Speciation and transport of anthropogenic iodine-129 and natural iodine-127 in surface and subsurface environments
Schwehr, Kathleen A
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Iodine is a biophilic element with one natural long-lived isotope, super(129)I(t sub(1/2) = 15.6 million years), and one stable isotope, super(127)I. The inventory of super(129)I in surface environments has been overwhelmed by anthropogenic releases over the past 50 years. The objective of this study is to utilize the elevated concentration and biophilic nature of super(129)I and the isotopic ratio of iodine ( super(129)I/ super(127)I) as a tracer of water mass movement and organic matter. Additionally, the significantly elevated values of super(129)I/ super(127)I could provide a geochronometer, similar to the way super(14)C is used, particularly for terrestrial organic matter that is less than 50 years old. A series of laboratory experiments and field investigations were carried out to characterize the dominant chemical forms of dissolved iodine, i.e., iodide (I super(-)), iodate (IO sub(3) super(-)), and organic iodine (DOI) in natural waters. Sensitive methods were developed for the analysis of nanomolar quantities of super(127)I species in a variety of environmental systems using high performance liquid chromatography (HPLC) and an organic iodine decomposition technique, dehydrohalogenation. The potential use of super(129)I/ super(127)I as a hydrological tracer was evaluated through measurements of super(129)I and super( 127)I, which were carried out in wells in the artificially recharged ground water basin of Orange County, California. Literature values of aquifer ages based on super(3)H/ super(3)He and delta super(18)O tracer data, as well as time-series data of chloride and Santa Ana River flow rates over the past decade were compared to values for super(129)I and super(127)I. The iodine isotopes demonstrated a conservative behavior in these aquifers, suggesting that the observed variations of these isotopes reflect past river flow conditions during the time of recharge. The feasibility of using super(129)I/ super(127)I ratios to trace terrestrial organic matter across an estuary was tested. A novel analytical technique to determine super(129)I/ super(127)I ratios in DOI was developed for this investigation. The results of a Galveston Bay transect clearly show that super(129)I/ super(127)I ratios in DOI can remain elevated up to salinity of about 15, but that super(129 )I/ super(127)I values of inorganic iodine species do not show any trend with change in salinity gradient due to fast isotopic and chemical equilibration in the estuarine waters.