Sediment capping effects on gas ebullition, hyporheic exchange and benthic microbial community structure

The research described in this dissertation added to the current body of literature on the effectiveness of active capping in mitigating ebullition facilitated contaminant fluxes and in lowering gas ebullition rates, during an active capping sediment remediation in the WBGCR. The three-year post-capping study provided a comprehensive dataset on ebullition rates and the influence of environmental parameters with reductions in gas ebullition rates of 84%, 63% and 61%. The post-capping gas fluxes were also strongly influenced by sediment temperature and water depth. Incubation experiments to assess gas production potential of the post-cap sediment layers showed that cumulative gas production was similar in the contaminated sediment (CSed) and new deposit (ND) layers, whereas the armor (GAL) and organoclay (OrgC) layers exhibited minimal gas production. These results provide further evidence that the CSed layer is ebullition active and thus continued ebullition is likely following capping. This study also evaluated active capping performance in mitigating ebullition-facilitated metal and PAH transport. Metal fluxes were lowered by 89-97% with re-suspension of surficial sediment being the primary mode of transport. PAH fluxes also fell sharply in the first year but increased to 60% of pre-capping levels in 2013 and followed again by a decrease in 2014. The rise and fall of PAH flux in 2013 and 2014 were accompanied by a rise and fall of sediment temperature although average gas fluxes were similar. This suggests that the higher temperatures stimulated increased gas production in the CSed layer thereby increasing PAH partition and transport for 2013. Increases in sediment temperatures could reactivate gas generation in the CSed layer, with subsequent potential for cap fracture, enhanced advective transport and lower design breakthrough times. The impact of capping on the sediment Archaeal community structure was evaluated by using phylogenetic analysis of 16 sRNA genes from pre- and post-capping sediment. The archaeal community structure was dominated by methanogens in both pre-and post-capping sediment. Capping resulted in a more diverse distribution of methanogens in the surficial zone, with evidence of methanogenesis occurring via the three major methanogenic pathways: hydrogenotrophic, acetoclastic and C1-methylotrophic methanogenesis where as pre-cap Archaea were dominated by acetoclastic and hydrogenotrophic methanogens. Field measured gas fluxes were also significantly correlated with Methanosaeta abundance in pre- and post-capping sediment, suggesting that acetoclastic methanogenesis controls gas production in the GCR. The analysis also revealed the presence of Ammonia Oxidizing Archaea (AOA) in increasing abundance with depth, suggesting a more important role for these newly discovered group in contaminated sediments. This research also explored the potential for using heat tracer methods to evaluate GW-SW interactions and the Darcy velocity in different layers of post-cap sediment. Two approaches utilizing amplitude/phase shift and forward modeling of temperature signal were used. The McCallum method provided a more comprehensive insight into the nature of GW-SW interaction in the top 25 cm, with GW fluxes strongly influenced by stream depth and storm events. The Darcy velocity were also found to decreased with depth in the OrgC and CSed layers, suggesting the presence of horizontal flow paths below the gravel layer. The higher velocities observed in the gravel layer suggest that the armor should not be viewed as additional protection against contaminant migration. Also, the higher seepage velocities can rapidly transport heat and nutrients to the subsurface thereby increasing the potential for gas production below the cap. Heat tracer methods if implemented properly can provide Darcy estimates with lower uncertainty compared to traditional methods.