The production of methyl mercury, an acute neurotoxin which readily
accumulates in the tissue of organisms, is a biologically mediated process facilitated by
sulfate reducing bacteria in aquatic sediments. In-situ capping is a frequently considered
risk management strategy for contaminated sediments. Since placement of an in-situ cap
will induce anaerobic conditions that are known to be favorable for the growth of sulfate
reducing bacteria, there is justifiable concern that capping could increase mercury
methylation in underlying sediments. This research builds an understanding of the
effects of in-situ capping on underlying biogeochemical processes and elucidates their
importance in controlling methyl mercury production. Laboratory experiments and
mathematical models were implemented to simulate mercury methylation in redox
conditions likely to be induced by capping using sediment from different environments. Mathematical descriptions of processes known to be involved in methylation were
incorporated into the model to quantify the effects of these processes.
Observations in both well-mixed slurry conditions and intact sediment columns
showed that methyl mercury concentrations are strongly dependent upon biogeochemical
conditions. Results from experiments with sediment spanning a range of redox
conditions and organic contents suggested that sulfate reduction rates, aqueous
speciation, and solid phase partitioning are involved in limiting methylation depending on
bulk geochemical characteristics. A model with a mechanistic basis that incorporates the
effects of these processes provides a useful means of qualitatively and quantitatively
considering their cumulative impact in limiting methyl mercury production. High methyl
mercury concentrations observed in some lab experiments suggest that there is reason to
be concerned about anoxic conditions induced by capping; however, not all anoxic
conditions led to equivalent increases in methyl mercury. Experimental and modeling
results suggest that in a high organic environment, in-situ capping may produce
conditions which accelerate methylation in (formerly) surficial sediment while in a low
organic environment, with an overall lower potential for methylation, capping can be
expected to have a less dramatic effect. Over time, two processes will temper capinduced
increases in methyl mercury. Increases will only last until sulfide builds up to
inhibitory levels in underlying sediment or until organic carbon is depleted and overall
bacterial activity slows. By providing a more fundamental understanding of the effects of
capping on mercury methylation, the results of this research will aid in identifying
situations and conditions in which cap-induced increases in methyl mercury have the
potential to limit the effectiveness of the management strategy. / text
| Identifer | oai:union.ndltd.org:UTEXAS/oai:repositories.lib.utexas.edu:2152/6550 |
| Date | 16 October 2009 |
| Creators | Johnson, Nathan William |
| Source Sets | University of Texas |
| Language | English |
| Detected Language | English |
| Format | electronic |
| Rights | Copyright is held by the author. Presentation of this material on the Libraries' web site by University Libraries, The University of Texas at Austin was made possible under a limited license grant from the author who has retained all copyrights in the works. |
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