Uranium and Thorium-Series Isotopes in Fresh Groundwater at the INEEL

We are using the short- and long-lived members of the U- and Th-decay series as natural analogues of contaminant transport at INEEL. We have thus far analyzed 32 groundwater samples for U isotopics and concentration, and 12 large-volume samples (∼ 1000L) for long- and short-lived isotopes of Th, Ra and Pa. Here we report on general trends in concentration and isotope data. Uranium concentrations range from 0.3 to 3.6 ppb and 234U/238U activity ratios range between 1.6 and 3.1. Groundwater nearest the local recharge supplied by Birch Creek and Little Lost River has high 234U/238U ratios, in contrast to the low to moderate values of groundwater dominated by the regional southwesterly flow of the Snake River Plain Aquifer. Contours of high 234U/238U ratios delineate preferential flow-paths extending southward from the Birch Creek and Little Lost River recharge areas. 'Low-isotope ratio zones' separate preferential flow zones. These are interpreted to be due to greater extent of reaction with the host basalt which, in turn, probably indicates longer groundwater residence times. Thorium concentrations are between 0.1 and 0.6 ppt and 230Th/232Th activities range from 0.87 to 1.42. Protactinium concentrations are between 0.14 and 0.01 fg/L. Protactinium-231 and uranium concentrations are positively correlated. Radium-226 concentrations range from 0.65 to 7.8 fg/L. Radium-226 concentrations are negatively correlated with both uranium concentrations and 234U/238U activities along one flow path toward a 'low-isotope ratio zone'. This also suggest greater extent of water/rock interaction in accordance with the uranium isotopic data. In general, waters entering the aquifer from the north show highest degrees of disequilibrium. Decreases in activity ratios away from the local recharge areas reflect interaction with the host basalt and mixing with regional groundwaters flowing from them the northeast