I-129 and I-127 transport in the Mississippi River

The watershed processes which control I-129/I-127 ratios, I-129 and I-127 c oncentrations, and speciation of iodine isotopes were studied through an in vestigation into the variability of these parameters in the Mississippi Riv er near New Orleans, undertaken in 1996-1998. Analyses of suspended particu late matter (SPM) revealed a greater percent association of I-127 than of I -129, resulting in lower I-129/I-127 ratios in SPM than in surrounding wate r. Furthermore, crossflow ultrafiltration showed that organo-iodine was the dominant form for both isotopes, with 70-85% of these isotopes found in th e 0.45 mum filter-passing fraction associated with colloidal macromolecular organic matter, I-129 showed a weak correlation, I-127 no correlation, and I-129/I-127 ratios a strong inverse correlation with river flow rate. Inve rse correlations between I-129/I-127 ratios and river flow rates can be bes t explained by rainwater and evapotranspiration dominated ratios at base fl ow and a lowering of the isotope ratios during higher flow due to extra inp uts of I-127 from soil weathering. We postulate that different equilibratio n times for I-127 and I-129 as well as for bomb-produced I-129 and reproces sing-produced I-129 are responsible for these fractionation effects and the differential mobilities of these isotopes in the Mississippi River watersh ed.