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The ability to measure [sup.238]U/[sup.235]U to high precision presents an important new opportunity to study the fate and transport of uranium in the environment. The ratio of [sup.238]U/[sup.235]U was determined by multicollector--inductively coupled plasma--mass spectrometer in six uranium ore samples representing two different classes of deposits. Significant offsets in [sup.238]U/[sup.235]U are observed between uranium ores precipitated from groundwaters at low temperature versus hydrothermal deposits precipitated at high temperatures, reinforcing an observation made previously but lacking the needed precision. Specifically, tabular sandstone-type uranium deposits were found to be depleted in [sup.235]U, with a total offset between low-temperature deposits and higher temperature deposits of [approximately equal to] 1.0 [per thousand]. We attribute this offset to reflect a temperature-dependent fractionation related to the nuclear field shift effect during chemical reduction of uranium in ambient temperature groundwaters.

Groundwater in aquifers is generally older than expected on the basis of flow velocity, and this observation has important implications for interpreting radiometric age determinations. Hydrologists commonly account for the aging of water as it flows along streamtubes, but not for the effects of mixing old water from aquitards (or confining layers) into aquifers, because the rate of mass exchange between aquifers and aquitards can in many cases be assumed to be small. We show, however, that the effect on age of such mixing does not depend on the mixing rate; this is the paradox of groundwater age. Surprisingly, the contribution of aquitards to the age of groundwater in aquifers depends only on the ratio of fluid volume in aquitards to aquifers. This result has broad importance for understanding the relationship between groundwater flow and the distribution of radiometric age. Keywords: groundwater age, residence time, radiometric age dating.

Preferential flow paths are expected in many groundwater systems and must be located because they can greatly affect contaminant transport. The fundamental characteristics of radiogenic isotope ratios in chemically evolving waters make them highly effective as preferential flow path indicators. These ratios tend to be more easily interpreted than solute-concentration data because their response to water-rock interaction is less complex. We demonstrate this approach with groundwater [sup.87]Sr/[sup.86]Sr ratios in the Snake River Plain aquifer within and near the Idaho National Engineering and Environmental Laboratory. These data reveal slow-flow zones as lower [sup.87]Sr/[sup.86]Sr areas created by prolonged interaction with the host basalts and a relatively fast flowing zone as a high [sup.87]Sr/[sup.86]Sr area. Keywords: groundwater tracers, isotope ratios, Snake River Plain, aquifer properties, aqueous solutions.

The ability to measure 238U/235U to high precision presents an important new opportunity to study the fate and transport of uranium in the environment. The ratio of 238U/235U was determined by multicollector-inductively coupled plasma-mass spectrometer in six uranium ore samples representing two different classes of deposits. Signifi cant offsets in 238U/235U are observed between uranium ores precipitated from groundwaters at low temperature versus hydrothermal deposits precipitated at high temperatures, reinforcing an observation made previously but lacking the needed precision. Specifically, tabular sandstone-type uranium deposits were found to be depleted in 235U, with a total offset between low-temperature deposits and higher temperature deposits of ≈1.0%. We attribute this offset to reflect a temperature-dependent fractionation related to the nuclear field shift effect during chemical reduction of uranium in ambient temperature groundwaters. [ABSTRACT FROM AUTHOR]