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Redox and oxygen control on water quality and mercury cycling in the profundal zone of California reservoirs

Creative Commons 'BY-NC-SA' version 4.0 license

California reservoirs are vital resources that serve as a source of raw water for drinking water production, as well as providing other beneficial uses such as flood control, recreation, and wildlife habitat. These benefits can be limited by the excessive growth of algae and poor water quality associated with nutrients enrichment and eutrophication. In eutrophic reservoirs, nutrients can be released from profundal sediment when oxygen is depleted in the bottom water. These nutrients can exacerbate eutrophication if mixed into surface waters. Hypolimnetic anoxia is associated with a variety of additional water quality issues including reduced water treatment efficiency and the release of toxins.

The first objective of this dissertation was to investigate the economic implications and greenhouse gas emissions associated with hypolimnetic oxygenation, a system designed to prevent anaerobic conditions in bottom waters to improve water quality and treatability. This study performed an economic analysis and a life cycle assessment (LCA) of a hypolimnetic oxygenation system (HOS) installed in Upper San Leandro (USL), a drinking water reservoir in Oakland, California. The assessment compared water treatment and reservoir management operations under two scenarios, past conditions without a HOS and current conditions where a HOS is operated to improve water treatability. The largest economic benefit and reduction in greenhouse gas emissions (GHG) for the HOS scenario relative to previous operations was obtained from the reduction in ozone use for disinfection in the water treatment process. Construction costs had a more substantial impact than the cost of oxygen use in the net present value (NPV) of the HOS over its lifetime. In contrast, the LCA revealed that HOS oxygen use contributed the larger proportion of GHG emissions. Overall, results indicate that the implementation of HOS in USL was profitable and will lead to reduced GHG emissions over the assumed 60-year life of the system.

The second objective of this research was to investigate mercury cycling in Hodges Reservoir, a hypereutrophic sulfate-rich reservoir in San Diego, California. Anaerobic conditions at the sediment-water interface are associated with the buildup of methylmercury (MeHg), an organic form of mercury produced by anaerobic bacteria, in profundal sediment and water. MeHg poses a threat to ecosystem and human health due to its ability to bioaccumulate in aquatic food webs. The overarching goal of this work was to gain deeper insight into MeHg biogeochemical cycling during anaerobic conditions in the profundal zone to inform reservoir management about key mechanisms involved in mercury production, with the aim of lowering mercury bioaccumulation and protecting human and wildlife health.

The first study in this effort tracked MeHg production, degradation, and release into hypolimnetic water while anaerobic conditions were prevalent at the sediment-water interface. This involved tracking MeHg in the sediment, porewater, and water column, as well as associated parameters of interest such as redox acceptors and organic matter. Sediment-associated MeHg was greatest at the onset of anaerobic conditions, suggesting MeHg production in profundal sediment may be greatest during the oxic-anoxic transition, due to the abundance of redox acceptors to fuel anaerobic respiration. The depletion of bioavailable iron-oxide occurred simultaneously with the liberation of MeHg from the sediment, suggesting iron-oxide dissolution led to the release of organic-matter-associated MeHg to overlay water. The activity of sulfate-reducing bacteria (SRB) in the late spring and early summer was associated with the build-up of MeHg in profundal sediment. The depletion of sulfate in the porewater during mid-summer appears to have led to methanogenesis and high levels of biodemethylation.

A second study used microcosm incubations of profundal sediment and bottom water to assess seasonal patterns of MeHg cycling under various chemical treatments. Treatments included addition of air, organic carbon, and microbial inhibitors. Both aeration and sodium molybdate, a SRB inhibitor, generally caused a decrease in MeHg concentration, likely by inhibiting SRB activity. A methanogenic inhibitor resulted in a significant increase in MeHg concentration, indicating a suppressive effect by methanogens on net MeHg production, potentially due to enhanced microbial demethylation. Pyruvate resulted in a significant increase in MeHg concentration in the spring when the sediment-water interface was moderately reduced, but caused a significant decrease in the fall under highly reduced conditions. Acetate resulted in a significant increase in MeHg concentration, likely due to the stimulation of acetogenic SRB.

Collective results highlight the complex temporal dynamics of MeHg cycling at the sediment-water interface, which is regulated by organic matter composition, redox acceptor availability, and sediment microbial community structure and activity. MeHg production and release at the sediment-water interface was elevated under moderately reduced conditions, which may be the primary period of MeHg entry into the water column and aquatic food web entry. This indicates that management strategies to repress mercury bioaccumulation should focus on the key window of moderately reducing conditions during the onset of anaerobic conditions. Hypolimnetic oxygenation, if it is designed and operated to maintain an oxygenated sediment-water interface, could be a viable approach to repress the MeHg release observed with iron-oxide dissolution and MeHg production in the surficial sediment by SRB. The study also showed that that the sediment-water interface transitioned to be a net sink for MeHg during highly reduced conditions in the fall, likely due to enhanced demethylation by methanogens. Thus, highly reduced conditions appear to be a sink for MeHg in the profundal zone. This indicates that care must be taken to ensure the sediment-water interface is fully oxygenated to prevent mildly reducing conditions associated with MeHg production and release. Together, the results of this comprehensive research suggest that hypolimnetic oxygenation can be an economically viable and environmentally sound solution to enhancing source water treatability while potentially reducing MeHg bioaccumulation.

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