Selenium Stable Isotope Ratios in Wetlands: Insights Into Biogeochemical Cycling and How a Diffusive Barrier Affects the Measured Fractionation Factor

Wetlands remove many dissolved pollutants, such as SeO4 2-, Cr(IV), NO3-, and perchlorate, from surface waters via reduction, sorption, and assimilation. Isotope fractionation studies can potentially distinguish reduction from other removal processes, and possibly quantify the amount of reduction, assuming that the correct isotope fractionation factor, alpha, is applied. Typically, alpha is determined under laboratory conditions and subsequently applied to field data. In Chapter 2, I present effective fractionation factors, alphaeff, determined for a series of selenate reduction experiments in which reduction occurred within sediments and the fractionation was measured in overlying waters. The experiments showed that alphaeff measured in open waters overlying an intact sediment core was significantly less than that measured in a continuously-mixed, anoxic slurry of the same sediments. This difference can be attributed to the effect of a diffusion zone separating the reactant pool from the reaction sites in the sediments. Consequently, degree of reduction and dominant reduction pathways may be miscalculated when isotopic fractionation factors obtained from laboratory experiments are directly applied to field data. Chapter 3 presents results of the first comprehensive, multi-year (2003--2005) isotope and concentration survey of dissolved Se oxyanions and reduced Se in a wetland/lake setting. Overall, no strong isotopic contrast was observed, with the mean values of Se oxyanions and major reduced reservoirs in plants and sediments of delta80/76SeSRM = 1.84‰ to 2.65‰. These data are consistent with the results in Chapter 2 in that Se isotopes did not provide a means of detecting Se(VI) reduction or of discerning the dominant removal pathway for Se(VI). Concentration and isotope trends suggest most of the dissolved Se(IV) in the lake is from oxidation of reduced forms of Se within the wetland/lake system, not reduction of Se(VI). Release of Se from weathered shale, and delivery into surface water, induces only minor isotopic fractionation Limited earlier reports that phytoplankton and macrophytes are slightly depleted in heavier isotopes relative to dissolved Se(VI) are confirmed, though macrophytes may contain a minor, more strongly depleted component. Selenium in zooplankton and fishes may be slightly depleted relative to Se in phytoplankton.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Geology

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2015

Author dc:creator

• Clark, Scott Kelvin

Contributors dc:contributor

• Johnson, Thomas M.

• Biogeochemistry

Language dc:language

eng

Identifier

(MiAaPQ)AAI3301119

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/86538