution via desorption and dissolution. A model involving the derivation of the excess ^(230)Th from the in situ decay of dissolved ^(234)U in the groundwaters indicates the operation of an adsorption mechanism on the time scale of 10–10^3 years. The results reported here may have broader application to the assessment and management of hazardous chemical species in natural environments.

Additional details