Temporal variability of uranium concentrations and 234U/ 238U activity ratios in the Mississippi river and its tributaries

Abstract

234U/ 238U activity ratios and total dissolved uranium concentrations in the Lower Mississippi River at New Orleans, LA have been analyzed on a bi-weekly basis over 2003–2004. During this time period, the range in 234U/ 238U activity ratios is approximately 17% (from 1.241 to 1.473), while uranium concentrations span an even greater range of 130% (0.28 to 1.33 ppb). There is no correlation between uranium activity ratio and discharge, and only a very weak correlation between uranium concentration and discharge. In order to examine the cause of the substantial variability in the lower river concentrations and 234U/ 238U activity ratios, we sampled the four major tributaries of the Mississippi River (Ohio, Missouri, Upper Mississippi, and Arkansas Rivers). Each river was sampled four times, each time representing a unique discharge season. In general, lithological and climatological parameters in the tributary sub-basins exert the greatest control on Lower Mississippi River uranium concentration and isotope signatures. These parameters may also be influenced by groundwater infiltration below the confluences of the major tributaries. Our temporal analysis suggests that if one were to estimate Mississippi River uranium fluxes to the ocean based on single-point sampling during 2003–2004, significant over- or under-estimations of these values would ensue. Our two-year average Mississippi River uranium-flux values suggest that previous best estimates of the contribution of Mississippi River uranium to the global average riverine 234U/ 238U activity ratio are too high by 36%. If other major world rivers are as poorly constrained with respect to uranium systematics as the Mississippi River, then the global average riverine U flux and 234U/ 238U activity ratio need to be revised.