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At Idaho National Laboratory, Cr(VI) concentrations in a groundwater plume once exceeded regulatory limits in some monitoring wells but have generally decreased over time. This study used Cr stable isotope measurements to determine if part of this decrease resulted from removal of Cr(VI) via reduction to insoluble Cr(III). Although waters in the study area contain dissolved oxygen, the basalt host rock contains abundant Fe(II) and may contain reducing microenvironments or aerobic microbes that reduce Cr(VI). In some contaminated locations, 53Cr/52Cr ratios are close to that of the contaminant source, indicating a lack of Cr(VI) reduction. In other locations, ratios are elevated. Part of this shift may be caused by mixing with natural background Cr(VI), which is present at low concentrations but in some locations has elevated 53Cr/53Cr. Some contaminated wells have 53Cr/53Cr ratios greater than the maximum attainable by mixing between the inferred contaminant and the range of natural background observed in several uncontaminated wells, suggesting that Cr(VI) reduction has occurred. Definitive proof of reduction would require additional evidence. Depth profiles of 53Cr/53Cr suggest that reduction occurs immediately below the water table, where basalts are likely least weathered and most reactive, and is weak or nonexistent at greater depth. [ABSTRACT FROM AUTHOR]

Examines the isotopic ratios of Uranium 234 and 238 in a freshwater, fracture-flow aquifer, the eastern Snake River Plain aquifer in Idaho. Interpretation of results in conjunction with strontium isotope and solute concentration data; Implications for groundwater flow patterns and geochemical evolution of the aquifer.

Examines the effectiveness of radiogenic isotopes ratios in tracing groundwater preferential flow paths of Snake River Plain aquifer in Idaho. Characteristic of radiogenic isotope ratios; Role of flow paths in contaminants transportation; Applicability of the ratio as compared to solute-concentration data; Identification of path slow and fast-flow zones.