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

Contaminated aquatic sediments pose health risks to fish, wildlife, and humans and can limit recreational and economic uses of surface waters. Technical and cost effective in situ approaches for sediment management and remediation have been identified as a research need. Subaqueous in situ capping is a promising remedial approach; however, little is known regarding its impact on underlying sedimentary processes and the feasibility of bioaugmented caps at sites subject to contaminated groundwater seepage. This work specifically addresses (1) the impact of capping on biogeochemical processes at the sediment-water interface, (2) the ability and degree to which indigenous sediment microorganisms colonize an overlying cap, (3) the effect of advective flow direction on redox conditions within a cap, (4) natural contaminant bioattenuation processes within capped sediment, and (5) limitations toward a functional bioreactive in situ cap. Laboratory-scale experiments with capped sediment columns demonstrated that emplacement of a sand-based in situ cap induced an upward, vertical shift of terminal electron accepting processes into the overlying cap while simultaneously conserving redox stratification. Upflow conditions simulating a groundwater seep compressed anaerobic processes towards the cap-water interface. Microorganisms indigenous to the underlying sediment colonized cap material and spatial population differences generally reflected redox stratification. Downflow of oxic surface water through the cap, simulating tidally-induced recharge, created fully oxic conditions within the cap, demonstrating that flow direction strongly contributes to redox conditions. Experiments simulating capped sediment subject to contaminated groundwater seepage revealed a reduction of natural bioattenuation processes with time, stemming from the elimination of labile organic matter deposition to the sediment and a subsequent lack of electron donor. Thus, parent contaminants within groundwater seeps will be subject to minimal biotransformations within the sediment before entering a reducing cap. A bioreactive cap, inoculated with microorganisms capable of reductive dehalogenation, was established to reductively dechlorinate tetrachloroethene present in the groundwater; however electron donor amendments to sediment effluent were required to achieve complete dechlorination of tetrachloroethene to non-toxic ethene. Results from this work improve understanding of biogeochemical and bioattenuation processes within capped aquatic sediments and should aid in the development of active capping technologies.