Downstream Environments Research Articles

Etching of Si3N4 induced by electron beam plasma from hollow cathode plasma in a downstream reactive environment

An etching system based on the interaction of electrons extracted from a direct current hollow cathode (HC) Ar plasma and injected toward an Si3N4 covered silicon substrate located in the downstream reactive environment created by an additional remote CF4/O2 plasma source was developed and evaluated. By controlling the properties of the injected beam electrons, this approach allows to deliver energy to a surface functionalized by exposure to reactive species and initiate surface etching. The energy of the primary beam electrons is controlled by the acceleration voltage relative to the HC discharge. Ar atoms flow from the high-pressure HC discharge into the low pressure downstream reactive environment in the process chamber. For an acceleration voltage greater than the ionization potential of Ar and/or process gas species, the energetic primary beam electrons produce a secondary plasma in the process chamber and can also cause additional dissociation. The authors have characterized the properties of the secondary plasma and also surface etching of Si3N4 as a function of process parameters, including acceleration voltage (0–80 V), discharge current of the HC discharge (1–2 A), pressure (3.5–20 mTorr), source to substrate distance (1.5–5 cm), and feed gas composition (20% and 80% O2 in CF4/O2). The electron energy probability function measured with a Langmuir probe about 2.5 cm below the extraction ring suggests several major groups of electrons for this situation, including high energy primary beam electrons with an energy that varies as the acceleration voltage is changed and low-energy electrons produced by beam electron-induced ionization of the Ar gas in the process chamber. When a remote CF4/O2 plasma is additionally coupled to the process chamber, Si3N4 surfaces can be functionalized, and by varying the energy of the beam electrons, Si3N4 etching can be induced by electron-neutral synergy effect with plasma-surface interaction. For conditions without beam electron injection, the remote plasma etching rate of Si3N4 depends strongly on the O2 concentration in the CF4/O2 processing gas mixture and can be suppressed for O2-rich process conditions by the formation of an SiONF passivation layer on the Si3N4 surface. The combination of the HC electron beam (HCEB) source with the remote plasma source makes it possible to induce Si3N4 etching for O2-rich remote plasma conditions where remote plasma by itself produces negligible Si3N4 etching. The electron enhanced etching of Si3N4 depends strongly on the O2/CF4 mixing ratio reflecting changing arrival rates of O and F species at the surface. Optical emission spectroscopy was used to estimate the ratio of gas phase F and O densities and found to be controlled by the gas mixing ratio and independent of HCEB operating conditions. At this time, the detailed sequence of events operative in the etching mechanism is unclear. While the increase of the electron energy is ultimately responsible for initiating surface etching, presently, the authors cannot rule out a role of ions from the simultaneously produced secondary plasma in plasma-surface interaction mechanisms.

Optimization analysis of seepage control design in Mopanshan Reservoir, northeastern China

Anti-seepage calculation is key to the design of reservoir dams, and the uncertainty parameter makes the sensitivity analysis a useful tool in the design process. A reservoir constructed in northeastern China was selected to analyze optimal anti-seepage designs, and numerical models have been established with visual MODflow software. The results of analyzing seepage field characteristics show that under different schemes, anti-seepage capability is controlled by the length of an anti-seepage body on the left riverbank and the depth of curtain grouting, and that seepage decreases with increasing length of the anti-seepage body and depth of the curtain grouting. The results of the sensitivity analysis show that, according to the sensitivity analysis, the permeability coefficient had different effects on the seepage gradient under incomplete and complete anti-seepage. Under incomplete anti-seepage, the seepage increased with permeability coefficient and the seepage gradient increased; and under complete anti-seepage, the seepage increased slightly with the permeability coefficient, and the seepage gradient decreased, the permeability coefficient of the anti-seepage body clearly influences the anti-seepage effects. From a project investment standpoint, complete anti-seepage on the left riverbank with a curtain grouting standard of 5 Lugeon units (Lu) is the preferred anti-seepage control scheme. This scheme not only guarantees the safe operation of the dam but also ensures that seepage is less than the acceptable seepage limit, is close to the natural groundwater flow rate, and that the scheme will not significantly impact the downstream hydrogeological environment after the construction of the dam.

Hydrological Impact of Ilisu Dam on Mosul Dam; the River Tigris

The Ilisu Dam is part of the Turkish Southeastern Anatolia Project (GAP) and is the largest dam on the Tigris River in Turkey. It is located on the main river course 65 km upstream of the Syrian and Iraqi border. The Ilisu Dam watershed is the same as that of the Mosul Dam in Iraq. Sharing the same watershed with the Mosul Dam and located upstream, the Ilisu Dam will usurp most of the watershed and deprive the Mosul Dam of most of its current inflow. This paper presents an assessment of the hydrological impact (basically predicts changes on future inflow) of the Ilisu Dam on the Mosul Dam. The assessment is based on the worst-case scenario. The analyses that are employed include geographic information system (GIS) techniques and regression models, along with statistical analyses to numerate expected future impacts on the Mosul Dam’s inflow distribution. Results reveal that the Ilisu will have a drastic impact on the inflow regime of the Mosul Dam. A reduction as high as 78% of the inflow of the Mosul Dam may occur if the operation of the Ilisu and the Cizre Dams is conducted with no consideration of downstream hydrological and environment impacts.

Electron beam injection from a hollow cathode plasma into a downstream reactive environment: Characterization of secondary plasma production and Si3N4 and Si etching

A material etching system was developed by combining beam electron injection from a direct current hollow cathode (HC) electron source with the downstream reactive environment of a remote CF4/O2 low temperature plasma. The energy of the injected beam electrons is controlled using an acceleration electrode biased positively relative to the HC argon discharge. For an acceleration voltage greater than the ionization potential of Ar, the extracted primary electrons can produce a secondary plasma in the process chamber. The authors characterized the properties of the secondary plasma by performing Langmuir probe measurements of the electron energy probability function (EEPF) 2.5 cm below the extraction ring. The data indicate the existence of two major groups of electrons, including electrons with a primary beam electron energy that varies as the acceleration voltage is varied along with low energy electrons produced by ionization of the Ar gas atoms in the process chamber by the injected beam electrons. When combining the HC Ar beam electron with a remote CF4/O2 electron cyclotron wave resonance plasma, the EEPF of both the low energy plasma electron and beam electron components decreases. Additionally, the authors studied surface etching of Si3N4 and polycrystalline Si (poly-Si) thin films as a function of process parameters, including the acceleration voltage (0–70 V), discharge current of the HC discharge (1–2 A), pressure (2–100 mTorr), source to substrate distance (2.5–5 cm), and feed gas composition (with or without CF4/O2). The direction of the incident beam electrons was perpendicular to the surface. Si3N4 and polycrystalline silicon etching are seen and indicate an electron-neutral synergy effect. Little to no remote plasma spontaneous etching was observed for the conditions used in this study, and the etching is confined to the substrate area irradiated by the injected beam electrons. The electron etched Si3N4 surface etching rate profile distribution is confined within a ∼30 mm diameter circle, which is slightly broader than the area for which poly-Si etching is seen, and coincides closely with the spatial profile of beam electrons as determined by the Langmuir probe measurements. The magnitude of the poly-Si etching rate is by a factor of two times smaller than the Si3N4 etching rate. The authors discuss possible explanations of the data and the role of surface charging.

Numerical simulation of sediment-laden flow on the Yarlung Zangbo River incorporating the climate change

The Yarlung Zangbo River originates from the Qinghai-Tibet Plateau, and runoff consists of the rainfall, melting of snow and glaciers in the upper high-altitude areas. The flow and sediment transport induced by flow in the Yarlung Zangbo River is highly sensitive to changes in climatic conditions. Changes in flow and sediment transport will also have an impact on the downstream wetland ecosystem. In order to study the changes of flow and sediment transport in the Yarlung Zangbo River, Niyang River wetlands under the changes of climatic conditions is used as the considered case. The two-dimensional shallow water equation and the non-equilibrium bedload sediment transport model are proposed in the Yarlung Zangbo-Niyang River. The model is discretized using the unstructured grid finite volume method (FVM). The model includes approximately 355 km length and covers an area of approximately 673 km2. According to the measured time series of flow data at Nuxia hydrology station and the simulated data under the climate change conditions by the climate change model, the water level, flow velocity, erosion and siltation, and sediment gradation change in the river channel are calculated. The calculation results show that by the year 2025, the water level of Niyang River wetlands will increase by 2m under RCP8.5 (Representative Concentration Pathway), and accordingly the total area of the wetlands will decrease by 5%. The results show that climate change has a significant impact on the flow and sediment transport and downstream ecological environment of the Yarlung Zangbo River.

River dam impacts on biogeochemical cycling

The increased use of hydropower is currently driving the greatest surge in global dam construction since the mid-20th century, meaning that most major rivers on Earth are now dammed. Dams impede the flow of essential nutrients, including carbon, phosphorus, nitrogen and silicon, along river networks, leading to enhanced nutrient transformation and elimination. Increased nutrient retention via sedimentation or gaseous elimination in dammed reservoirs influences downstream terrestrial and coastal environments. Reservoirs can also become hotspots for greenhouse gas emission, potentially impacting how ‘green’ hydropower is compared with fossil-fuel burning. In this Review, we discuss how damming changes nutrient biogeochemistry along river networks, as well as its broader environmental consequences. The influences of construction and management practices on nutrient elimination, the emission of greenhouse gases and potential remobilization of legacy nutrients are also examined. We further consider how regulating hydraulic residence time and environmental flows (or e-flows) can be used in planning and operation from dam conception to deconstruction. River damming can harness hydropower, control flooding and store water, but can also alter biogeochemistry in reservoirs and downstream environments. In this Review, the impacts of dams on nutrient cycling and greenhouse production are discussed, emphasizing the need to consider biogeochemical cycling at all stages of dam lifespan.

Reducing the Consumption of Antibiotics: Would That Be Enough to Slow Down the Dissemination of Resistances in the Downstream Environment?

Reducing the Consumption of Antibiotics: Would That Be Enough to Slow Down the Dissemination of Resistances in the Downstream Environment?

Use of pre-industrial baselines to monitor anthropogenic enrichment of metals concentrations in recently deposited sediment of floodplain lakes in the Peace-Athabasca Delta (Alberta, Canada)

Well-designed monitoring approaches are needed to assess effects of industrial development on downstream aquatic environments and guide environmental stewardship. Here, we develop and apply a monitoring approach to detect potential enrichment of metals concentrations in surficial lake sediments of the Peace-Athabasca Delta (PAD), northern Alberta, Canada. Since the ecological integrity of the PAD is strongly tied to river floodwaters that replenish lakes in the delta, and the PAD is located downstream of the Alberta oil sands, concerns have been raised over the potential transport of industry-supplied metals to the PAD via the Athabasca River. Surface sediment samples were collected in September 2017 from 61 lakes across the delta, and again in July 2018 from 20 of the same lakes that had received river floodwaters 2 months earlier, to provide snapshots of metals concentrations (Be, Cd, Cr, Cu, Ni, Pb, V, and Zn) that have recently accumulated in these lakes. To assess for anthropogenic enrichment, surficial sediment metals concentrations were normalized to aluminum and compared to pre-industrial baseline (i.e., reference) metal-aluminum linear relations for the Athabasca and Peace sectors of the PAD developed from pre-1920 measurements in lake sediment cores. Numerical analysis demonstrates no marked enrichment of these metals concentrations above pre-1920 baselines despite strong ability (> 99% power) to detect enrichment of 10%. Measurements of river sediment collected by the Regional Aquatics- and Oil Sands-Monitoring Programs (RAMP/OSM) also did not exceed pre-1920 concentrations. Thus, results presented here show no evidence of substantial oil sands-derived metals enrichment of sediment supplied by the Athabasca River to lakes in the PAD and demonstrate the usefulness of these methods as a monitoring framework.

Water evaporation at wet-cooled nuclear power plants on river banks: Application to the French Rhône river

Nuclear power plants (NPP) installed on a river bank use once-through or recirculating wet cooling systems. While the quantitative assessment of mainly local evaporation of river water by a recirculating cooling system (RC) does not involve any major difficulty, the same cannot be said for the once-through cooling system (OTC) for which evaporation (called forced evaporation) occurs in the downstream aquatic environment. Our study applies an energy-balance temperature model to the Rhône River downstream from Lake Geneva. Our results show the approximate figure of 1.5 m3/MWh of forced evaporation per electricity produced (put forward in 2002 by EPRI and widely used in the literature) is debatable for setting a freshwater balance in a watershed because it does not consider the saltiness of the river downstream. The retention time of water bodies in the downstream freshwater stretch, the cross-section shape and regional weather characteristics appear to be key factors to take into account. In particular, for the three NPP with OTC systems installed along the French Rhône, we find an average forced evaporation between 0.3 and 0.7 m3/MWh of net electricity generated in the freshwater river stretch through to Arles between 1994 and 2015. These values are between three to eight times lower than those obtained for the evaporation generated by the plants with RC system installed along the river stretch.

Photophysical and photochemical insights into the photodegradation of tricyclazole and pymetrozine in water bodies of a rice field

Environmental contextPhotodegradation, a natural process for removing pesticides from water, is particularly important in rice fields with their large surface area and sunlight exposure. By studying two commonly used pesticides, we show that the properties of rice field water, such as dissolved organic matter, facilitate indirect photolytic processes while reducing direct photolysis. A better understanding of the natural degradation mechanisms for pesticides will inform their safer use in agriculture. AbstractRice paddy fields (RFs) make ecologically important contributions to agricultural wetland ecosystems in south China. In recent years, the elevated use of pesticides has resulted in concerns over the water quality in RFs and their downstream aquatic environment. Photodegradation is important for the natural elimination of nonpoint source agricultural contaminants, and RF water provides an ideal environment for the photochemical removal of pesticides, owing to its large surface area and photic conditions. Therefore, in this study, the physical and photochemical properties of water collected from the irrigation area were investigated, along with the direct and indirect photodegradation of two commonly used pesticides in RFs, tricyclazole (TC) and pymetrozine (PYM). Results indicated that allochthonous-like dissolved organic matter (DOM) was abundant in the irrigation water, possibly originating from the decomposition of rice straw. Moreover, the light screen effect and the formation rate of singlet oxygen (1O2) and the triplet excited state of DOM (3DOM*) were generally increased in RF water, while the formation of HO• was marginally reduced. Evapoconcentration and photobleaching processes were found to correlate with crop leaf coverage and had a significant impact on the levels and compositions of water constituents. Finally, the photodegradation kinetics of TC and PYM further confirmed that RF water would facilitate indirect photolysis and reduce the direct photolysis of pesticides.

Review of ecological mercury and arsenic bioaccumulation within historical gold mining districts of Nova Scotia

Gold mining in Nova Scotia dates back to the mid-1800s. Historical industrial practices generated over 3 000 000 tonnes of finely ground mine wastes (tailings), the majority of which were deposited into lakes, streams, wetlands, and low-lying areas close to the mill sites. These legacy tailings typically contain high concentrations of mercury (Hg) and arsenic (As) and continue to impact downstream environments to this day. The objective of this review is to critically examine and summarize existing knowledge on the transfer and bioaccumulation of Hg and As in aquatic and terrestrial organisms exposed to legacy gold mine tailings in Nova Scotia. This review reveals that as of mid-2019, 23 previous studies have been completed on this subject. Several of these studies were based on small sample sizes, or had other limitations, such as missing identification of biological species. Despite these limitations, the data in these publications clearly indicate that both Hg and As from abandoned gold mine sites in Nova Scotia are bioaccumulating in plants, fungi, freshwater and terrestrial invertebrates, marine mollusks, amphibians, fish, and mammals. In many cases, concentrations of MeHg exceed Canadian Council of Ministers of the Environment safety guideline values for consumption of wildlife; however, no guideline values are in place for either total Hg or As. No studies were found examining tailings-related Hg or As accumulation in lichen, reptiles, birds, or large mammals. This review concludes that further research on bioaccumulation and biomagnification of tailings-related Hg and As is needed to understand the overall impact of historical tailings on the aquatic and terrestrial ecosystems and species of Nova Scotia. More detailed studies are vital for guiding risk-management decisions and future land-use practices for these contaminated sites.

Pesticide Application Practices and Knowledge among Small-Scale Local Rice Growers and Communities in Rwanda: A Cross-Sectional Study

Background: Agriculture contributes a third of Rwanda’s GDP and is the main source of income for rural households, with 80% of the total population involved in crop and/or livestock production. The Government of Rwanda established the Muvumba rice project in 2011 amidst a policy shift towards rice as a national staple crop. However, the indiscriminate use of pesticides by local, low-income rice growers has raised concerns about potential human, animal and ecosystem health impacts as pesticide distribution and application are not strictly regulated. Although pesticide use can directly influence farmer health and ecosystems, little is known about small-scale farmers’ pesticide application practices and knowledge. We aimed to assess local application practices and understanding of pesticides to identify gaps in farmers’ knowledge on safe pesticide use and deviations from established standards and recommended practices. Methods: We conducted a cross-sectional study consisting of observations of pesticide practices and interviews with 206 small-scale rice growers in Nyagatare District, Rwanda, in March 2017. Descriptive statistical analyses (sample means, standard deviation and range) were performed, and we evaluated the association between farmers’ personal protective equipment (PPE) use and their education level and literacy status. Results: Over 95% of observed farmers did not comply with minimum standards for safe pesticide use, and 80% of respondents reported that they stored pesticides in their homes without personal protection measures. Education and literacy level were not significantly associated with PPE use. Additionally, 90% of respondents had experienced adverse health effects after using pesticides including intense headache, dizziness, stomach cramps, skin pain and itching, and respiratory distress. All respondents also reported animals in and around the rice scheme (cattle, birds, and fish) behaving abnormally or with signs consistent with pesticide exposure in the six months preceding the study, which may be linked to pesticide-contaminated water. Conclusions: Our study demonstrates potential for high exposure to pesticides for farmers, their families, and animals sharing rice-growing or downstream environments and points to the need for training on safe and effective pesticide use.

Spatial and seasonal variability in grain size, magnetic susceptibility, and organic elemental geochemistry of channel-bed sediments from the Mekong Delta, Vietnam: Implications for hydro-sedimentary dynamic processes

Understanding the relationship between depositional processes and their products in the fluvial–marine transition zone in tide-dominated depositional systems is fundamental to improving environmental and stratigraphic interpretations of long-term sedimentary records. The distributary channels of the Mekong Delta represent a typical example of a complex sedimentary environment characterized by strong fluvial and tidal interactions, and seasonal variability in hydro-sedimentary dynamic processes, particularly the migration of a salt wedge front linked to seasonal fluctuations in freshwater discharge. For this study, we analyzed grain size, magnetic susceptibility, and organic elemental geochemistry of channel-bed surficial sediments collected in 2015 from all distributary channels during the dry season and from the Mekong–Co Chien River during the flood season. This paper reports spatial and seasonal differences in the characteristics of channel-bed surficial sediments, which are closely linked to sediment sources and depositional processes during the dry and flood seasons.Distributary channels are characterized by coarse-grained sediments in the upper reaches and fine-grained sediments in the lower reaches, in both dry and flood seasons, reflecting the difference between the upstream fluvial-dominated environment and the downstream tide-dominated environment. Flood season samples from the Co Chien River show higher magnetic mineral contents and a terrestrial source of organic carbon compared with dry season samples, indicating sourcing from the drainage basin and trapping of suspended materials in the distributary channel due to the barrier effect of the saltwater wedge at the river mouth. Compared with the flood season, during the dry season the mud content was lower in the downstream reach close to the river mouth and higher in the upstream reach of the Co Chien River, and the content of magnetic minerals was lower in both reaches, indicating dissolution of magnetic minerals during early diagenesis. Furthermore, the lower C/N ratios (<10) and slightly enriched δ13C values of the dry season samples from the Co Chien River imply an additional contribution of organic carbon from a marine source. These various features imply a landward import of mud into the Co Chien River induced by stronger salinity intrusion during the dry season. Along the downstream tract of the Bassac River, two mud-dominated reaches were identified during the dry season. The magnetic mineral contents, C/N ratios and δ13C values indicate that the mud-dominated reach near the Bassac River mouth is also supplied from resuspended sediments by estuarine circulation and tidal pumping, whereas the mud in the reach that is transitional from fluvial to tide dominance has a riverine supply. The results suggest that the Mekong distributary channels are important sinks for suspended sediments during both the flood and dry seasons, controlled by a sediment trapping mechanism provided by either estuarine processes or the morphological configuration of the channels, both of which are controlled by fluvial–tidal dynamics. These depositional processes may be compensating for the incision of channels induced by sand mining in recent decades.

Weathering Dynamics Under Contrasting Greenland Ice Sheet Catchments

Chemical weathering dynamics in Greenland Ice Sheet (GrIS) catchments are largely unknown, due to a scarcity of field data. This paper presents the most comprehensive study to date of chemical weathering rates from four GrIS catchments of contrasting size. Cationic denudation rates varied greatly between catchments studied (2.6 to 37.6 tons km-2 a-1, world mean = 11.9 tons km -2 a -1), but were of the same order of magnitude to the world non-glacial riverine mean, and are greater than those documented in some major temperate rivers catchments (e.g. Mississippi (1.3 tons km-2 a-1) and Nile (0.4 tons km-2 a-1) rivers). These high chemical denudation rates indicate that the GrIS is a potential source of solute to downstream environments. Dissolved silica yields, indicative of silicate weathering rates, also varied by an order of magnitude, with upper values similar to the world mean (0.2 to 3.8 tons km-2 a-1, world mean = 3.53 tons km-2 a-1). Elevated chemical weathering rates in GrIS catchments are strongly influenced by the specific discharge, which drives flushing of the subglacial environment and physical erosion of the ice sheet bed. The direct relationship between specific discharge and chemical denudation rates supports the hypothesis that Greenland Ice Sheet chemical weathering rates and solute fluxes are likely to increase with enhanced melt rates in a warming climate.

Exploration of the antibiotic resistome in a wastewater treatment plant by a nine-year longitudinal metagenomic study

The spread of antibiotic resistance genes (ARGs) is a growing global problem. Activated sludge (AS) in wastewater treatment plants (WWTPs) has been proposed as a hotspot for ARGs. However, few studies have been conducted to uncover the temporal dynamics of the resistome of AS in WWTPs by long-term longitudinal sampling. In this study, we quantified ARGs and identified their host microbiome in a Hong Kong WWTP in 97 monthly AS samples spanning 9 years. Throughout this analysis, we demonstrated that both the abundance and structures of the resistome changed significantly every two to three years, implying that there was a successive selection of resistomes in the AS system over the study period. The detection of genes of antibiotic-resistant pathogens that are emerging major threats to public health in the AS samples, including mcr, CRE (carbapenem-resistant Enterobacteriaceae) and MRSA (methicillin-resistant Staphylococcus aureus)-related genes, highlight the role of WWTPs as reservoirs of ARGs. In addition, the core resistome (abundant and persistent genes) in AS were found to overlap with those in other ecosystems such as urban sewage, livestock feces, and fishpond sediments, revealing the broad dissemination of ARGs in WWTPs and other environments. Annual variation of resistomes were explained via structural equation modeling (SEM), which deciphered the structural linkages of determining factors such as the operational parameters, microbial community composition and horizontal gene transfer (HGT). Specifically, potentially relevant antibiotic resistance bacteria (ARBs) were explored and discussed based on assembly-based analyses and network correlations. Moreover, consistent with the clear relationship between resistomes and mobile genetic elements (MGEs), it was found that there was a relatively high potential for gene exchange in AS in comparison with soil genomes, which could be explained by the engineering features of WWTPs. Based on these findings, longitudinal monitoring of WWTPs is warranted for risk assessment to reveal emerging ARGs, resistome evolution, correlations with ARBs, and the potential for spread in downstream environments and concomitant exposure risks for humans.

Antibiotic resistome in a landfill leachate treatment plant and effluent-receiving river.

Landfills leachate contained diverse antibiotic resistance genes (ARGs). Treated landfill leachate effluent could enter into the downstream environments, leading to the dissemination of ARGs, which might pose a health risk to public. Here, we used high-throughput qPCR to characterize the resistome and 16S rRNA-based Illumina sequencing to analyze the bacterial community in a leachate treatment plant and the river near the landfill. A total of 91 ARGs and 5 mobile genetic elements were detected. Leachate treatment process significantly changed the profiles of resistome and bacterial community structures. Similar bacterial community structure and ARG profiles were detected between effluent and downstream river, which were both dominated by multidrug and beta-lactams resistance genes and harbored higher ARG relative abundance than that in upstream river. In particular, seven ARGs were detected both in effluent and downstream river samples but not detected in upstream river, including genes encoding resistance to vancomycin (vanXD and vanSB) and carbapenem (cphA and blaGES), which implied the effects of the effluent on its receiving river. This study highlights the risk of discharge of processed landfill leachate in dissemination of antibiotic resistance determinants to the environments, and suggests an urgent need for surveillance of ARGs and development of techniques to mitigate the risk.

Comparative diversity of microbiomes and Resistomes in beef feedlots, downstream environments and urban sewage influent

BackgroundComparative knowledge of microbiomes and resistomes across environmental interfaces between animal production systems and urban settings is lacking. In this study, we executed a comparative analysis of the microbiota and resistomes of metagenomes from cattle feces, catch basin water, manured agricultural soil and urban sewage.ResultsMetagenomic DNA from composite fecal samples (FC; n = 12) collected from penned cattle at four feedlots in Alberta, Canada, along with water from adjacent catchment basins (CB; n = 13), soil (n = 4) from fields in the vicinity of one of the feedlots and urban sewage influent (SI; n = 6) from two municipalities were subjected to Illumina HiSeq2000 sequencing. Firmicutes exhibited the highest prevalence (40%) in FC, whereas Proteobacteria were most abundant in CB (64%), soil (60%) and SI (83%). Among sample types, SI had the highest diversity of antimicrobial resistance (AMR), and metal and biocide resistance (MBR) classes (13 & 15) followed by FC (10 & 8), CB (8 & 4), and soil (6 & 1). The highest antimicrobial resistant (AMR) gene (ARG) abundance was harboured by FC, whereas soil samples had a very small, but unique resistome which did not overlap with FC & CB resistomes. In the beef production system, tetracycline resistance predominated followed by macrolide resistance. The SI resistome harboured β-lactam, macrolide, tetracycline, aminoglycoside, fluoroquinolone and fosfomycin resistance determinants. Metal and biocide resistance accounted for 26% of the SI resistome with a predominance of mercury resistance.ConclusionsThis study demonstrates an increasing divergence in the nature of the microbiome and resistome as the distance from the feedlot increases. Consistent with antimicrobial use, tetracycline and macrolide resistance genes were predominant in the beef production system. One of the feedlots contributed both conventional (raised with antibiotics) and natural (raised without antibiotics) pens samples. Although natural pen samples exhibited a microbiota composition that was similar to samples from conventional pens, their resistome was less complex. Similarly, the SI resistome was indicative of drug classes used in humans and the greater abundance of mercury resistance may be associated with contamination of municipal water with household and industrial products.

Acquisition of Extracellular DNA by Acinetobacter baylyi ADP1 in Response to Solar and UV-C254nm Disinfection.

Extracellular DNA (eDNA) cannot be effectively removed by most of the existing wastewater treatment technologies and can contribute to the gain of new functional traits when transformed into competent bacteria present in downstream environments. This study evaluates the contributions of solar and UV-C254nm irradiation to the transformation of eDNA in Acinetobacter baylyi ADP1. Solar irradiation was evaluated because it is a natural environmental stressor to which eDNA would be exposed during wastewater reuse. UV-C254nm was evaluated as an alternative to a chlorine-based disinfection strategy. Our findings showed that solar disinfection increased the natural transformation frequency by up to 2.0-fold after irradiance at 153 mJ/cm2. This was largely mediated by reactive oxygen species generation, which was correlated with an upregulation of both DNA repair (recA and ddrR) and competence (comA and pilX) genes. In contrast, even though UV-C254nm exposure was accompanied by the upregulation of DNA repair (recA, ddrR, and uvrB) genes and, hence, possibly higher integration rates of eDNA, we observed a concentration-dependent decrease in transformation rates. This decrease in transformation was likely due to the UV dimerization of eDNA, which resulted in the integration of damaged genes that cannot be transcribed into any functional gene products. These results imply that even though sunlight stimulates eDNA uptake and integration in the natural environment, UV disinfection implemented at a treatment plant can potentially minimize subsequent detrimental effects by damaging the extracellular genetic material and ensuring that there is no substantial expression of these transformed genes.

Numerical study on crack thermal resistance effect on thermo-mechanical coupled behavior of concrete structure at room temperature

Cracking behavior in massive concrete structure is inescapable. Cracks would impair the thermal conduction ability of concrete and indirectly induce temperature redistributions, which, in this study, is termed “Crack Thermal Resistance (CTR) effect”. In order to evaluate the influence of CTR on massive concrete structure, a specific thermo-mechanical (TM) coupled model at room temperature, featuring with the evolution of concrete thermal conductivity during material fracture, is proposed based on the thermodynamic principle and concrete damage plastic model. This coupled model is coded as subroutines in ABAQUS and the implementation validated by an uniaxial cyclic test of a single element and a double edge notched specimen (DENS) test. In the DENS numerical test, it is observed that CTR effect can cause heat flux turbulences around cracking area, leading to redistributions of temperature. As a consequence, the stress field and crack opening change around the cracking area as well. In the end, this model is applied to investigate the influence of CTR on the TM coupled behavior of a gravity dam under a long term operation period thermal load. The comparison between the simulations with and without CTR effect demonstrates that (a) CTR effect has a negative effect on structure interior crack propagation, because thermal resistances near dam toe protect the dam from the downstream environment temperature shock; (b) CTR effect can induce redistributions of stress field around the crack area, but the difference in stress field does not affect the compressive stress state on dam-foundation interface.

Factors Affecting Soil Total Mercury in Seasonal Pond Basins within a Northern Hardwood Forest in Minnesota, USA

Forest canopies are sites for mercury (Hg) deposition, and forests can act as Hg sources to downstream aquatic environments. This study examined soil total Hg (THg), carbon (C), and nitrogen (N) to 15 cm in 10 seasonal pond basins in a northern hardwood forest in Minnesota. Pools (mass per area) and concentrations (mass per soil mass) of THg to 15 cm were lower in uplands than in ponds, indicating downslope transport. In uplands, THg concentrations were the same at 0‐ to 2‐cm and 2‐ to 5‐cm depths and then decreased, whereas THg density (mass per volume) peaked at 2 to 5 cm, highlighting the importance of bulk density on mass. Carbon and N trends were similar to THg. Apart from pond centers, strong positive relationships between THg and C were observed. Upland slope length, graminoid cover, basin area, and tree height accounted for over half of THg variance at pond edge. Understanding the distribution, trends, and contributing factors of soil THg can further efforts toward immobilization and sequestration, thus minimizing the potential for bioaccumulation.Core Ideas THg uniform among basins in uplands with pond centers variable by substrate. THg greatest at pond center and uniform among upland landscape positions. Positive THg relationships with C and N, except organic pond centers. Upland THg concentration peaked at 0 to 5 cm, with mass peaking at 2 to 5 cm. Fifty‐six percent of THg pool was explained by slope length, graminoid cover, basin area, and tree height.