Concentrated Stream Research Articles

Near-extinction transient behaviors of counterflow nonpremixed flames in a porous spherical burner at very low stretch rate

Near extinction transient behaviors of CH4/N2 versus O2/N2 counterflow nonpremixed flames were investigated by adopting a porous spherical burner with a large radius of curvature to achieve a very low buoyant stretch rate. By varying the oxygen concentrations in the oxidizer stream, four regimes were identified by three limits of sooting, instability, and extinction. The sooting limit was sensitive to flame temperature, and was characterized by two indexes of fuel injection velocity uF and oxygen concentration XO2. When decreasing uF, the sooting limit was different from high stretch flames because of appreciable heat loss toward the burner surface. In the instability regime, periodic holes were observed whose frequency decreased with XO2. Periodic holes were also found to occur with N2 dilution in the fuel stream, which were more regular with larger frequencies compared with the periodic holes in the diluted oxidizer streams. The edge propagation behaviors for periodic holes in diluted fuel streams were analyzed. The statistical average of the propagation speed of advancing edge (hole closing) Se decreased with increasing fuel concentration gradient, and the ratio of Se with the stoichiometric laminar burning velocity SLst of Se/SLst was larger than unity. Also, the stretch rate significantly affected the propagation speed of advancing edge flames. Determination of the propagation speed of the retreating edge (hole opening) had intrinsic difficulty in estimating the transient radial flow velocity, however, the approximated |Se/SLst| was larger than unity and decreased with the fuel mole fraction. Nitrogen dilution in the oxidizer stream was proven to be more effective than in the fuel stream for flame suppression of methane. This study could enrich the understanding of very low-stretch nonpremixed flames in a counterflow burner and provide information regarding low-stretch flame suppression, which may be relevant to microgravity flame behaviors.

Electrical conductivity as a reliable indicator for assessing land use effects on stream N2O concentration

While it is widely acknowledged that different categories of terrestrial land use significantly affect N2O emissions from streams and rivers, a suitable indicator to comprehensively reflect such influences is still lacking. This study examined the effectiveness of stream environmental and microbial factors in reflecting the influence of land use types on stream N2O concentrations, taking into account both endogenous N2O production and exogenous N2O inputs. Our results showed that human-disturbed (urban and agricultural) streams had nearly four times higher excess N2O concentrations (ΔN2O) than forest streams (5.01 versus 1.32 nmol/L N2O). By combining our observations with previous studies, we identify electrical conductivity as a significant and even the strongest predictor of (excess) N2O concentration and flux in streams across different land uses, surpassing the predictive power of nitrogen content. Elevated ΔN2O was observed at higher conductivities (r = 0.69, p < 0.01), which coincided with higher carbon and nutrient levels across different types of land use. Also, conductivity was positively correlated with the abundance of N2O-producing microbes (p < 0.05), including both nitrifying and denitrifying microbes. Denitrifiers adapted to human disturbance and the genetic potential for net N2O production, as evidenced by the positive correlation between conductivity and nir:nosZ ratio, were enhanced under high-conductivity conditions (p < 0.05). Furthermore, electrical conductivity had great potential to qualitatively indicate the magnitude of exogenous N2O input across different land uses. Overall, our results highlight the value of electrical conductivity as a reliable indicator for assessing the effect of land use types on stream N2O concentrations. This opens opportunities for the development of simple field-based assessments of riverine N2O emissions across regional landscapes.

The seasonal and spatial variability of ammonium uptake in a hilly watershed

Elevated nutrient loading can cause deleterious impacts on aquatic ecosystems such as eutrophication. Seasonal variability and land use change often lead to varied nutrient uptake from streams. However, the impacts of seasonal and spatial variation on stream nutrient transport within the same watershed haven’t been fully understood. Here, we conducted nutrient addition experiments using the Tracer Additions for Spiraling Curve Characterization (TASCC) approach within the Xiaogang Watershed, Zhejiang Province, China. Six of the experiments were conducted in one stream every other month and eight releases in eight different streams to quantify ammonium uptake kinetics across different seasons and land uses. Our findings suggest that the uptake capability increases with both discharge and ambient concentration (C0): seasonal variability of discharge shows small impacts on uptake metrics, except the ambient uptake length (Sw-amb), which increases with discharge; while the change in C0 could have more significant effects on both ambient areal uptake (Uamb) and maximum areal uptake rate parameter (Umax). Downstream the river network, the increase in discharge led to a significant increase in Uamb, ambient uptake velocity (Vf-amb), and Umax. On the other hand, the change in C0 is less influential than discharge along the river network, which may be correlated with the hydraulic geometry. Additionally, there is an optimal temperature (T) for uptake in our study region around 17°C, which may be explained by the growth of chlorophyll a. The positive correlation between Vf-amb and Q at the spatial scale may be attributed to the increase of dissolved organic carbon (DOC) and river chlorophyll a downstream with stream width. In addition, land use had an important effect on ion concentration in streams, and the proportion of agricultural land was positively correlated with nutrient concentration. Our findings could help provide scientific support for land use management and water quality regulation.

Trace Elements and Consequent Ecological Risks in Mining-Influenced Streams of Appalachia.

Appalachian (eastern USA) coal surface mines fracture geologic materials, causing release of both major ions and trace elements to water via accelerated weathering. When elevated above natural background, trace elements in streams may produce adverse effects on biota via direct exposure from water and sediment and via dietary exposure in food sources. Other studies have found elevated water concentrations of multiple trace elements in Appalachia's mining-influenced streams. Except for Se, trace-element concentrations in abiotic and biotic media of Appalachian mining-influenced streams are less well known. We analyzed environmental media of headwater streams receiving alkaline waters from Appalachian coal mines for eight trace elements (Al, As, Cd, Cu, Ni, Sr, V, and Zn) and assessed the potential consequent ecological risks. Streamwater, particulate media (sediment, biofilm, leaf detritus), and benthic macroinvertebrates (primary consumers, secondary consumers, crayfish) were sampled from six mining-influenced and three reference streams during low-flow conditions in two seasons. Dissolved Cu, Ni, and Sr were higher in mining-influenced streams than in reference streams; Ni, Sr, and Zn in fine sediments and Ni in macroinvertebrates were also elevated relative to reference-stream levels in samples from mining-influenced streams. Seasonal ratios of mining-influenced stream concentrations to maximum concentrations in reference streams also demonstrated mining-influenced increases for several elements in multiple media. In most media, concentrations of several elements including Ni were correlated positively. All water-column dissolved concentrations were below protective levels, but fine-sediment concentrations of Ni approached or exceeded threshold-effect concentrations in several streams. Further study is warranted for several elements (Cd, Ni, and Zn in biofilms, and V in macroinvertebrates) that approached or exceeded previously established dietary-risk levels. Environ Toxicol Chem 2023;42:2651-2665. © 2023 SETAC.

Graph-Based Deep Learning Model for Forecasting Chloride Concentration in Urban Streams to Protect Salt-Vulnerable Areas

In cold-climate regions, road salt is used as a deicer for winter road maintenance. The applied road salt melts ice and snow on roads and can be washed off through storm sewer systems into nearby urban streams, harming the freshwater ecosystem. Therefore, aiming to develop a precise and accurate model to determine future chloride concentration in the Credit River in Ontario, Canada, the present work makes use of a “Graph Neural Network”–“Sample and Aggregate” (GNN-SAGE). The proposed GNN-SAGE is compared to other models, including a Deep Neural Network-based transformer (DNN-Transformer) and a benchmarking persistence model for a 6 h forecasting horizon. The proposed GNN-SAGE surpassed both the benchmarking persistence model and the DNN-Transformer model, achieving RMSE and R2 values of 51.16 ppb and 0.88, respectively. Additionally, a SHAP analysis provides insight into the variables that influence the model’s forecasting, showing the impact of the spatiotemporal neighboring data from the network and the seasonality variables on the model’s result. The GNN-SAGE model shows potential for use in the real-time forecasting of water quality in urban streams, aiding in the development of regulatory policies to protect vulnerable freshwater ecosystems in urban areas.

Widespread increases in soluble phosphorus concentrations in streams across the transboundary Great Lakes Basin

Widespread increases in soluble phosphorus concentrations in streams across the transboundary Great Lakes Basin

Energy, environmental, and economic analysis of a new configuration multi-stage flash distillation unit coupled with steam power plant

This study focuses on the utilization of three waste streams from a power plant for the provision of cooling water and required steam for heating purposes in a multi-stage flash desalination (MSF) unit. The mathematical model for a new configuration of MSF desalination is presented. In conventional MSF desalination unit, a stream of salt water is used for desalination and system coolant. A new structure for the desired desalination process, proportional to the salt concentration of the feed streams and based-on un-mixing of the target streams has been proposed. The proposed system required 18 m2 of cross-sectional area and also 87 m2/(kg/s) of specific heat transfer area in the power plant with seven stages. The optimal value of top brine temperature (TBT) has 110 °C and the capacity of the proposed MSF desalination unit is 92.43 m3 of fresh water/day. The gain output ratio (GOR), energy intensity and water cost of the system are calculated to be 3.12, 706.8 kJ/kg, and 0.69 $/m3, respectively. By application of proposed MSF desalination, the annual emission of 1167.5 tons CO2 into the atmosphere can be obviated and the cost savings resulting from equal fuel consumption amount to $116,315.5 per year.

Stormwater Suspended Solids and Pollutant Concentrations in an Urban Stream

The chemical and ecological quality of small rivers or streams is not normally classified, but any urban stormwater discharges will result in their impairment. To assist in understanding the resulting applied environmental stress to benthic communities, the empirical relationships between in-stream suspended solids and metal pollutants, initially identified by principal component analysis, are explored using data from a small urbanised upland catchment. When data are amalgamated from individual storms, a correlation for each metal pollutant with suspended solids is obtained, starting very strongly, and decreasing in the order Fe > Al > Mn > Pb > Cr > Zn > Cu > Ni. Comparison of pollutant/solids correlations for individual storms is, for some metals, excellent, but differing intercepts and/or gradients can be obtained for an individual metal between storms. The practice of assessing their correlation based on event mean concentrations or longer-term averages across several storm events may conceal strong correlations for individual storms, exemplifying the impact of variable catchment characteristics and storm conditions; Cd and Be are poorly correlated with suspended solids and are likely affected by changes in background pollution levels. The analysis of covariance also highlights a significant difference between certain storm events for specific metals, which calls into question the validity of using combined storm-event datasets. The relationships identified here enable the determination of stormwater pollutant loads from suspended solids concentrations for heavy metals and are useful for developing water quality assessments.

Douglas-fir plantations impact stream and groundwater chemistry in western Europe: Insights from three case studies in France and Luxembourg

In rural areas, nitrate concentrations in surface waters most often originate from the leaching of excess N fertilizer in agricultural lands, whereas forested catchments often have good water quality. However, Douglas-fir plantations may induce nitrogen cycle unbalances which may lead to an excess of nitrate production in the soil. We hypothesize that the excess of production of nitrate in the soil and nitrate leaching to streamwater is greater in catchments planted with Douglas fir. We used paired catchments in both France and Luxembourg with different land covers (Douglas-fir, Spruce, Deciduous, Grassland and clearcut) which were monitored over a 3–5 year period in order to assess the effect of Douglas-fir plantations on the chemical composition of surface water. Nitrate concentration in the soil and groundwater were also monitored. The results show that nitrate concentrations in streams draining Douglas-fir catchments were two to ten times higher than in streams draining other land covers, but were similar to the clearcut catchment. Nitrate concentrations under Douglas-fir in groundwater (up to 50 mg L−1) and in the soil were also higher than under all other land covers. Soil nitrate concentration was related to stream nitrate concentration. This suggests that soil processes, through excessive nitrate production under Douglas-fir, are driving the nitrate concentration in the stream water and our hypothesis of a transfer of a fairly large proportion of this excessive production from the soil to the stream is supported. This study also shows that nitrate concentrations in surface and ground waters in rural areas could also originate from Douglas fir forested catchments. The impact of Douglas-fir is nevertheless reduced downstream through a dilution effect: mixing tree species at the catchment scale could thus be a solution to mitigate the effect of Douglas-fir on nitrate concentration in surface waters.

Impact of Iron Impurities on the Performance of PEM Water Electrolyzers

Hydrogen gas is a promising green energy solution, with enormous potential for using hydrogen fuel cells to power vehicles, homes, and for portable power applications [1]. Proton exchange membrane water electrolyzers (PEMWEs) are a viable way for the production of green hydrogen, when used in conjunction with renewable energy sources such as wind and solar. A crucial component to PEMWE is the membrane electrode assembly (MEA). MEAs are susceptible to degradation from transition metal cation impurities such as Fe 2+ [2,3,4]. The presence of iron impurities is common and can come from the feed water, cell components, and piping in the system. Concentrations of parts per million (PPM) of iron impurities have been shown to be extremely detrimental to cell performance [3, 4]. However, little work has been done to show the effects of concentrations in the parts per billion (PPB) range. Understanding the impact of Fe2+ impurities with very low concentrations in the water stream in PEMWEs on their performance is needed to assess and improve the durability of PEMWEs.In this work, several cells assembled with MEAs with active area of 5 cm2 have been evaluated, and the impact of low-level iron impurities on their performance has been comprehensively studied. A baseline performance was established by testing a cell in pure DI water before the feedstock was replaced with stock solutions containing Fe2+ ions with various concentration in the low PPB range. The cells were tested at 50 oC, and constant current of 1.8 A/cm2 for up to 500 hrs. Diagnostic tests were taken every 25 hours, which included polarization curves and electrochemical impedance spectroscopy (EIS) measurements. Water samples were taken every day and analyzed by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), to monitor the Fe concentration in the feedstock water. In addition, water samples from the cathode were taken to investigate the fluoride emission rate (FER), to better understand membrane degradation. The findings from these experiments help to better understand the performance of PEMWEs, as well as the degradation mechanisms governing the performance loss in the MEA.

Advancements in Thin, Reinforced Proton Exchange Membranes for Water Electrolysis

Proton exchange membrane water electrolysis (PEMWE) is emerging as one of the most promising methods to produce clean hydrogen at industrial scale. New PEMWE stack designs require high voltage efficiency and differential pressure operation without sacrificing low hydrogen in oxygen concentrations in the anode stream. Membranes used in current stack designs, such as Chemours Nafion™ N115, are thick to provide strong mechanical stability. However, employing a thinner and mechanically supported membrane can enhance both the electrochemical performance and mechanical properties. With the demand for cheaper hydrogen, these new membrane designs are needed to achieve advanced performance metrics. In this presentation, recent improvements in membrane design at Chemours will be summarized.Novel architectures comprise a thin, reinforced composite membrane approach, utilizing a lower equivalent weight (EW) polymer that can surpass the performance of the current commercial membranes. For example, overpotential is reduced by >150 mV at 3 A/cm2 for novel membranes compared to Nafion™ N115 at 80°C. However, one concern with having a thinner film is the increase in hydrogen crossover, since the lower explosive limit (LEL) for hydrogen in oxygen is only 4 vol %. PEMWE differential pressures of up to 30 bar on the cathode side can exacerbate these hydrogen crossover challenges. To mitigate this hazard, gas recombination catalyst (GRC) additives have been implemented within the membrane where the GRC recombines hydrogen and oxygen to form water. A reduction in anodic hydrogen in oxygen content of >50x has been observed, assuring the safe operation at high differential pressures for these thin membranes. Polarization curves across different membrane constructions were analyzed as well as hydrogen crossover for various GRC designs. Comparisons will be presented demonstrating the improved performance and reduced hydrogen crossover. Figure 1

Concentration‐Discharge Patterns Reveal Catchment Controls Over the Stoichiometry of Carbon and Nutrient Supply to Boreal Streams

AbstractCarbon (C), nitrogen (N), and phosphorus (P) export from catchments is strongly regulated by interactions between hydrological flowpaths and their terrestrial use/storage. While concentration‐discharge (c‐Q) relationships have been widely used to understand this interplay for C, N, and P individually, how flow regulates the relative supply of these resources across spatial and temporal scales is not well documented. Here, we analyze c‐Q relationships from 12 years of data to test how seasonal flow regulates the concentrations of inorganic N (Dissolved inorganic nitrogen [DIN]) and P (Dissolved inorganic phosphorus [DIP]), dissolved organic N (DON) and C (dissolved organic carbon [DOC]) and their respective ratios across 12 streams in a boreal landscape. We observed opposing c‐Q relationships between organic and inorganic solutes. DOC and DON tended toward transport limitation with little year‐to‐year change, whereas ammonium (NH4) and DIP were increasingly source limited over time. These different c‐Q relationships translated into large (up to three‐fold) shifts in resource ratios (e.g., DOC:DIN) in response to changes in flow. Our results also highlight strong influences of catchment structure on c‐Q patterns, regardless of solute, season, and longer‐term directional changes. Here, the organic solute c‐Q responses became less transport limited over time; while inorganic solute responses became less source limited with increasing mire/decreasing forest cover. Overall, differences in timing of catchment exports for C, N, and P, create dynamic variation in solute concentrations in streams with subsequent impacts on resource stoichiometry that is central to aquatic ecological processes.

Comparing Drivers of Spatial Variability in U.S. Lake and Stream Phosphorus Concentrations.

Decision makers need to know the drivers of surface water phosphorus (P) concentrations, the environmental factors that mediate P loading in freshwater systems, and where pollution sources and mediating factors are co-located to inform water quality restoration efforts. To provide this information, publicly available spatial data sets of P pollution sources and relevant environmental variables, like temperature, precipitation, and agricultural soil erodibility, were matched with >7,000 stream and lake total P observations throughout the conterminous United States. Using three statistical approaches, consisting of (a) correlation, (b) regression, and (c) machine learning techniques, we identified likely drivers of P concentrations. Surface water concentrations in streams were more strongly correlated and effectively predicted by annual fertilizer and manure input rates and agricultural legacy sources compared to that of lakes. This observation suggests that streams may be more immediately responsive to improvements in agricultural nutrient management. In contrast, lake concentrations, though still positively associated with agricultural input and surplus variables, may be more influenced by historic erosional inputs, internal lake recycling, and other environmental factors. Thus, lake TP concentrations may not be as immediately responsive as streams to improvements in phosphorus management. Both stream and lake P concentrations will potentially increase because of warming temperatures and forest recovering from past acidification, putting even further pressure on existing water quality restoration efforts to meet nutrient loading reduction targets. The identified spatial data sets and relationships elucidated in this effort can inform the placement and development of watershed restoration strategies to reduce excess P in aquatic systems.

Assessment of nutrients and conductivity in the Wachusett Reservoir watershed: An investigation of land use contributions and trends

The quality of drinking water for the Boston Metropolitan Area, supplied by the Quabbin-Wachusett system, is impacted by environmental trends. The objectives of this study are to increase understanding of the role that small streams may play in degradation of reservoir quality by characterizing seasonal constituent patterns from 1998 to 2020 in the Wachusett Reservoir watershed and by developing enhanced modeling frameworks. Previous monitoring (1998–2012) exhibited increased loads due to increasing flows despite declining solute concentration. This present study analyzed seasonal nitrate (NO3) and total phosphorus (TP) concentration and load trends from 2012 to 2020 across 11 tributaries. Specific conductivity (SC) was also assessed to evaluate the impacts of road salt application. From 2012 to 2020, statistical results for mean nutrient concentrations suggest static or declining temporal trends, while SC in all tributaries exhibited increasing trends. Land use data suggest association with altered drainage landscapes as potential sources of increased constituent transport. Subbasins with the highest concentrations of TP, NO3, and SC have the largest percentage of impervious and cultivated areas, two to three times greater than other subbasins. Daily flows were modeled using the airGR hydrological model, subsequently used to calculate loads. Overall, flow magnitude was a more important load driver than long-term nutrient concentrations, thus, showing that stream discharge controlled load variability. On the other hand, persistently high SC levels controlled the increasing SC load trends. Finally, many nutrient reduction management strategies demonstrated an important impact from 1998 to 2020. Despite watershed programs aimed at reducing salt applications, concentrations in streams are increasing, indicating a long-term legacy of salt accumulation. Although smaller tributaries represent a modest portion of the system, addressing these sources has the potential to further reduce the long-term ecological impacts of reservoir constituent loading.

GRiMeDB: the Global River Methane Database of concentrations and fluxes

Abstract. Despite their small spatial extent, fluvial ecosystems play a significant role in processing and transporting carbon in aquatic networks, which results in substantial emission of methane (CH4) into the atmosphere. For this reason, considerable effort has been put into identifying patterns and drivers of CH4 concentrations in streams and rivers and estimating fluxes to the atmosphere across broad spatial scales. However, progress toward these ends has been slow because of pronounced spatial and temporal variability of lotic CH4 concentrations and fluxes and by limited data availability across diverse habitats and physicochemical conditions. To address these challenges, we present a comprehensive database of CH4 concentrations and fluxes for fluvial ecosystems along with broadly relevant and concurrent physical and chemical data. The Global River Methane Database (GriMeDB; https://doi.org/10.6073/pasta/f48cdb77282598052349e969920356ef, Stanley et al., 2023) includes 24 024 records of CH4 concentration and 8205 flux measurements from 5029 unique sites derived from publications, reports, data repositories, unpublished data sets, and other outlets that became available between 1973 and 2021. Flux observations are reported as diffusive, ebullitive, and total CH4 fluxes, and GriMeDB also includes 17 655 and 8409 concurrent measurements of concentrations and 4444 and 1521 fluxes for carbon dioxide (CO2) and nitrous oxide (N2O), respectively. Most observations are date-specific (i.e., not site averages), and many are supported by data for 1 or more of 12 physicochemical variables and 6 site variables. Site variables include codes to characterize marginal channel types (e.g., springs, ditches) and/or the presence of human disturbance (e.g., point source inputs, upstream dams). Overall, observations in GRiMeDB encompass the broad range of the climatic, biological, and physical conditions that occur among world river basins, although some geographic gaps remain (arid regions, tropical regions, high-latitude and high-altitude systems). The global median CH4 concentration (0.20 µmol L−1) and diffusive flux (0.44 mmolm-2d-1) in GRiMeDB are lower than estimates from prior site-averaged compilations, although ranges (0 to 456 µmol L−1 and −136 to 4057 mmolm-2d-1) and standard deviations (10.69 and 86.4) are greater for this larger and more temporally resolved database. Available flux data are dominated by diffusive measurements despite the recognized importance of ebullitive and plant-mediated CH4 fluxes. Nonetheless, GriMeDB provides a comprehensive and cohesive resource for examining relationships between CH4 and environmental drivers, estimating the contribution of fluvial ecosystems to CH4 emissions, and contextualizing site-based investigations.

Spatial Distribution of Nitrogen and Phosphorus Nutrients in the Main Stream and Typical Tributaries of Tuojiang River and Fujiang River

The Tuojiang River and Fujiang River, two important tributaries of the upper reaches of the Yangtze River, have serious water pollution problems, among which nitrogen (N) and phosphorus (P) are the most important pollutants. Therefore, the aim of this study was to identify the influencing factors of water quality in different spaces and provide a scientific basis for the prevention and control of surface water pollution in the upper reaches of the Yangtze River and its tributaries. Water samples of trunk and tributaries in the Tuojiang River and Fujiang River were collected, and the spatial distribution characteristics of water N and P were analyzed. The results showed that the Tuojiang River and Fujiang River showed serious pollution of total nitrogen (TN), with a water quality worse Ⅴ-section proportion as high as 94% and 50%, respectively. Both rivers showed that TN and TP concentrations in the tributaries were higher than those in the main stream. For both rivers, total phosphorus (TP), with moderate pollution, was mainly concentrated in Ⅱ, Ⅲ, and Ⅳ class water quality, whereas the P pollution was more serious for the Fujiang River compared to that of the Fujiang River. For the Tuojiang River, nitrate nitrogen (NN) concentration from upstream to downstream showed a trend of decreasing after the first increase, with the maximum concentration of ammonium nitrogen (AN) exhibiting at the upstream site. In particular, TP concentration increased significantly after rivers flowed through a city. For the Fujiang River trunk stream, TN and NN concentration exhibited a gradually increasing trend from the middle to lower reaches. Generally, our study revealed that TN, TP, and NN in the rivers were affected by water pH and water temperature (T). Therefore, the control of N and P pollution in rivers should pay attention to the influence of water environmental factors.

Watershed characteristics and chemical properties govern methyl mercury concentrations within headwater streams of boreal forests in Ontario, Canada

Methyl mercury (MeHg) concentrations in boreal headwater streams are influenced by complex natural processes and disturbances such as forestry management. Understanding drivers of MeHg within boreal streams in Ontario, Canada, is of particular interest as there are legacy MeHg concerns. However, models accounting for the complexity of underlying processes have not yet been developed. We assessed how catchment characteristics and stream water chemistry influence MeHg concentrations within 19 watersheds of the Dryden – Wabigoon Forest in Ontario, Canada, using a structural equation modelling (SEM) approach. Despite the study area encompassing a large variation of boreal forest watersheds in the Canadian Shield, our SEM had substantial explanatory power across the region (χ251 = 45.37, p-value = 0.70, R2 = 0.75). Nitrate concentrations (p-value <0.001), water temperature (p-value = 0.002), and the latent watershed characteristic (p-value <0.001) had a positive influence on MeHg concentrations once variable interactions were accounted. Due to the inherent strengths of applying an SEM approach, we describe two plausible pathways driving MeHg concentrations: 1) indirect effect of forest-derived nutrients increases in-situ MeHg production in Dryden – Wabigoon Forest streams, and 2) direct supply of MeHg from inundated soils following consistent precipitation and inundation events (i.e., fill, sit, and spill).

Integrated capture and solar-driven utilization of CO2 from flue gas and air

Integrated capture and solar-driven utilization of CO2 from flue gas and air

Understanding links between water-quality variables and nitrate concentration in freshwater streams using high frequency sensor data.

Real-time monitoring using in-situ sensors is becoming a common approach for measuring water-quality within watersheds. High-frequency measurements produce big datasets that present opportunities to conduct new analyses for improved understanding of water-quality dynamics and more effective management of rivers and streams. Of primary importance is enhancing knowledge of the relationships between nitrate, one of the most reactive forms of inorganic nitrogen in the aquatic environment, and other water-quality variables. We analysed high-frequency water-quality data from in-situ sensors deployed in three sites from different watersheds and climate zones within the National Ecological Observatory Network, USA. We used generalised additive mixed models to explain the nonlinear relationships at each site between nitrate concentration and conductivity, turbidity, dissolved oxygen, water temperature, and elevation. Temporal auto-correlation was modelled with an auto-regressive-moving-average (ARIMA) model and we examined the relative importance of the explanatory variables. Total deviance explained by the models was high for all sites (99%). Although variable importance and the smooth regression parameters differed among sites, the models explaining the most variation in nitrate contained the same explanatory variables. This study demonstrates that building a model for nitrate using the same set of explanatory water-quality variables is achievable, even for sites with vastly different environmental and climatic characteristics. Applying such models will assist managers to select cost-effective water-quality variables to monitor when the goals are to gain a spatial and temporal in-depth understanding of nitrate dynamics and adapt management plans accordingly.

Distributions of trace elements within MSWI bottom and combined ash components: Implications for reuse practices

Concentrations of 25 inorganic elements were measured in both bulk ash and individual ash components from residuals at three municipal solid waste incineration (MSWI) facilities in the US (two combined ash (CA) and one bottom ash (BA)). Concentrations were assessed based on particle size and component to understand the contribution from each fraction. The results found that among facilities, the finer size fractions contained elevated concentrations of trace elements of concern (As, Pb, Sb) when compared to the coarse fraction, but concentrations varied among facilities depending on the type of ash and differences in advanced metals recovery processes. This study focused on several constituents of potential concern, As, Ba, Cu, Pb, and Sb, and found that the main components of MSWI ash (glass, ceramic, concrete, and slag) are sources of these elements in the ash streams. For many elements, concentrations were significantly higher in CA bulk and component fractions opposed to BA streams. An acid treatment procedure and scanning electron microscopy/energy-dispersive x-ray spectroscopy analysis revealed that some elements, such as As in concrete, are result of the inherent properties of the component, but other elements, such as Sb, form on the surface during or after incineration and can be removed. Some Pb and Cu concentrations were attributed to inclusions in the glass or slag introduced into the material during the incineration process. Understanding the contributions of each ash component provides critical information for developing strategies to reduce trace element concentrations in ash streams to promote reuse opportunities.