Profundal lake sediment is an important site of toxic methylmercury (MeHg) production by anaerobic bacteria including sulfate-reducing bacteria (SRB). This study tracked sediment MeHg production, release and degradation in hypereutrophic Hodges Reservoir, USA. Sediment-associated MeHg was elevated in the early spring, suggesting production in profundal sediment during moderately reduced conditions. Later in the spring, sediment MeHg was released into hypolimnetic water, coincidental with the loss of bioavailable iron-oxides, suggesting iron-oxide dissolution led to the co-release of organic matter-associated MeHg. SRB activity in the late spring and early summer, as demonstrated by a drop in porewater sulfate, was associated with sediment buildup of MeHg, likely due to enhanced sorption to biogenic sulfides and organic matter. In the fall, sediment MeHg decreased, potentially a result of demethylation by methanogenic bacteria. Shortly afterwards, MeHg increased in the hypolimnetic water column, indicating an upward shift in the zone of SRB methylation. Our study suggests two “hot moments” of MeHg entry into the water column where it is susceptible to uptake into the pelagic food web: a spring window of mildly reduced conditions that promote MeHg release from sediment, and a fall window where MeHg is produced in the upper hypolimnetic water column.

Methylmercury (MeHg) produced by anaerobic bacteria in lakes and reservoirs poses a threat to ecosystem and human health due to its ability to bioaccumulate in aquatic food webs. This study used 48-hour microcosm incubations of profundal sediment and bottom water from a sulfate-rich, hypereutrophic reservoir to assess seasonal patterns of MeHg cycling under various treatments. Treatments included addition of air, inorganic mercury, organic carbon, and microbial inhibitors. Both aeration and sodium molybdate, a sulfate-reducing bacteria (SRB) inhibitor, generally decreased MeHg concentration in microcosm water, likely by inhibiting SRB activity. The methanogenic inhibitor bromoethanesulfonate increased MeHg concentration 2- to 4-fold, suggesting that methanogens were potent demethylators. Pyruvate increased MeHg concentration under moderately reduced conditions, likely by stimulating SRB, but decreased it under highly reduced conditions, likely by stimulating methanogens. Acetate increased MeHg concentration, likely due to the stimulation of acetogenic SRB. Results suggest that MeHg production at the sediment-water interface is elevated under moderately reduced conditions (-100 to -50 mV). In contrast, it is suppressed under oxic conditions due to low SRB activity, and under highly reduced conditions (> -100 mV) due to enhanced demethylation by methanogens.

Almaden, Calero, and Guadalupe reservoirs (San Jose, CA, USA) are small (volume < 13 million m3) surface water reservoirs polluted by the former New Almaden Mining District, North America’s most productive historical mercury (Hg) mine. Fish tissue mercury (fish Hg) concentrations in Guadalupe Reservoir are among the highest in the USA, up to 4.9 mg/kg (muscle tissue, wet weight) in 35 cm length-standardized bass. Stevens Creek Reservoir (Cupertino, CA, USA) also has elevated fish Hg concentrations, but no historical mining source. We report a 15-year dataset to evaluate the effectiveness of line diffuser hypolimnetic oxygenation systems (HOSs) in reducing methylmercury (MeHg) concentrations in reservoir water and fish after five consecutive years of operation. HOSs were installed in each reservoir to increase dissolved oxygen concentrations in bottom water, thereby suppressing the activity of anaerobic bacteria (e.g., sulfate-reducing bacteria) known to produce MeHg. Before HOS operation, MeHg concentrations increased in bottom waters of all four reservoirs during periods of thermal stratification and profundal anoxia. MeHg concentrations decreased significantly in bottom waters during HOS operation, with mean reductions of 63%-85% below pre-oxygenation concentrations. However, MeHg concentrations were unchanged or increased in surface waters. This could be the result of enhanced mixing between surface and bottom waters as a result of line diffuser oxygenation, or continued Hg methylation occurring in the oxic water column and littoral sediments. Despite little change in whole water column MeHg concentrations, we observed modest but significant declining trends in fish tissue Hg in Guadalupe and Stevens Creek reservoirs. Results suggest that oxygenation, rather than directly lowering MeHg in water, may have mixed nutrients into surface waters, thereby enhancing primary productivity and indirectly affecting Hg bioaccumulation by diluting concentrations in phytoplankton.

Anaerobic co-digestion of food waste (FW) from municipal solid waste and wastewater solids (WWS) from domestic wastewater shows potential to enhance methane-rich biogas production at existing wastewater treatment facilities while diverting solid waste from landfills. This study assessed the potential to implement co-digestion of FW and WWS at Yosemite National Park (YNP), which is upgrading its wastewater treatment facilities. A 35-day biochemical methane potential test was performed in triplicate incubations with varying %FW by volatile solids. The anaerobic co-digestion process was stable across all treatments, but methane production rates appeared lowest in 90 and 100 %FW treatments. Specific methane yield increased linearly with increasing %FW with a peak at 75 %FW (239 mL CH4/g VS at day 35), while volumetric methane yield increased exponentially. Results highlight the potential for co-digestion of WWS and FW to enhance methane production and suggest 75 %FW as an upper limit for optimal biogas production for the organic wastes in this study. YNP produces 5.1 million kg/d of WWS and 1.0 million tons of FW waste, yielding a 70 %FW co-digestion feed stock. Co-digesting this feed stock in existing anaerobic digestion facilities would increase methane production from 28,000 m3/yr (WWS only) to 91,000 m3/yr (70 %FW). Harvesting energy from this enhanced methane production via a combined heat and power system could produce enough energy to both heat digestor facilities and power the treatment plant.

The byproducts of anaerobic biogeochemical processes, many mediated at the sediment–water interface, can degrade water quality in the bottom of reservoirs. Using both experimental sediment–water chambers and field monitoring, this study assessed nutrient and metals cycling in the profundal zone of hypereutrophic Hodges Reservoir, San Diego (maximum depth = 29.1 m; maximum surface area = 6.0 km2 ). A focus of the study was methylmercury (MeHg), a toxic form of mercury that bioaccumulates in aquatic food webs. Results confirmed that oxic conditions repressed release of redox-sensitive compounds (e.g., MeHg, ammonia, phosphate, manganese) from profundal sediment, though high sediment oxygen demand complicated experimental efforts to maintain a well-oxygenated sediment–water interface. In both experimental chambers and in the reservoir, patterns of MeHg cycling correlated with manganese cycling, suggesting that moderately low redox conditions that stimulate reductive dissolution of manganese also enhance net Hg methylation. Sediment MeHg flux was tightly coupled with sulfate uptake under moderately reduced conditions, reinforcing the link between sulfate-reducing bacteria and Hg methylation. Results highlight the fact that hypolimnetic oxygenation in Hodges Reservoir using pure oxygen gas, which is planned for spring 2020, must maintain high oxygen concentrations at the profundal sediment–water interface. In addition, results indicate that the presence of manganese in surface water can be used as an indicator of oxygenation’s effectiveness in lowering rates of internal nutrient and MeHg loading.

In the present study, mercury (Hg) concentrations were investigated in lichens (Flavoparmelia caperata (L.) Hale, Parmelia saxatilis (L.) Ach., and Xanthoria parietina (L.) Th.Fr.) collected in the surrounding of the dismissed Abbadia San Salvatore Hg mine (Monte Amiata district, Italy). Results were integrated with Hg concentrations in tree barks and literature data of gaseous Hg levels determined by passive air samplers (PASs) in the same area. The ultimate goal was to compare results obtained by the three monitoring techniques to evaluate potential mismatches. Lichens displayed 180–3600 ng/g Hg, and Hg concentrations decreased exponentially with distance from the mine. Mercury concentration was lower than in Pinus nigra barks at the same site. There was a moderate correlation between Hg in lichen and Hg in bark, suggesting similar mechanisms of Hg uptake and residence times. However, correlation with published gaseous Hg concentrations (PASs) was moderate at best (Kendall Tau = 0.4–0.5, p > 0.05). The differences occurred because a) PASs collected gaseous Hg, whereas lichens and barks also picked up particulate Hg, and b) lichens and bark had a dynamic exchange with the atmosphere. Lichen, bark, and PAS outline different and complementary aspects of airborne Hg content and efficient monitoring programs in contaminated areas would benefit from the integration of data from different techniques.

Low dissolved oxygen (DO) in the sediments of Camanche Reservoir, California (513 million m3, 31 km2), produced toxic hydrogen sulfide (H2S). Direct hypolimnetic oxygenation suppressed H2S without provoking early destratification and cold-water fish problems downstream. A cool, bubble-free plume directed horizontally over the sediments was chosen over a rising bubble plume. A submerged, down-flow contact oxygenator (Speece cone) pumped anoxic water from 5 m above the sediments to the top of a 7 m high cone to dissolve a counterflow of rising pure oxygen bubbles. The bubble-free, highly oxygenated discharge (80 mg/L) was diluted to fish-safe levels (8 mg/L) and directed up-reservoir via jets in a 45 m long manifold. Placing the cone on the bottom near the dam increased hydraulic pressure and doubled oxygen solubility. Poor-quality hypolimnion water (DO <2 mg/L, redox 18–100 mV, H2S odors) was converted to good-quality water (DO 3–7 mg/L, redox >300 mV, no H2S odors). Comparing preoxygenation hypolimnion DO decline (0.1 mg/L/d) with oxygenation temporarily switched off (0.23 mg/L/d) gave a full-scale estimate of induced hypolimnion oxygen demand. In 1994, the oxygenated plume moved 4.5 km upstream at 0.1 cm/s via natural water motion. No long pipes were needed. About 18% of the bottom hypolimnion was directly oxidized in the cone and 1.8 times the total volume was indirectly oxidized via entrainment in the plume. After 10 yr, oxygen additions were reduced by >50% with no deleterious effects.

This study evaluated the ability of hydrous ferric oxide reactive filtration (HFO-RF) to remove mercury (Hg) from municipal secondary effluent at four study sites. Pilot HFO-RF systems (136 m3 /day) at two sites demonstrated total Hg concentration removal efficiencies of 96% (inflow/outflow mean total Hg: 43.6/1.6 ng/L) and 80% (4.2/0.8 ng/L). A lightly loaded medium-scale HFO-RF system (950 m3 /day) had a concentration removal efficiency of 53% (0.98/0.46 ng/L) and removed 0.52 mg/day of total Hg and 2.2 μg/day of methyl-Hg. A full-scale HFO-RF system (11,400 m3 / day) yielded a total Hg concentration removal efficiency of 97% (87/2.7 ng/L) and removed an estimated 0.36 kg/year of Hg. Results suggest that the quality of secondary effluent, including dissolved organic matter content, affects achievable minimum total Hg concentrations in effluent from HFO-RF systems. Low HFO-RF effluent concentrations (<1 ng/L) can be expected when treating secondary effluent from suspended-growth biological treatment systems. 

A long-term improvement in water quality due to a hypolimnetic oxygenation system (HOS) used without other management actions was shown for Camanche Reservoir in California (volume = 514 million m3). Prior to oxygenation, the reservoir was eutrophic with low water clarity, high chlorophyll a (Chl-a), and blooms of cyanobacteria primarily caused by internal nutrient loading from anoxic sediments. All 4 trophic state indicators showed major improvement, most pointing to a new oligotrophic state. Days after HOS startup in July 1993, hypolimnion soluble phosphate and ammonium concentrations declined and, unexpectedly, nitrate remained low. This report covers pre-HOS (1988 to mid-1993) and 12 yr of post-HOS measurements (mid-1993 to mid-2005). For surface waters during the growth season (Apr–Oct) after HOS, average Chl-a decreased 79% (14.6–3.1 μg/L) and peak Chl-a decreased sixfold (from 49 to 8 μg/L). Average Secchi depth increased 10-fold (0.47 to 4.9 m) and peak Secchi depth doubled (4.2–8 m). Cyanobacteria (Aphanizomenon and Dolichospermum, formerly Anabaena) decreased >95% while the previously absent Merismopedia, an indicator of clean water, appeared. The large colonial diatom Fragilaria also decreased by >90%. Zooplankton (rotifers and crustaceans) appeared unchanged. After HOS, late winter surface nutrients prior to the next spring bloom were much reduced (total phosphorus, TP, by 58%, total inorganic nitrogen, TIN, by 88%). The TIN:TP ratio dropped from 6 to 1.6. In 2004, the last full year of measurements, further declines were found for nitrate nitrogen (42–3 μg/L) and Chl-a (3.1–2.8 μg/L) while TP was unchanged (14 μg/L).

The upper Columbia River and associated valley systems are highly contaminated with metal wastes from nearby smelting operations in Trail, British Columbia, Canada (Teck smelter), and to a lesser extent, Northport, Washington, USA (Le Roi smelter). Previous studies have investigated depositional patterns of airborne emissions from these smelters, and documented the Teck smelter as the primary metal contamination source. However, there is limited research directed at whether these contaminants are bioavailable to aquatic organisms. This study investigates whether smelter derived contaminants are bioavailable to freshwater zooplankton. Trace metal (Zn, Cd, As, Sb, Pb and Hg) concentrations and Pb isotope compositions of zooplankton and sediment were measured in lakes ranging from 17 to 144 km downwind of the Teck smelter. Pb isotopic compositions of historic ores used by both smelters are uniquely less radiogenic than local geologic formations, so when zooplankton assimilate substantial amounts of smelter derived metals their compositions deviate from local baseline compositions toward ore compositions. Sediment metal concentrations and Pb isotope compositions in sediment follow significant (p < 0.001) negative exponential and sigmoidal patterns, respectively, as distance from the Teck smelting operation increases. Zooplankton As, Cd, and Sb contents were related to distance from the Teck smelter (p < 0.05), and zooplankton Pb isotope compositions suggest As, Cd, Sb and Pb from historic and current smelter emissions are biologically available to zooplankton. Zooplankton from lakes within 86 km of the Teck facility display isotopic evidence that legacy ore pollution is biologically available for assimilation. However, without water column data our study is unable to determine if legacy contaminants are remobilized from lake sediments, or erosional pathways from the watershed.