The growing need for sustainable municipal solid waste treatment and energy production has driven the development of new waste management methods like co-digestion. Anaerobic co-digestion of food waste (FW) and wastewater solids (WWS) has been implemented at a few wastewater treatments plants to efficiently treat organic wastes and produce methane-rich biogas as an energy source. Yosemite National Park has an opportunity to design a new co-digestion facility with an upcoming upgrade to their local wastewater treatment plant in El Portal, California. The Park annually produces approximately 5 million tons of primary WWS and 1 million tons of FW waste, with a volatile solid ratio of 70:30 FW to WWS, or 70% FW. Diverted FW is currently sent to the Mariposa County landfill’s compost facility. To measure the possible increase in biogas production associated with FW addition to WWS, a biochemical methane potential (BMP) test was done over 35 days under mesophilic conditions with treatment mixing ratios ranging from 0% to 100% FW on a volatile solids basis. Calculated annual methane production increased 3.25 times from 0% FW scenario (WWS only) versus a 70% FW scenario, translating to a potential increase in methane production at the wastewater treatment plant of 28,000 to 91,000 m3/yr. Results showed that if the wastewater treatment plant also implemented combined heat and power to combust the increased biogas from 70% FW co-digestion, potentially 920,000 kWh/yr could be produced to cover all electricity and heating needs. This research demonstrates that Yosemite National Park could combine FW and WWS to sustainably manage their organic waste in line with their Zero Landfill Initiative, as well as produce enough energy to fully power the El Portal wastewater treatment plant.

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.

Microorganisms are pivotal in mangrove ecosystems, serving as recyclers and transformers of nutrients in these typically nutrient-limited areas. Auckland’s Waitemata Harbor houses several mangrove stands that are polluted with a slew of hydrocarbons and heavy metals. The primary objective of this study is to identify the level of PAH/heavy metals from Waitemata Harbor mangrove stands that may impact fungal growth. Secondarily, this experiment sought to characterize the microorganisms naturally present within the sediments. Two sites were selected from around the Harbor: Hobson Bay (highly polluted) and Herald Island (less polluted), the sediments from which were either autoclaved or non-autoclaved, inoculated with a growth medium (MEA) along a dilution gradient, inoculated with three species of fungi, and incubated on petri dishes for approximately 20 days. Plates with autoclaved sediment were used to determine the sediment to agar ratio that would allow the most successful fungal growth over time. Plates with non-autoclaved sediment were used to culture the microorganisms naturally present in the sediments. Results indicated that Hobson Bay sediments in higher concentrations are detrimental to Ileodictyon cibarium growth. Trichoderma, Bacillus, Talaromyces and Penicillium species grew on plates with non-autoclaved Hobson Bay sediment, while only Talaromyces species grew on Herald Island sediments. This experiment provides insight into the fungal community composition of the two sites, and may serve as an introductory examination of the bioremediation potential of fungi within the Harbor ecosystem.

Reservoirs are vital components of California’s water infrastructure that allow a population of 40 million to thrive in its drought-prone climate. These engineered impoundments provide additional benefits beyond storing water for irrigation and potable supply. By containing runoff during the rainy season, reservoirs provide incidental flood protection downstream. Reservoirs can benefit the environment by storing water that maintains consistent outflow to creeks that would otherwise go dry. Reservoirs also serve recreational needs of communities by providing local opportunities for fishing, boating, picnicking, and hiking. Many of those who fish in reservoirs rely on their catch to provide themselves and their families with an inexpensive, nutritious meal. However, California’s legacy of mining has contaminated fish throughout the state, putting anglers and their families at risk for mercury poisoning.

About half of reservoirs in California exceed regulatory standards for safe mercury levels in fish set by the State Water Resources Control Board. Some coastal California reservoirs are contaminated by waste material from legacy mercury mining operations. Others have received quicksilver lost to the environment during the California Gold Rush. Even seemingly pristine lakes can host fish with high mercury concentrations that result from deposition of mercury released into the atmosphere from fossil fuel combustion around the world.Though mercury is typically present in trace concentrations in reservoir sediments, reservoirs create conditions that are conducive to the production and bioaccumulation of neurotoxic methylmercury. When inflow decreases and air temperatures rise in the springtime, reservoirs undergo thermal stratification. The warm, buoyant epilimnion floats atop the cold, dense hypolimnion, blocking off bottom waters from oxygen input from the atmosphere and photosynthesis. Aerobic bacteria consume the remaining dissolved oxygen in the hypolimnion, creating anoxic conditions in bottom waters and at the sediment-water interface. Under anoxic conditions, some anaerobic bacteria can convert inorganic mercury present in the water and sediments into methylmercury. Methylmercury is a potent neurotoxin that can cause neurological damage, cardiovascular disease, and reproductive impairment in humans and wildlife. Methylmercury is released into the water column where it concentrates in phytoplankton by millions of times its concentration in water. Methylmercury is concentrated further up the food chain in zooplankton, planktivorous fish, and predatory fish, reaching dangerously high (>0.5 ppm) concentrations in fish that are consumed by humans and wildlife. The complexity of mercury cycling and bioaccumulation in reservoir systems presents a wide array of potential management strategies. Reservoir managers could decrease methylmercury levels in fish by limiting the microbial conversion of inorganic mercury to methylmercury, by lowering its introduction into the food web, or by controlling trophic transfer between organisms. This dissertation investigated these three different aspects of reservoir mercury management in a series of field and laboratory studies. The overall objective of this dissertation was to investigate methods for controlling methylmercury production and bioaccumulation in reservoirs contaminated by the former New Almaden Mining District, North America’s largest historical mercury mine.

Chapter 1 is a manipulated field experiment to evaluate the effects of hypolimnetic oxygenation on mercury cycling and fish tissue mercury in contaminated reservoirs. Using a 15-year dataset consisting of water quality and fish tissue measurements collected prior to and during reservoir oxygenation, I conducted statistical analyses to identify key changes in water quality and fish tissue data. Results indicated declining trends in fish mercury in two of four reservoirs, likely the result of increased primary productivity and not due to the inhibition of methylmercury production. Chapter two consists of a set of sediment slurry incubations aimed at evaluating the use of manganese oxide and activated carbon sediment amendments for mercury control through redox buffering and sorption. Manganese oxide and activated carbon amendments were equally effective in decreasing methylmercury concentrations in sediment porewater. Manganese oxide amendments were rapidly reduced to Mn2+, warranting further optimization to increase longevity. Chapter three is an observational study aimed at characterizing plankton dynamics and methylmercury bioaccumulation in the pelagic food web. An increased understanding of bioaccumulation in lower-trophic level organisms is necessary when considering potential ecological mercury management options (e.g., fish stocking). Overall, results demonstrated the seasonal patterns of MeHg introduction into the pelagic food web and how complex and counteracting factors may control the magnitude of MeHg bioaccumulation. Collectively, the three chapters of this dissertation provide key information that will inform future mercury management efforts in the study reservoirs.

The historic gold rush era and atmospheric deposition in California have led to mercury (Hg) contamination in surface waters. The organic form of Hg, methylmercury (MeHg), bioaccumulates in the food chain and poses a negative health effect for wildlife and humans consuming contaminated fish, seafood, and rice. Controlling MeHg production is challenging due to its dependency on multiple site-specific and interrelated environmental conditions. In this dissertation, I examined the factors contributing to MeHg production, the methylation window, in two aquatic environments: Hodges Reservoir (a lake-like environment), and Cache Creek Settling Basin, CCSB (a wetland-like environment). Additionally, I explored two mercury remediation techniques, a hypolimnetic oxygenation system (HOS) and coagulation with metal-based salts. In Hodges Reservoir, a hypereutrophic sulfate-rich water supply reservoir located in San Diego, CA, field monitoring revealed that before oxygenation, winter precipitation and changes in hypolimnion volume mechanistically affected the biogeochemistry processes of the reservoir, especially the Hg cycle (Chapter 1). During wet years, a spring methylation window opened as precipitation provided additional mass of oxygen due to a larger hypolimnion. This maintained mildly reduced conditions that enhanced Hg methylation in bottom waters. But during dry years, the methylation window closed earlier in the season because of the development of highly reduced conditions favoring demethylation relative to methylation, and due to the buildup of sulfide concentration decreasing inorganic Hg bioavailability. Following HOS implementation, oxygenation appeared to suppress anaerobic bacteria activity responsible for MeHg production, indicating the potential usefulness of techniques designed to keep bottom lake water oxidized for repressing MeHg production. In CCSB, located in the Sacramento Valley, CA, the United States Geological Survey (USGS) studied the use of the coagulants ChitoVan™ (organic/shell amino-based), Ferralyte® (ferric-sulfate-based), and Ultrion® (polyaluminum chloride-based) to immobilize Hg from Cache Creek into the sediment. Bench-scale sediment-water slurry incubations with CCSB soils from USGS field experiments demonstrated that organic matter is the main driver for MeHg production (Chapter 2). The three different coagulant-treated soils were not a source of MeHg under rewetted conditions (standard test). However, under elevated organic matter loading (stress test), inorganic Hg could be potentially released. In a follow-up experiment using CCSB non-treated soils and water under a gradient of algal organic matter (Spirulina powder), the addition of more than 0.1 g of algal organic matter to 250 ml sediment-water slurries appeared to activate a diverse microbial community that led to an ephemeral window of MeHg production (Chapter 3). Analysis of dissolved organic matter (DOM) optical properties using fluorescence spectroscopy indicated that some regions of the DOM (F, B, and FI index) could correlate to the window of MeHg production. This suggests that monitoring DOM optical characteristics in the field could hold promise in better understanding MeHg production and bioaccumulation in aquatic ecosystems. Overall, the outcomes of these studies indicate that oxygenation and controlling organic matter loading are viable management strategies to mitigate Hg contamination in managed aquatic ecosystems. Note, Appendix includes supplementary material and raw data for Chapters 2 and 3.