Phosphorus (P) supply (concentration and flux) is an important driver for biological activity in flowing waters and needs to be managed to avoid eutrophication impacts associated with urbanisation ...and agricultural intensification. This paper examines the role of in-stream retention and cycling in regulating river P concentrations in order to better understand the links between P sources and their ecological impacts. In terms of their composition (solubility and concentration), patterns of delivery (mode and timing) and therefore ecological relevance, P sources entering rivers are best grouped into wastewater discharges > runoff from impervious surfaces (roads, farmyards) > runoff from pervious surfaces (forestry, cultivated land and pasture). The localized impacts of soluble P discharges during ecologically sensitive periods can be distinguished from the downstream impacts associated with particulate P discharges under high flows due to the different processes by which these sources are retained, transformed and assimilated within the river channel. The range of physico-chemical processes involved in P cycling and the variable importance of these processes in different river environments according to stream size, stream geomorphology and anthropogenic pressures are summarised. It is concluded that the capacity to retain (process) P within the river channel, and hence regulate the downstream delivery of P without stressing the aquatic communities present, is considerable, especially in headwaters. To help achieve good water quality, there is scope to better manage this ecosystem service through regulation of P supply whilst optimising in-stream P retention according to subsidy-stress theory. Further research is needed to develop in-stream management options for maximising P subsidies and to demonstrate that regulation of downstream P delivery will reduce the incidence of eutrophication in connected waterbodies.
Quantitative X-ray tomography Maire, E.; Withers, P. J.
International materials reviews,
20/1/1/, Letnik:
59, Številka:
1
Journal Article
Recenzirano
Odprti dostop
X-ray computer tomography (CT) is fast becoming an accepted tool within the materials science community for the acquisition of 3D images. Here the authors review the current state of the art as CT ...transforms from a qualitative diagnostic tool to a quantitative one. Our review considers first the image acquisition process, including the use of iterative reconstruction strategies suited to specific segmentation tasks and emerging methods that provide more insight (e.g. fast and high resolution imaging, crystallite (grain) imaging) than conventional attenuation based tomography. Methods and shortcomings of CT are examined for the quantification of 3D volumetric data to extract key topological parameters such as phase fractions, phase contiguity, and damage levels as well as density variations. As a non-destructive technique, CT is an ideal means of following structural development over time via time lapse sequences of 3D images (sometimes called 3D movies or 4D imaging). This includes information needed to optimise manufacturing processes, for example sintering or solidification, or to highlight the proclivity of specific degradation processes under service conditions, such as intergranular corrosion or fatigue crack growth. Besides the repeated application of static 3D image quantification to track such changes, digital volume correlation (DVC) and particle tracking (PT) methods are enabling the mapping of deformation in 3D over time. Finally the use of CT images is considered as the starting point for numerical modelling based on realistic microstructures, for example to predict flow through porous materials, the crystalline deformation of polycrystalline aggregates or the mechanical properties of composite materials.
To better understand the relationship between the nucleation and growth of defects and the local stresses and phase changes that cause them, we need both imaging and stress mapping. Here, we explore ...how this can be achieved by bringing together synchrotron X-ray diffraction and tomographic imaging. Conventionally, these are undertaken on separate synchrotron beamlines; however, instruments capable of both imaging and diffraction are beginning to emerge, such as ID15 at the European Synchrotron Radiation Facility and JEEP at the Diamond Light Source. This review explores the concept of three-dimensional crack-tip X-ray microscopy, bringing them together to probe the crack-tip behaviour under realistic environmental and loading conditions and to extract quantitative fracture mechanics information about the local crack-tip environment. X-ray diffraction provides information about the crack-tip stress field, phase transformations, plastic zone and crack-face tractions and forces. Time-lapse CT, besides providing information about the three-dimensional nature of the crack and its local growth rate, can also provide information as to the activation of extrinsic toughening mechanisms such as crack deflection, crack-tip zone shielding, crack bridging and crack closure. It is shown how crack-tip microscopy allows a quantitative measure of the crack-tip driving force via the stress intensity factor or the crack-tip opening displacement. Finally, further opportunities for synchrotron X-ray microscopy are explored.
Without post-manufacture HIPing the fatigue life of electron beam melting (EBM) additively manufactured parts is currently dominated by the presence of porosity, exhibiting large amounts of scatter. ...Here we have shown that the size and location of these defects is crucial in determining the fatigue life of EBM Ti-6Al-4V samples. X-ray computed tomography has been used to characterise all the pores in fatigue samples prior to testing and to follow the initiation and growth of fatigue cracks. This shows that the initiation stage comprises a large fraction of life (>70%). In these samples the initiating defect was often some way from being the largest (merely within the top 35% of large defects). Using various ranking strategies including a range of parameters, we found that when the proximity to the surface and the pore aspect ratio were included the actual initiating defect was within the top 3% of defects ranked most harmful. This lays the basis for considering how the deposition parameters can be optimised to ensure that the distribution of pores is tailored to the distribution of applied stresses in additively manufactured parts to maximise the fatigue life for a given loading cycle.
X-ray computed tomography has been used to track the behaviour of individual pores found in selective electron beam melted additive manufactured titanium. Porosity was found to shrink below the ...detection limit of X-ray microtomography (<5μm) upon hot isostatic pressing. Spherical argon containing gas pores, which have a high internal gas pressure following hot isostatic pressing, have been found to progressively reappear and grow in proportion to their original as-built size during high temperature (β-anneal) treatments, whereas larger irregular low pressure pores did not reappear. The implications of these observations in terms of additive manufacturing are discussed.
Display omitted
Natural and manufactured materials rely on complex hierarchical microstructures to deliver a suite of interesting properties. To predict and tailor their performance requires a joined-up knowledge of ...their multiphase microstructure, interfaces, chemistry and crystallography from the nanoscale to the macroscale. This Perspective reflects on how recent developments in correlative characterization can bring together multiple image modalities and maps of the local chemistry, structure and functionality to form rich multimodal and multiscale correlated datasets. The automated collection and digitization of multidimensional data is an essential part of the picture for developing multiscale modelling and 'big data'-driven machine learning approaches. These are needed to both improve our understanding of existing materials and exploit high-throughput combinatorial synthesis, processing and testing methods to develop materials with bespoke properties.
Septic tank systems (STS) are a potential source of nutrient emissions to surface waters but few data exist in the UK to quantify their significance for eutrophication. We monitored the impact of STS ...on nutrient concentrations in a stream network around a typical English village over a 1-year period. Septic tank effluent discharging via a pipe directly into one stream was highly concentrated in soluble N (8–63mgL−1) and P (<1–14mgL−1) and other nutrients (Na, K, Cl, B and Mn) typical of detergent and household inputs. Ammonium-N (NH4N) and soluble reactive P (SRP) fractions were dominant (70–85% of total) and average concentrations of nitrite-N (NO2N) were above levels considered harmful to fish (0.1mgL−1). Lower nutrient concentrations were recorded at a ditch and a stream site, but range and average values downstream of rural habitation were still 4 to 10-fold greater than those in upstream sections. At the ditch site, where flow volumes were low, annual flow-weighted concentrations of NH4N and SRP increased from 0.04 and 0.07mgL−1, respectively upstream to 0.55 and 0.21mgL−1 downstream. At the stream site, flow volumes were twice as large and flow-weighted concentrations increased much less; from 0.04 to 0.21mgL−1 for NH4N and from 0.06 to 0.08mgL−1 for SRP. At all sites, largest nutrient concentrations were recorded under low flow and stream discharge was the most important factor determining the eutrophication impact of septic tank systems. The very high concentrations, intercorrelation and dilution patterns of SRP, NH4-N and the effluent markers Na and B suggested that soakaways in the heavy clay catchment soils were not retaining and treating the septic tank effluents efficiently, with profound implications for stream biodiversity. Water companies, water regulators and rural communities therefore need to be made more aware of the potential impacts of STS on water quality so that their management can be optimised to reduce the risk of potential eutrophication and toxicity to aquatic ecosystems during summer low flow periods.
► First direct UK evidence that nutrient emissions from septic tank systems affect water quality in rural areas. ► Septic tank soakaways on impermeable soils failed to adeqautely treat the septic tank effluent. ► The downstream eutrophication impact of septic tank systems largely depends on stream discharge volumes. ► Septic tank systems act as mini-point sources and need to better managed in catchment management planning.
Foreign object damage (FOD) can seriously shorten the fatigue lives of components. On the other hand, laser shock peening improves fatigue life by introducing deep compressive residual stress into ...components. In this paper we examine how the non-uniform steep residual stress profile arising from FOD of laser peened aerofoil leading edges varies as a function of fatigue crack growth under high cycle fatigue and mixed high and low cycle fatigue conditions. The ballistic FOD impacts were introduced by impacting a cube edge head-on (at an angle of 0°) to the leading edge. The residual stress distributions have been mapped by synchrotron X-ray diffraction prior to cracking and subsequent to short (∼1mm) and long (up to 6mm) crack growth. The results suggest that the local residual stress field is highly stable even to the growth of relatively long cracks.
Reactive nitrogen (N) and phosphorus (P) inputs to surface waters modify aquatic environments, affect public health and recreation. Source controls dominate eutrophication management, whilst ...biological regulation of nutrients is largely neglected, although aquatic microbial organisms have huge potential to process nutrients. The stoichiometric ratio of organic carbon (OC) to N to P atoms should modulate heterotrophic pathways of aquatic nutrient processing, as high OC availability favours aquatic microbial processing. Heterotrophic microbial processing removes N by denitrification and captures N and P as organically-complexed, less eutrophying forms. With a global data synthesis, we show that the atomic ratios of bioavailable dissolved OC to either N or P in rivers with urban and agricultural land use are often distant from a “microbial optimum”. This OC-deficiency relative to high availabilities of N and P likely overwhelms within-river heterotrophic processing. We propose that the capability of streams and rivers to retain N and P may be improved by active stoichiometric rebalancing. Although autotrophic OC production contributes to heterotrophic rates substantial control on nutrient processing from allochthonous OC is documented for N and an emerging field for P. Hence, rebalancing should be done by reconnecting appropriate OC sources such as wetlands and riparian forests that have become disconnected from rivers concurrent with agriculture and urbanisation. However, key knowledge gaps require research prior to the safe implementation of this approach in management: (i) to evaluate system responses to catchment inputs of dissolved OC forms and amounts relative to internal production of autotrophic dissolved OC and aquatic and terrestrial particulate OC and (ii) evaluate risk factors in anoxia-mediated P desorption with elevated OC scenarios. Still, we find stoichiometric rebalancing through reconnecting landscape beneficial OC sources has considerable potential for river management to alleviate eutrophication, improve water quality and aquatic ecosystem health, if augmenting nutrient source control.
Display omitted
•Eutrophication assessment should examine balances of the macronutrients C, N and P.•C:N and C:P stoichiometry was assessed in global catchment sources and river waters.•Excesses of NP relative to organic C was associated with agriculture and urbanisation.•Restoration of catchment C sources (e.g. wetlands) may drive microbial NP sequestration.
The impact of solar flares on the upper atmosphere of Mars presents an opportune environment for investigating the ionosphere's response to the rapid and high‐energy dynamics of our Sun, with ...implications for characterizing atmospheric escape and evolution. The ionosphere of Mars is strongly influenced both by the levels of ionizing solar irradiance it receives and the associated behavior of the neutral thermosphere within which it is embedded. Here, we characterize density and composition changes in the topside ionosphere of Mars due to the X8.2‐class solar flare on 10 September 2017 from MAVEN spacecraft observations roughly 90 min after the onset of flare irradiance at Mars, by which time the neutral thermosphere had experienced significant thermal expansion. By contrasting altitude and pressure‐based representations of the ionospheric changes during the flare with the neutral thermospheric changes in both representations and basic photochemical expectation, we interpret and quantify the ionospheric behavior through a lesser explored lens of the neutral thermosphere's response to a flare. We find that the high‐altitude ionospheric density enhancements during this flare are larger than many identified from other flares, and that the neutral thermosphere's expansion accounts for a majority of the cumulative ion density changes observed. The ion density and compositional changes exhibit corresponding behavior with the neutral thermosphere's changes from both thermal expansion and photochemical effects. The identified characteristics of the ionosphere's response to a solar flare provide observational benchmarks for assessing and aiding numerical modeling efforts in accurately reproducing the behavior of the environment of Mars.
Key Points
At high altitudes, ion density as much as doubles due to this solar flare in contrast with the little change identified for other flares
Ion species exhibit flare‐driven changes unique both from one another and between altitude and pressure‐based representations
Thermal expansion of the thermosphere accounts for the majority of ion density and composition changes during these flare event observations
PDF
