Ebullition Rates Research Articles

Methane dynamics and thermal response in impoundments of the Rhine River, Germany

River impoundments have been identified as important emission hot spots of the greenhouse gas methane. In this study, we investigated methane dynamics of five river impoundments within a two year period using a variety of methods ranging from sediment incubations for measuring methane formation rates (MF), automatic bubble-traps and echo-sounding surveys to assess ebullition fluxes, and estimated diffusive methane fluxes via dissolved concentrations in the water and calculated transport coefficients via wind speed. MF was found to be predominantly acetoclastic, and higher porewater acetate concentrations were associated with higher MF. Moreover, sediment MF showed consistent depth profiles, and when depth-integrated, MF was comparable to bubble-trap ebullition time-series measurements. Thermal response analysed for our systems and a wide range of literature data demonstrated a consistent mean value, but a large range of temperature coefficient Q10 (1.6 to 7.0) for different studies. Annual mean ebullition rates varied over more than one order of magnitude from site to site (0.03 to 1.85 mgCH4 l−1 d−1), demonstrating that river impoundments are not all hot-spots. Future work should investigate the role of sediment delivery, deposition patterns and management on methane emissions by ebullition.

Using Noble Gases to Compare Parameterizations of Air‐Water Gas Exchange and to Constrain Oxygen Losses by Ebullition in a Shallow Aquatic Environment

AbstractAccurate determination of air‐water gas exchange fluxes is critically important for calculating ecosystem metabolism rates from dissolved oxygen in shallow aquatic environments. We present a unique data set of the noble gases neon, argon, krypton, and xenon in a salt marsh pond to demonstrate how the dissolved noble gases can be used to quantify gas transfer processes and evaluate gas exchange parameterizations in shallow, near‐shore environments. These noble gases are sensitive to a variety of physical processes, including bubbling. We thus additionally use this data set to demonstrate how dissolved noble gases can be used to assess the contribution of bubbling from the sediments (ebullition) to gas fluxes. We find that while literature gas exchange parameterizations do well in modeling more soluble gases, ebullition must be accounted for in order to correctly calculate fluxes of the lighter noble gases. In particular, for neon and argon, the ebullition flux is larger than the differences in the diffusive gas exchange flux estimated by four different wind speed‐based parameterizations for gas exchange. We present an application of noble gas derived ebullition rates to improve estimates of oxygen metabolic fluxes in this shallow pond environment. Up to 21% of daily net oxygen production by photosynthesis may be lost from the pond via ebullition during some periods of biologically and physically produced supersaturation. Ebullition could be an important flux of oxygen and other gases that is measurable with noble gases in other shallow aquatic environments.

Quantification of Methane Ebullition Flux from Small Ponds Using the Inverted-Funnel Method

To quantify the ratio of CH4 ebullition to total flux in subtropical shallow ponds, the CH4 flux at the water-air interface was measured using the inverted-funnel and water equilibrium methods in two small ponds in Quanjiao, Anhui Province from July 28 to August 13, 2016. The average CH4 ebullition fluxes were 121.78 and 161.08 mg·(m2·d)-1 and the average diffusion fluxes were 3.38 and 3.79 mg·(m2·d)-1 over pond A and pond B, respectively. The ebullition flux accounted for 97.5% and 96.4% of the total flux over pond A and pond B, respectively. Methane ebullition ranged from 0.11 to 446.90 mg·(m2·d)-1 over pond A and from 0.05 to 607.51 mg·(m2·d)-1 over pond B. Gas ebullition rate during the day was higher than that at night and was controlled by wind speed. Methane ebullition flux was influenced by wind speed over the shallow pond at hourly scale and by water depth and wind speed at daily scale, with positive correlation with wind speed and negative correlation with water depth. Varying with latitude, methane ebullition flux was higher for the water bodies in the mid-latitude region compared to those in the high-latitude region. Direct observations of the methane ebullition flux over small ponds provide data support and theoretical reference to precisely estimate the contribution of inland water bodies to regional and global carbon cycle.

Synergy between nutrients and warming enhances methane ebullition from experimental lakes

Lakes and ponds are important natural sources of the potent greenhouse gas methane (CH4), with small shallow waters identified as particular hotspots1,2. Ebullition (bubbles) of CH4 makes up a large proportion of total CH4 flux3,4. However, difficulty measuring such episodic events 5 makes prediction of how ebullition responds to nutrient enrichment and rising temperatures challenging. Here, the world’s longest running, mesocosm-based, shallow lake climate change experiment was used to investigate how the combination of warming and eutrophication (that is, nutrient enrichment) affects CH4 ebullition. Eutrophication without heating increased the relative contribution of ebullition from 51% to 75%. More strikingly the combination of nutrient enrichment and experimental warming treatments of +2–3 °C and +4–5 °C had a synergistic effect, increasing mean annual ebullition by at least 1900 mg CH4-C m−2 yr−1. In contrast, diffusive flux showed no response to eutrophication and only a small increase at higher temperatures (average 63 mg CH4–C m−2 yr−1). As shallow lakes are the most common lake type globally, abundant in highly climate sensitive regions 6 and most vulnerable to eutrophication, these results suggest their current and future contributions to atmospheric CH4 concentrations may be significantly underestimated. The combination of nutrient enrichment and warming has a synergistic effect on rates of methane ebullition from experimental lakes. This suggests methane emissions from shallow lakes may be significantly underestimated.

Importance of sediment organic matter to methane ebullition in a sub-tropical freshwater reservoir

This paper summarises the results of an extensive field campaign which demonstrates that high sediment organic matter is the primary driver of methane ebullition in a sub-tropical, freshwater reservoir. Methane emissions from freshwater reservoirs represent an important global methane source. Whilst diffusive methane fluxes are commonly used for predicting total emissions, recent studies show that the under-reported ebullitive fluxes can vary by over three orders of magnitude within an individual system and have a greater relative contribution compared to diffusion. Key to predicting this variability in emission rates is to better understand the primary drivers of different emission pathways, and in particular ebullition. Zones of catchment derived organic material as well as elevated water column chlorophyll a concentration have been associated with increased rates of ebullition. Little Nerang Dam (LND), a subtropical freshwater reservoir in Queensland Australia, consistently experiences high rates of ebullition adjacent to major inflow arms that are both deposition zones of catchment organic material as well as areas with elevated water column chlorophyll a concentration. A year-long study of emission rates was undertaken on LND during which water column chlorophyll a concentration, bottom water temperature and sediment organic matter content were assessed for methane ebullition potential. This included a transect with high-spatial sampling (259 sites) from the dam wall to the ebullition zones to explore the relationship between water column chlorophyll a concentration, sediment organic matter content and methane ebullition. These results showed that ebullition was associated with the large forest litter deposits adjacent to major inflow sites where sediment organic matter content was significantly higher compared with the main body of the reservoir.

Deconstructing Methane Emissions from a Small Northern European River: Hydrodynamics and Temperature as Key Drivers.

Methane (CH4) emissions from small rivers and streams, particularly via ebullition, are currently under-represented in the literature. Here, we quantify the methane effluxes and drivers in a small, Northern European river. Methane fluxes are comparable to those from tropical aquatic systems, with average emissions of 320 mg CH4 m-2 d-1. Two important drivers of methane flux variations were identified in the studied system: 1) temperature-driven sediment methane ebullition and 2) flow-dependent contribution suspected to be hydraulic exchange with adjacent wetlands and small side-bays. This flow-dependent contribution to river methane loading is shown to be negligible for flows less than 4 m3 s-1 and greater than 50% as flows exceed 7 m3 s-1. While the temperature-ebullition relationship is comparable to other systems, the flow rate dependency has not been previously demonstrated. In general, we found that about 80% of the total emissions were due to methane bubbles. Applying ebullition rates to global estimates for fluvial systems, which currently are not considered, could dramatically increase emission rates to ranges from lakes or wetlands. This work illustrates that small rivers can emit significant methane and highlights the need for further studies on the link between hydrodynamics and connected wetlands.

Diploptene <i>δ</i><sup>13</sup>C values from contemporary thermokarst lake sediments show complex spatial variation

Abstract. Cryospheric changes in northern high latitudes are linked to significant greenhouse gas flux to the atmosphere, for example, methane that originates from organic matter decomposition in thermokarst lakes. The set of pathways that link methane production in sediments, via oxidation in the lake system, to the flux of residual methane to the atmosphere is complex and exhibits temporal and spatial variation. The isotopic signal of bacterial biomarkers (hopanoids, e.g. diploptene) in sediments has been used to identify contemporary ocean-floor methane seeps and, in the geological record, periods of enhanced methane production (e.g. the PETM). The biomarker approach could potentially be used to assess temporal changes in lake emissions through the Holocene via the sedimentary biomarker record. However, there are no data on the consistency of the signal of isotopic depletion in relation to source or on the amount of noise (unexplained variation) in biomarker values from modern lake sediments. We assessed methane oxidation as represented by the isotopic signal of biomarkers from methane oxidising bacteria (MOB) in multiple surface sediment samples in three distinct areas known to emit varying levels of methane in two shallow Alaskan thermokarst lakes. Diploptene was present and had δ13C values lower than −38 ‰ in all sediments analysed, suggesting methane oxidation was widespread. However, there was considerable variation in δ13C values within each area. The most 13C-depleted diploptene was found in an area of high methane ebullition in Ace Lake (diploptene δ13C values between −68.2 and −50.1 ‰). In contrast, significantly higher diploptene δ13C values (between −42.9 and −38.8 ‰) were found in an area of methane ebullition in Smith Lake. δ13C values of diploptene between −56.8 and −46.9 ‰ were found in the centre of Smith Lake, where ebullition rates are low but diffusive methane efflux occurs. The small-scale heterogeneity of the samples may reflect patchy distribution of substrate and/or MOB within the sediments. The two ebullition areas differ in age and type of organic carbon substrate, which may affect methane production, transport, and subsequent oxidation. Given the high amount of variation in surface samples, a more extensive calibration of modern sediment properties, within and among lakes, is required before down-core records of hopanoid isotopic signatures are developed.

Intense methane ebullition from open water area of a shallow peatland lake on the eastern Tibetan Plateau

Methane fluxes from a shallow peatland lake (3450m a.s.l., 1.6km2 in area, maximum depth <1m) on eastern Tibetan Plateau were measured with floating chamber method during May to August, 2009. The overall average of methane emission rate during the study period was 34.71±29.15mg CH4 m−2 h−1. The occurrence of ebullition among the overall methane flux from Lake Medo was about 74%. The average rate of ebullition was 32.45±28.31mg CH4 m−2 h−1, which accounted for 93% of the overall average of methane emission. Significant seasonal variation was found for occurrence (P<0.05) and rate (P<0.01) of ebullition, both peaking synchronously in mid-summer. Both the occurrence and rate of ebullition were found positively related to sediment temperature but negatively related to lake water depth. The high methane production in the lake sediment was likely fueled by organic carbon loaded from surrounding peatlands to the lake. The shallowness of the water column could be another important favorable factor for methane-containing bubble formation in the sediment and their transportation to the atmosphere. The methane ebullition must have been enhanced by the low atmospheric pressure (ca. 672hPa) in the high-altitude environment. For a better understanding on the mechanism of methane emission from alpine lakes, more lakes on the Tibetan Plateau should be studied in the future for their methane ebullition.

Methane oxidation following submarine permafrost degradation: Measurements from a central Laptev Sea shelf borehole

Submarine permafrost degradation has been invoked as a cause for recent observations of methane emissions from the seabed to the water column and atmosphere of the East Siberian shelf. Sediment drilled 52 m down from the sea ice in Buor Khaya Bay, central Laptev Sea revealed unfrozen sediment overlying ice-bonded permafrost. Methane concentrations in the overlying unfrozen sediment were low (mean 20 µM) but higher in the underlying ice-bonded submarine permafrost (mean 380 µM). In contrast, sulfate concentrations were substantially higher in the unfrozen sediment (mean 2.5 mM) than in the underlying submarine permafrost (mean 0.1 mM). Using deduced permafrost degradation rates, we calculate potential mean methane efflux from degrading permafrost of 120 mg m−2 yr−1 at this site. However, a drop of methane concentrations from 190 µM to 19 µM and a concomitant increase of methane δ13C from −63‰ to −35‰ directly above the ice-bonded permafrost suggest that methane is effectively oxidized within the overlying unfrozen sediment before it reaches the water column. High rates of methane ebullition into the water column observed elsewhere are thus unlikely to have ice-bonded permafrost as their source.

Pumping methane out of aquatic sediments – ebullition forcing mechanisms in an impounded river

Abstract. Freshwater systems contribute significantly to the global atmospheric methane budget. A large fraction of the methane emitted from freshwaters is transported via ebullition. However, due to its strong variability in space and time, accurate measurements of ebullition rates are difficult; hence, the uncertainty regarding its contribution to global budgets is large. Here, we analyze measurements made by continuously recording automated bubble traps in an impounded river in central Europe and investigate the mechanisms affecting the temporal dynamics of bubble release from cohesive sediments. Our results show that the main triggers of bubble release were pressure changes, originating from the passage of ship lock-induced surges and ship passages. The response to physical forcing was also affected by previous outgassing. Ebullition rates varied strongly over all relevant timescales from minutes to days; therefore, representative ebullition estimates could only be inferred with continuous sampling over long periods. Since ebullition was found to be episodic, short-term measurement periods of a few hours or days will likely underestimate ebullition rates. Our results thus indicate that flux estimates could be grossly underestimated (by up to ~50%) if the correct temporal resolution is not used during data collection.

Plant-mediated transport and isotopic composition of methane from shallow tropical wetlands

The methane (CH4) content of gas bubbles in sediments and CH4 emission from plants in the Pantanal wetland of Brazil was examined across a depth gradient to ascertain the relative importance of plant-mediated CH4 emission versus ebullition. Sediment bubbles contained >50% CH4 at sites with water depths >1 m but often contained <50% CH4 in shallower waters. Previous work has shown that bubble % CH4 is related to ebullition rates, and therefore these patterns are suggestive of greater CH4 emission by ebullition from deeper waters compared to waters <1 m in depth. Stable isotope ratios of carbon and hydrogen in the CH4 showed evidence for significant CH4 oxidation but no apparent relation with water depth. Shallower waters typically contain rooted emergent plants with erect, vertical stems or petioles, whereas in waters deeper than ~1 m, emergent aquatic plants are limited to rooted forms with long stems that float at the water surface. Rooted emergent plants typical of deeper waters had low rates of CH4 emission (0.0–0.2 mg d−1 CH4 for individual plants) compared to species typical of inundation depths <1 m (1.6–7.5 mg d−1 CH4). These observations suggest that tropical savanna floodplains like the Pantanal with inundation depths >1 m have low rates of CH4 emission via rooted emergent and floating-leaf plants, in contrast to results from shallower rice fields and temperate wetlands in which plant-mediated transport dominates CH4 emission from vegetated waters.

Estimation of N2 and N2O ebullition from eutrophic water using an improved bubble trap device

Ebullition pathway of N2 and N2O emission and its importance on nitrogen loss were quantified during a survey of a eutrophic pond located at the subtropical climate zone in China. Using an improved bubble trap device, in situ collection of N2 bubbles was achieved by avoiding the contamination of N2 in the air. Measurements using the device indicated very high ebullition rates (36.3–366.7mlm−2h−1) and N2 ebullition flux (0.025–0.297gm−2h−1) at warmer months of September and October. The ebullition rates and N2 ebullition fluxes dropped sharply in colder months of December and January, ranged 2.5–15.9mlm−2h−1 and 0.002–0.016gm−2h−1, respectively. Distinct spatial variation of ebullition rates, and N2 and N2O ebullition fluxes were observed, with the highest rate at the heavy sediment location. Ebullition of N2O was a very minor fraction of total gaseous nitrogen released to air. The data demonstrated that ebullition could contribute greatly to biogenic N2 fluxes in eutrophic waters with significant bubble emission.

Methane production and ebullition in a shallow, artificially aerated, eutrophic temperate lake (Lake Elsinore, CA)

Methane is an important component of the gases released from lakes. Understanding the factors influencing the release is important for mitigating this greenhouse gas. The volume of methane (CH4) and other gases in sediments, and the rate of CH4 ebullition, were determined for an artificially aerated, shallow, eutrophic freshwater lake in Southern California. Gas volume was measured at 28 sites in July 2010, followed by monthly sampling at 7 sites through December 2011. Gas volumes measured in July 2010 at the 28 sites exhibited a complex dependence on sediment properties; the volume of CH4 and other gases was negligible in very coarse-textured sediment with low water and organic carbon contents. Gas volumes increased strongly with increased silt content, and were highest in sediments with intermediate water contents (60 to 70%), organic carbon contents (2 to 3%) and depths (approximately 4m). Methane was the dominant gas collected from sediment (80 to 90%), while carbon dioxide comprised roughly 2 to 3% of sediment gas in the lake. Gas sampling during cool winter months revealed very low or undetectable volumes of gas present, while sediment gas volumes increased markedly during the spring and early summer months, and then declined in late summer and fall. The rate of CH4 ebullition, quantified with an echosounder, also varied markedly across the lake and seasonally. High rates of ebullition were measured at all 7 sites in July 2011 (up to 96mmolCH4m−2d−1), while the rates were >50% lower in September and negligible in December 2010. Ebullition rates were inversely correlated with depth and most other sediment properties, but strongly positively correlated with sand content. No simple relationship between ebullition rate and sediment gas volume across the set of sites was found, although ebullition rates at individual sites were strongly related to gas volume.

Seasonal variations of methane fluxes from an unvegetated tidal freshwater mudflat (Hammersmith Creek, GA)

Wetlands are estimated to contribute nearly 40 % of global annual methane (CH4) emissions to the atmosphere. However, because CH4 fluxes from these systems vary spatially, seasonally, and by wetland type, there is a large uncertainty associated with scaling up the CH4 flux from these environments. We monitored seasonal patterns of CH4 cycling from tidal mudflat wetland sediments adjacent to a vegetated freshwater wetland in coastal Georgia between 2008 and 2009. CH4 emissions were significantly correlated with CH4 production and sediment saturation state with respect to CH4 but not with temperature. CH4 cycling displayed distinct seasonal patterns. Winter months were characterized by low CH4 production and emissions. During the spring, summer and fall, CH4 fluxes exceeded CH4 production in the top 40 cm. Comparison of CH4 sources and sinks in conjunction with the interpretation of CH4 concentration profiles using a 1D reactive transport model indicated that CH4 delivered via lateral tidal pumping likely provided additional CH4 to the upper sediment column. Seasonally high CH4 ebullition rates reflected increased CH4 production and decreased CH4 solubility. The annual CH4 flux was estimated to be on the order of 10 mol CH4 m−2 y−1 which is 2–4 times the global average for wetland CH4 emissions. Thus, even though tidal freshwater mudflats are of limited spatial extent, these environments may serve as globally significant sources of CH4 to the atmosphere. This study highlights the importance of these dynamic environments to the global CH4 cycle and their relevance to climate change.

Ebullition rates and mercury concentrations in St. Lawrence river sediments and a benthic invertebrate

Ebullition, the release of gas from anaerobic decomposition in sediments, was recorded in a mercury-contaminated depositional zone (Zone 1) of the St. Lawrence River Area of Concern in Cornwall, Ontario, Canada. The aim of the present study was to test if this disturbance affected the bioavailability of total mercury (THg) and methylmercury (MeHg) in surficial sediments to a benthic invertebrate (Echinogammarus ischnus). Ebullition rates ranged from <1 to 2,800 ml/m(2) daily, with methane gas comprising 29 to 84% of the total. No direct effects of ebullition were found on either abiotic (sediment or pore water THg or MeHg concentrations) or biotic (amphipod THg or MeHg concentrations) variables measured. Instead, amphipod MeHg concentrations were best predicted by pore water THg and MeHg concentrations, organic matter of surficial sediments, and water depth and location. Trend surface analyses demonstrated that a shallow, southwestern part of Zone 1 was most contaminated with pore water mercury, which decreased in a gradient toward the northeast. Further study is needed to determine if the amount of sediment resuspended by ebullition affects the spatial distribution of mercury.

Biases in discrete CH4 and N2O sampling protocols associated with temporal variation of gas fluxes from manure storage systems

Manure slurry storage systems are possible sources of methane (CH4) and nitrous oxide (N2O), both of which are strong greenhouse gases (GHG). The most commonly used methods to measure these emissions rely on chamber techniques deployed at discrete intervals (i.e. non-continuously). Due to long-term and diurnal variations in GHG emissions, discrete sampling may yield biased estimates when integrating fluxes over time. This research quantified the effect of sampling interval and the ‘time-of-day’ of sample collection on total emission estimates from discrete sampling compared to continuous monitoring, and characterized temporal flux variations in relation to environmental conditions. Methane and N2O emissions were measured continuously over the 6mo warm storage season in 2010 from 6 pilot-scale dairy manure slurry storage tanks using flow-through steady state chambers. Discrete sampling was simulated by extracting data from hourly flux time series at 13 sampling intervals ranging from 1 to 21d. For each sampling interval, 24 datasets were generated, one for each hour of the day. When there were high rates of CH4 ebullition, and crusts did not completely cover the surface, diurnal flux variations were linked with the diurnal surface temperature (T0) cycle, whereby bubbles that accumulated at the surface overnight burst after sunrise. With complete manure surface crusting, episodic and unpredictable flux events were more common. Diurnal variation in N2O emissions was strongly linked with variation in T0 when crusts >2mo old were present. For CH4, sampling between 1800 and 0800h at intervals ≤7d yielded ±10% deviation between discrete and continuous monitoring with a frequency of 96%. The frequency of achieving ±10% deviation for N2O was 50% when sampling at ∼2000h. If sampling during these times is not possible, manual measurements should be made in early morning (before 0900h) and late afternoon (after 1700h).

A new pathway of freshwater methane emissions and the putative importance of microbubbles

Continental freshwater systems have now been shown to be globally significant sources of methane, but there are still large uncertainties associated with freshwater methane fluxes. Studies to date have mainly focused on either ebullition of bubbles originating from sediments or on diffusive fluxes of dissolved methane across the air–water interface. We examined the potential influence of a new mode of methane emission from freshwater systems by comparing the gas exchange velocities derived from carbon dioxide and methane fluxes in a set of diverse systems and environmental conditions. In more than 90% of 260 measurements, methane exhibited higher evasion rates than strictly Fickian diffusive processes would suggest. The portion of flux associated with this non-Fickian diffusion, which we attribute to the presence of semistable microbubbles, was closely related and directly proportional to the degree of methane supersaturation relative to the atmosphere. On average, microbubble-mediated flux contributed about half of the total measured diffusive efflux from the systems and could be modeled as an additive gas piston velocity of, on average, 2.1 m d−1. The microbubble-mediated flux is completely absent in calculations of diffusive methane fluxes derived from ambient pCH4, and is also not necessarily captured in current approaches used to determine ebullition rates. Our results suggest that methane evasion rates based only on measured partial pressures and exchange velocities characteristic of Fickian diffusive processes will grossly underestimate methane losses from these boreal ecosystems.

Quantification of ebullitive and diffusive methane release to atmosphere from a water storage

Accurately quantifying total freshwater storage methane release to atmosphere requires the spatial–temporal measurement of both diffusive and ebullitive emissions. Existing floating chamber techniques provide localised assessment of methane flux, however, significant errors can arise when weighting and extrapolation to the entire storage, particularly when ebullition is significant. An improved technique has been developed that compliments traditional chamber based experiments to quantify the storage-scale release of methane gas to atmosphere through ebullition using the measurements from an Optical Methane Detector (OMD) and a robotic boat. This provides a conservative estimate of the methane emission rate from ebullition along with the bubble volume distribution. It also georeferences the area of ebullition activity across entire storages at short temporal scales. An assessment on Little Nerang Dam in Queensland, Australia, demonstrated whole storage methane release significantly differed spatially and throughout the day. Total methane emission estimates showed a potential 32-fold variation in whole-of-dam rates depending on the measurement and extrapolation method and time of day used. The combined chamber and OMD technique showed that 1.8–7.0% of the surface area of Little Nerang Dam is accounting for up to 97% of total methane release to atmosphere throughout the day. Additionally, over 95% of detectable ebullition occurred in depths less than 12 m during the day and 6 m at night. This difference in spatial and temporal methane release rate distribution highlights the need to monitor significant regions of, if not the entire, water storage in order to provide an accurate estimate of ebullition rates and their contribution to annual methane emissions.

Characterization, performance modeling, and design of an active capping remediation project in a heavily polluted urban channel

Collateral Channel is a heavily polluted former navigation slip to the Chicago Sanitary and Ship Canal (Illinois, USA). Characterization of sediment cores taken in the channel show high levels of heavy metals, polycyclic aromatic hydrocarbons (PAHs) and other contaminants in deposited sediment dating back to the 1800's. Of these, PAHs were the contaminants of greatest concern based upon exceedance of sediment contamination criteria (Σ 16 PAHs up to 1500 mg/kg). Benthic animal counts revealed a lack of biodiversity, with relatively low levels of small tubificid oligochaetes (generally < 3000/m 2) in surficial sediments. Comparison of surficial sediment contaminant levels between 1995 and 2005 showed few decreases in contaminant levels, indicating a lack of “natural recovery” processes occurring in the channel. These results led to an analysis of sediment amendments for an active capping demonstration project in the channel using transport models developed in our previous work (Viana et al., 2008). Based on the sediment characterization and modeling results, the active capping design will be focused on organic contaminant sequestration through the use of organoclay. A site-specific difficulty is the substantial rates of gas ebullition from anaerobic organic matter biodegradation in the sediments, particularly in the summer months. These gases can open advective channels that may result in substantial pollution release and compromise cap effectiveness, and thus the capping scenario must control for such releases. The active capping layer will underlay a sloped sand layer and a high permeability gas venting system to allow biogenically-produced gas migration to shoreline collectors through an innovative support grid. The cap will include an overlaying wetland to remove nutrients from the adjoining Chicago River and provide a public recreational space.

Extreme Methane Emissions from a Swiss Hydropower Reservoir: Contribution from Bubbling Sediments

Methane emission pathways and their importance were quantified during a yearlong survey of a temperate hydropower reservoir. Measurements using gas traps indicated very high ebullition rates, but due to the stochastic nature of ebullition a mass balance approach was crucial to deduce system-wide methane sources and losses. Methane diffusion from the sediment was generally low and seasonally stable and did not account for the high concentration of dissolved methane measured in the reservoir discharge. A strong positive correlation between water temperature and the observed dissolved methane concentration enabled us to quantify the dissolved methane addition from bubble dissolution using a system-wide mass balance. Finally, knowing the contribution due to bubble dissolution, we used a bubble model to estimate bubble emission directly to the atmosphere. Our results indicated that the total methane emission from Lake Wohlen was on average >150 mg CH(4) m(-2) d(-1), which is the highest ever documented for a midlatitude reservoir. The substantial temperature-dependent methane emissions discovered in this 90-year-old reservoir indicate that temperate water bodies can be an important but overlooked methane source.