Contaminant Dispersion Research Articles

Mixing dynamics at river confluences governed by intermodal behaviour

The extent to which flows mix at confluences is pivotal for determining spatial patterns of water quality and biodiversity. Because mixing processes are complex, predicting rates and characteristic scales of mixing is difficult. Here we introduce a theory for confluence mixing dynamics of shallow flows in which the mixing process is controlled by the switching between two modes of behaviour—one similar to a wake behind an obstacle and the other similar to a mixing layer between two parallel flows. Dye visualizations of mixing in field-based controlled experiments agree with theoretical predictions and support transitional behaviour between the two mixing modes. According to our theoretical framework, the mixing interface can grow rapidly in wake mode, when large vortices are shed from a zone of stagnant flow within the confluence, but lateral shear between incoming flows is negligible. This rapid growth occurs even though flow curvature and shallowness inhibit growth through advective and turbulent lateral exchange of momentum and through bed friction. Our findings provide insight into the importance of different modalities of flow structure in controlling mixing at river confluences, thereby contributing to practical knowledge on the role of confluences in dispersal of contaminants in river systems.

Contaminant Dispersion and Breakthrough in Groundwater Flow: Case Study in Maui, Hawaii

Contaminant Dispersion and Breakthrough in Groundwater Flow: Case Study in Maui, Hawaii

Portable X-ray Fluorescence (pXRF) as a Tool for Environmental Characterisation and Management of Mining Wastes: Benefits and Limits

Portable X-ray fluorescence (pXRF) is one of the main geochemical techniques employed in multi-elemental analysis screening for contaminated sites management. As the confidence of pXRF analyses are matrix-specific, efforts are made to provide studies of pXRF quality on different geochemical datasets, focusing on less investigated elements such as mercury (Hg) and antimony (Sb), to help both new and experienced users. The analysis of environmental solid samples from two decommissioned mining sites in NE Italy, characterised by Pb-Zn and (Hg-rich) Cu-Sb ore deposits, were prepared with two different protocols and compared with traditional destructive analyses. Sample composition was found strictly dependent to the occurrence of false positives and overestimation at low concentrations. In contrast, milling the sample did not produce major variations in the overall quality. Lead (Pb), Sb, and Zn reached the definitive data quality in at least one of the two datasets. Consequently, as far as a thorough QA/QC protocol is followed, pXRF can rapidly produce chemical data that is as accurate as that produced by destructive standard laboratory techniques, thus allowing to identify potential sources of contamination that could be reprocessed for the extraction of valuable elements and mitigating the dispersion of contaminants and ecological or health risks.

Effect of source location on contaminant dispersion pattern and occupants inhaled air quality in lecture room under displacement ventilation

AbstractDisplacement ventilation (DV) is considered an energy saving air conditioning system and is especially recommended for classrooms for providing cool clean air in the occupied zone. However, in the current pandemic situation, ventilation systems should be assessed in terms of infection‐spread prevention as well. Hence, this study aims at assessing the efficiency of displacement ventilation in minimizing infection spread in lecture halls through investigating the effect of source position on the contaminant concentration distribution. The settings of a mid‐size DV lecture hall in Osaka University, Japan was chosen as a case‐study. Ten single‐contaminant‐source cases were simulated using CFD software. The cases were analyzed in terms of temperature distribution, contaminant diffusion pattern and vertical distribution, and breathing air quality using indices, such as local air quality index and scale of ventilation efficiency. The results demonstrated a considerable variation between cases in terms of contaminant distribution pattern, number of occupants affected, average inhaled‐air quality, and diffusion radius. Accordingly, given the ventilation system design, best and worst case‐scenarios were pointed out and factors affecting the variation were highlighted and further investigated to conclude the impact of each factor.

Entrainment and vertical mixing of aquatic microplastics in turbulent flow: The coupled role of particle size and density.

Diversity in microplastics' characteristics, including their size, affects their transport and distribution in aquatic systems. Furthermore, turbulent induced mixing is often considered dominant in the dispersion of sediments and contaminants in marine and freshwater systems, which is also affected by particle size. The aim of this study is to investigate the effect of microplastics' size and polymer density on their mixing behaviour in response to turbulent structures. Using sediment analogy, several parameters are defined to describe entrainment patterns of microplastic particles of common polymers. Our results indicate that the level of mixing of microplastics in turbulent flow can vary several orders of magnitude. While large particles' vertical motion may be dominated by gravitational settling or rising, the motion of fine microplastics is mainly governed by the ambient turbulent flow. Our findings provide a plausible explanation for the presence of fine microplastics in remote areas.

Mycorrhizal inoculation effects on growth and the mycobiome of poplar on two phytomanaged sites after 7-year-short rotation coppicing.

Afforestation of trace-element contaminated soils, notably with fast growing trees, has been demonstrated to be an attractive option for bioremediation due to the lower costs and dispersion of contaminants than conventional cleanup methods. Mycorrhizal fungi form symbiotic associations with plants, contributing to their tolerance towards toxic elements and actively participating to the biorestoration processes. The aim of this study was to deepen our understanding on the effects of mycorrhizal inoculation on plant development and fungal community at two trace-element contaminated sites (Pierrelaye and Fresnes-sur-Escaut, France) planted with poplar (Populus trichocarpa x Populus maximowiczii). The 2 sites were divided into 4 replicated field blocks with a final plant density of 2200 tree h-1. Half of the trees were inoculated with a commercial inoculum made of a mix of mycorrhizal species. The sites presented different physico-chemical characteristics (e.g., texture: sandy soil versus silty-loam soil and organic matter: 5.7% versus 3.4% for Pierrelaye and Fresnes-sur-Escaut, respectively) and various trace element contamination levels. After 7 years of plantation, inoculation showed a significant positive effect on poplar biomass production at the two sites. Fungal composition study demonstrated a predominance of the phylum Ascomycota at both sites, with a dominance of Geopora Arenicola and Mortierella elongata, and a higher proportion of ectomycorrhizal and endophytic fungi (with the highest values observed in Fresnes-sur-Escaut: 45% and 28% for ECM and endophytic fungi, respectively), well known for their capacity to have positive effects on plant development in stressful conditions. Furthermore, Pierrelaye site showed higher frequency (%) of mycorrhizal tips for ectomycorrhizal fungi (ECM) and higher intensity (%) of mycorrhizal root cortex colonization for arbuscular mycorrhizal fungi (AMF) than Fresnes-sur-Escaut site, which translates in a higher level of diversity. Finally, this study demonstrated that this biofertilization approach could be recommended as an appropriate phytomanagement strategy, due to its capacity to significantly improve poplar productivity without any perturbations in soil mycobiomes.

Short- and long-term assessment of PAH, PCB, and metal contamination in the Belgian part of the North Sea

Dredging activities can result in the relocation of contaminants such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and metals from shipping channels, harbours, and ports into the marine environment where these components may exert a negative effect on the marine ecosystem. In this work, contamination associated with dredging is evaluated at and around dredged spoil disposal sites in the Belgian part of the North Sea, taking into account spatial as well as temporal variation. A homogeneous dispersion of the different contaminants was observed at the different sites. However, the contamination pattern was different for disposal sites that were linked with commercial ports, resulting in significantly higher PAH and PCB concentrations. When comparing concentrations with environmental assessment criteria, contamination of PAHs does not exceed these criteria in the Belgian Part of the North Sea, and for PCBs, only CB118 reveals a concerningly high concentration that exceeds the environmental assessment criteria. This is in contrast with metals where the contamination of As, Cr, Ni, and Zn exceeded the environmental limit values both at dredged spoil disposal sites and reference locations.

Experimental characterization of shear-enhanced dispersion in porous media

The shear flow generated by natural convection in porous media considerably influences the dispersion of contaminants in various geological formations. Previous studies have revealed that the dispersion coefficient depends on the Péclet number (Pe), shear rate, and complex pore structures. In this study, the dispersion behavior of a NaI tracer cloud was investigated under different shear rates ranging from 0.001 to 0.04 s−1 and Pe ranging from 10 to 200 using an innovative experimental apparatus via an X-ray technique. In contrast to uniform flow, a considerably enhanced dispersion was observed in shear flow along the longitudinal direction, and spreading in the longitudinal and transverse directions grew with time as ∼t3 and ∼t, respectively. In the power-law regime, the dispersion behaviors in the transverse and longitudinal directions were strengthened by shear action, with a larger dependence of the power law on the shear rates and Pe compared with that in the uniform flow. The strengthened dispersion promoted the formation of a hyper-mixing state, which was evaluated using the temporal evolution of the maximum concentration of the tracer plume, dilution index, and scalar dissipation rate. The results indicate that the dispersion behavior in a two-dimensional shear flow is quantitatively equivalent to that in a four-dimensional uniform-flow field. The findings of this study can provide a deeper understanding of the influence of shear flow on the hydrodynamic dispersion in porous media.

Blast damage zone influence on groundwater fluxes through backfilled open pits

The blast damage zone (BDZ) surrounding backfilled pits could significantly influence groundwater flow, increase flowrate through backfilled wastes, and potentially increase the dispersion of contaminants to the environment. A conceptual model of the fracture network of the BDZ was first proposed, which consisted of circular fracture oriented along the pit walls, and with a decreasing density with the distance to it. Realistic model parameters were also estimated based on the literature; BDZ equivalent permeability was analytically derived and used in 3D numerical models of conic open pits considering different backfilling scenarios. A total of 16 500 BDZ models were generated to assess the general influence of the BDZ on the flow and the influence of each parameter (fracture trace and attenuation length and fracture radius, aperture, and anisotropy). Results showed that the BDZ generally contributes to reduce (up to three orders of magnitude) the groundwater flow through backfilled tailings, but contributes to increase (by up to 250%) the flow in backfilled waste rocks, especially for wide and fractured BDZ in small pits. This study therefore suggests that the BDZ could be a self-sufficient containment structure to limit contaminant transports from backfilled tailings to the environment, but increases the risk considering waste rock.

Virus transport and infection evaluation in a passenger elevator with a COVID-19 patient.

Contaminant transport and flow distribution are very important during an elevator ride, as the reduced social distancing may increase the infection rate of airborne diseases such as COVID-19. Studying the airflow and contaminant concentration in an elevator is not straightforward because the flow pattern inside an elevator changes dramatically with passenger movement and frequent door opening. Since very little experimental data were available for elevators, this investigation validated the use of computational fluid dynamics (CFD) based on the RNG k- turbulence model to predict airflow and contaminant transport in a scaled, empty airliner cabin with a moving passenger. The movement of the passenger in the cabin created a dynamic airflow and transient contaminant dispersion that were similar to those in an elevator. The computed results agreed reasonably well with the experimental data for the cabin. The validated CFD program was then used to calculate the distributions of air velocity, air temperature, and particle concentration during an elevator ride with an index patient. The CFD results showed that the airflow pattern in the elevator was very complex due to the downward air supply from the ceiling and upward thermal plumes generated by passengers. This investigation studied different respiratory activities of the index patient, that is, breathing only, breathing, and coughing with and without a mask, and talking. The results indicated that the risk of infection was generally low because of the short duration of the elevator ride. If the index patient talked in the elevator, two passengers in the closest proximity to distance would be infected.

Risk index of contaminant dispersion in the sea, case study from HarmoNIA project

Risk index of contaminant dispersion in the sea, case study from HarmoNIA project

A GIS-based map of the Hg-impacted area in the Paglia River basin (Monte Amiata Mining District – Italy): An operational instrument for environmental management

In this study, we describe the elaboration of a GIS-based map of the Hg-impacted area of the upper part of the Paglia River basin. The map developed from a systematic collection of all existing geological and geochemical data, together with land cover and geomorphology data, integrated into a GIS-based geo-database. The Hg-impacted area was identified along the upper part of the Paglia River basin and was defined as a band within which there is at least one demonstrated occurrence of river/stream sediments and soils with concentrations of Hg ≥1 mg/kg. The Hg-impacted area covers about 20 km2 and the width roughly coincides with the Paglia River floodplain developed in the last 130 years. The identification of a Hg-impacted area will be a useful tool for the local communities in the Paglia River basin and for environmental national agencies when planning works involving the remobilization of river/stream sediments and soils to prevent the dispersal of contaminants. Considering that the extent of the Hg-contaminated area does not allow any remediation, the map can be used for site-specific risk assessment to ensure that anthropogenic activities occur at a minimum risk, considering the opportunity of revising local legislations regarding the permissible Hg-concentration in river/stream sediments in the area.

Pollution gradient leads to local adaptation and small-scale spatial variability of communities and functions in an urban marine environment.

Urbanization of coastal habitats, of which harbors and marinas are the paragon, has led to various ecological paradigms about their functioning. Harbor infrastructures offer new hard substrata that are colonized by a wide variety of organisms (biofouling) including many introduced species. These structures also modify hydrodynamism and contaminant dispersal, leading to strong disturbance gradients within them. Differences in sessile community structure have previously been correlated to these gradients at small spatial scale (<100 m). Local adaptation might be involved to explain such results, but as correlation is not causation, the present study aims to understand the causal link between the environmental gradients and community structure through a reciprocal transplant experiment among three sites of a marina (inner, middle, entrance). Our results highlighted strong small-scale spatial variations of contaminants (trace metals, PCB, pesticides, and PAH) in sediments and animal samples which have been causally linked to changes in community composition after transplant. But historical contingency and colonization succession also play an important role. Our results provided strong evidence for local adaptation since community structure, respiration, and pollutant uptake in Bugula neritina, as well as the metabolomes of B. neritina and Ciona intestinalis were impacted by the transplant with a disadvantage for individuals transplanted from the entrance to the inner location. The here observed results may thus indicate that the disturbance gradient in marinas might constitute a staple for selecting pollutant-resistant species and populations, causing local adaptation. This highlights the importance of conducting further studies into small scale local adaptation.

The shape and residual flow interaction of tidal oscillations

Tidal flows are seldom exactly sinusoidal, leading to a small discrepancy, or residual, over each tidal cycle. Although residuals are generally small in comparison with instantaneous currents, their cumulative effect is important for sediment transport and dispersion of contaminants. The meso-scale characteristics of tidal residual currents are investigated with a computational model of the Irish Sea. The role of the tidal oscillations, or eddies, in forcing the residual flow is considered first through theoretical considerations and secondly by calculating time mean flow quantities directly from the computational model. The tidal eddies contribute to the time mean vorticity balance through the tidal stresses which can be written in the form of a divergence of an eddy vorticity flux. In regions where the tidal flows are approximately horizontally non-divergent the anisotropy of the tidal eddies is strongly linked to their contribution to driving the residual flow. A measure of eddy anisotropy is proposed and this mirrors the shape and orientation of the tidal ellipses of the main tidal constituent. The vorticity balance of the residual flow is dominated by the frictional torque and the eddy vorticity flux divergence, with vorticity advection and vortex stretching by the residual flow generally being of secondary importance.

Assessment of mercury enrichment in lake sediment records from Alberta Oil Sands development via fluvial and atmospheric pathways

Exploitation of bitumen-rich deposits in the Alberta Oil Sands Region (AOSR) by large-scale mining and processing activities has generated widespread concern about the potential for dispersal of harmful contaminants to aquatic ecosystems via fluvial and atmospheric pathways. The release of mercury has received attention because it is a potent neurotoxin for wildlife and humans. However, knowledge of baseline mercury concentration prior to disturbance is required to evaluate the extent to which oil sands development has contributed mercury to aquatic ecosystems. Here, we use stratigraphic analysis of total mercury concentration ([THg]) in radiometrically dated sediment cores from nine floodplain lakes in the AOSR and downstream Peace-Athabasca Delta (PAD) and two upland lakes in the PAD region to establish pre-1900 baseline [THg] and evaluate if [THg] has become enriched via fluvial and atmospheric pathways since oil sands mining and processing began in 1967. Concentrations of THg in sediment cores from the study lakes range from 0.022–0.096 mg/kg (dry wt.) and are below the Canadian interim sediment quality guidelines for freshwater (0.17 mg/kg). Results demonstrate no enrichment of [THg] above pre-1900 baseline via fluvial pathways at floodplain lakes in the AOSR or PAD. Enrichment of [THg] was detected via atmospheric pathways at upland lakes in the PAD region, but this occurred prior to oil sands development and aligns with long-range transport of emissions from coal combustion and other anthropogenic sources across the northern hemisphere recognized in many other lake sediment records. The inventory of anthropogenic [THg] in the upland lakes in the AOSR is less than at the Experimental Lakes Area of northwestern Ontario (Canada), widely regarded as a “pristine” area. The absence of enrichment of [THg] in lake sediment via fluvial pathways is a critical finding for stakeholders, and we recommend that monitoring at the floodplain lakes be used to inform stewardship as oil sands operators prepare to discharge treated oil sands process waters directly into the Athabasca River upstream of the PAD.

Ventilation indices for evaluation of airborne infection risk control performance of air distribution

Air distribution is an effective engineering measure to fight against respiratory infectious diseases like COVID-19. Ventilation indices are widely used to indicate the airborne infection risk of respiratory infectious diseases due to the practical convenience. This study investigates the relationships between the ventilation indices and airborne infection risk to suggest the proper ventilation indices for the evaluation of airborne infection risk control performance of air distribution. Besides the commonly used ventilation indices of the age of air (AoA), air change effectiveness (ACE), and contaminant removal effectiveness (CRE), this study introduces two ventilation indices, i.e., the air utilization effectiveness (AUE) and contaminant dispersion index (CDI). CFD simulations of a hospital ward and a classroom served by different air distributions, including mixing ventilation, displacement ventilation, stratum ventilation and downward ventilation, are validated to calculate the ventilation indices and airborne infection risk. A three-step correlation analysis based on Spearman's rank correlation coefficient, Pearson correlation coefficient, and goodness of fit and a min-max normalization-based error analysis are developed to qualitatively and quantitatively test the validity of ventilation indices respectively. The results recommend the integrated index of AUE and CDI to indicate the overall airborne infection risk, and CDI to indicate the local airborne infection risk respectively regardless of the effects of air distribution, supply airflow rate, infectivity intensity, room configuration and occupant distribution. This study contributes to airborne transmission control of infectious respiratory diseases with air distribution.

Analysis of contaminant dispersion in open channel with two streambank-absorption boundaries.

Boundary absorption intensity can affect the contaminant depletion capacity within rivers, and the process of spatial contaminant cloud expansion is complicated with the consideration of irreversible absorption boundaries at riverbanks. Nonuniformity of concentration distribution appears in spatial concentration distribution, especially in the transverse direction, which is caused by the absorption capacity difference between two riverbanks. A model for illustrating the performance of environmental dispersion with two irreversible absorption banks in 2D space is given in this work. Furthermore, the position of the maximum concentration distributions shifts within the transverse directions with the change of the absorption intensities at two boundaries. The overall absorption capacity would also be affected by the ratio of two absorption intensities at the left and right riverbanks. The residual mass is left with a greater variation in the two bank-absorption intensity ratios. A detailed analysis of the spatial concentration distribution and contaminant depletion capacity with two bank-absorption boundaries would contribute to the construction of a wetland for water treatment. With a certain absorption capacity in total, the transverse distribution of concentration gets more heterogeneous as the ratio deviates from 1gradually, and the transverse concentration distribution appears to be symmetric to the center (0.5W) when the ratios of absorption intensities at two stream-banks are in accord with β0 : β1 = β1 : β0. The novelty of this work is to provide the analytical solution of two-dimensional concentration distribution with the ratio of two stream-bank absorptions, furthermore, a linear fitting equation([Formula: see text]) for crest position of transverse concentration distribution is given to show the shifting process of spatial contaminant cloud with the change of two stream-bank absorption ratios, and the correlation coefficients are all above 0.99, illustrating a good fit for the results.

Effect of Organic Amendment on the Physicochemical Characteristics of Tailings Dam Soil and Root Development of Tree Species, Fifteen Years After Planting

Among mine wastes, tailings are known to have the largest environmental impact, as they have high concentrations of trace elements and are susceptible to wind dispersal and water erosion. A tree plantation trial was installed at Kipushi tailing (DR Congo) in order to mitigate the contaminant dispersal in the surrounding areas. Fifteen years later, the present study was conducted for the purpose of investigating the macronutrient and metal content in amended holes and assessing the performance of tree species through root behavior in the tailings dams. Results show elevated available P, K, Ca, and Mg concentration in the surface and amended layers, which is higher than the unpolluted soil of the miombo woodland. Trace metals were manifold higher compared to the pedo-geochemical background of the region, with Cu and Co concentration tending to increase in the organic matter-rich layers, while Zn, Cd, Pb, and As remained higher in tailings. Compared to the tailing layer, roots grew well in the amended layers, but few roots ranging from very fine to big from all the surviving species were able to grow beyond the amended layers, indicating the possibility of tree survival on the tailings dams over many years. Acacia polyacantha and Psidium guajava are species that showed a higher quantity of roots in the unamended tailing layers. Leaves should be avoided for human or animal consumption, but as the concentration of Cu, Zn, and Co in guava was lower, there is no indication of hazards in case of their consumption. Therefore, the use of well-adapted tree species on the mix-up of the organic amendments with the uncontaminated topsoil seemed to be a good technique for the reclamation of larger polluted areas.

COVID-19 dispersion in naturally-ventilated classrooms: a study on inlet-outlet characteristics

Infectious aerosol dispersion poses significant infection risks (i.e., COVID-19) in classrooms due to dense and long occupancy. Natural ventilation is an effective strategy to reduce airborne infection transmission. The building-related parameters, particularly openings, determine the natural ventilation effectiveness in reducing contaminant dispersion, necessitating an inquiry due to complex dispersion and airflow patterns. This paper investigates the correlation between window height, natural ventilation, and COVID-19 dispersion. A simulation pipeline involving a parametric 3D design environment, computational fluid dynamics (CFD), and energy simulations is developed and implemented on nine design scenarios representing different inlet-outlet heights of a free-running (no heating, cooling or mechanical ventilation) classroom. The inlet height and the inlet-outlet height difference have a considerable impact on indoor infection risk confirming that stack ventilation and the Bernoulli effect decrease indoor contaminant concentration. Proximity to openings does not ensure lower contamination levels. Proximity to the contaminant does not result in higher contamination levels.

How much spatial and temporal variation in groundwater microbiology can occur following open dumping of municipal solid waste?

Abstract The effects of open dumping of municipal solid waste on local groundwater microbiology were assessed in Ranchi, India. Both the spatial and temporal variations of groundwater microbiology were studied. Groundwater was analyzed to determine heterotrophic plate count (HPC), total coliforms (TC) and faecal coliforms (FC). Highest HPC was 4.5 × 104 CFU/mL and the highest total coliform count was 3.7 × 104 CFU/mL. Faecal coliforms were detected in most of the groundwater samples. The highest faecal coliform count of 2.1 × 104 CFU/mL was from a hand pump located adjacent to the dumping site. Spatial variations revealed dispersion of bacterial contamination up to 5 km from the periphery of the municipal solid waste dump. The level of bacterial contamination decreased with an increase in distance from the municipal solid waste dump. The maximum bacterial count was detected in the monsoon whereas the lowest count was found in summer. Molecular characterization of morphologically similar faecal coliform colonies indicated the presence of Klebsiella pneumoniae (Klebsiella ssnkbit, KU647674) in one of the groundwater samples that showed maximum faecal coliform count. Results of this study indicated that groundwater in the area is highly contaminated with a bacterial load which can be a major health risk.