Erosion characteristics and floc strenght of Athabasca river cohesive sediments: towards managing sediment-related issues

Purpose: Most of Canada’s tar sands exploitations are located in the Athabasca river basin. Deposited cohesive sediments in Athabasca river and tributaries are a potential source of PAHs in the basin. Erosional behavior of cohesive sediments depends not only of fluid turbulence but on sediments structure and particularly the influence of organic content. This research tries to describe this behavior in Athabasca river sediments. Methods: An experimental study of cohesive sediments dynamics in one of the tributaries, the Muskeg river, was developed in a rotating annular flume. Variation of the shear stress allowed the determination of erosional strength for beds with different consolidation periods. Particle size measurements were made with a laser diffraction device operated in a continuous flow through mode. Optical analyses of flocs (ESEM and TEM) were performed with samples taken at the end of the experiments. Results: An inverse relationship between suspended sediment concentration (SS) and the consolidation period was found. The differences are related in this research to the increasing organic content of the sediments with consolidation period. The particle size measurements during the experiments showed differences on floc strength that are also related to changing organic content during different consolidation periods. ESEM and TEM observations confirm the structural differences for beds with different consolidation periods. The effects of SFGL on floc structure and in biostabilization of the bed are discussed. Conclusions: It is recommended in this paper that consolidation period should be taken into account for the modeling of erosion of cohesive sediments in the Athabasca river. Relating to transport models of pollutants (PAHs) it is highly recommended to consider flocs organic content, particularly algae, in the resuspension module.Environment Canada, CONACY

Perceptual Rivalry: Reflexes Reveal the Gradual Nature of Visual Awareness

Rivalry is a common tool to probe visual awareness: a constant physical stimulus evokes multiple, distinct perceptual interpretations (“percepts”) that alternate over time. Percepts are typically described as mutually exclusive, suggesting that a discrete (all-or-none) process underlies changes in visual awareness. Here we follow two strategies to address whether rivalry is an all-or-none process: first, we introduce two reflexes as objective measures of rivalry, pupil dilation and optokinetic nystagmus (OKN); second, we use a continuous input device (analog joystick) to allow observers a gradual subjective report. We find that the “reflexes” reflect the percept rather than the physical stimulus. Both reflexes show a gradual dependence on the time relative to perceptual transitions. Similarly, observers' joystick deflections, which are highly correlated with the reflex measures, indicate gradual transitions. Physically simulating wave-like transitions between percepts suggest piece-meal rivalry (i.e., different regions of space belonging to distinct percepts) as one possible explanation for the gradual transitions. Furthermore, the reflexes show that dominance durations depend on whether or not the percept is actively reported. In addition, reflexes respond to transitions with shorter latencies than the subjective report and show an abundance of short dominance durations. This failure to report fast changes in dominance may result from limited access of introspection to rivalry dynamics. In sum, reflexes reveal that rivalry is a gradual process, rivalry's dynamics is modulated by the required action (response mode), and that rapid transitions in perceptual dominance can slip away from awareness

Organic and organometallic compounds in estuarine sediments from the Gulf of Mexico (1993-1994)

700-709Sediment samples from 281 estuarine sites in the Gulf of Mexico were collected in 1993-1994 and analyzed for several classes of organic and organometallic compounds as part of the Environmental Monitoring and Assessment Program of the United States Environmental Protection Agency. Polynuclear aromatic hydrocarbons (PAHs) were the contaminant class found most frequently and in the highest concentrations; the sum of 24 congeners (SIGMAPAHs) ranged from < 5 ng g-1 to 15,500 ng g-1 (dry wt basis). A low percentage of samples (3.9%) exceeded 2000 ng g-1 SIGMAPAHs, and only six samples (2.1%) exceeded 4000 ng g-1, a level above which adverse biological effects may be expected to occur. Less than 4% of sediments exceeded 20 ng g-1 for the sum of 20 polychlorinated biphenyls (SIGMAPCBs) and only four samples (1.4%) exceeded 20 ng g-1 for the sum of several organochlorine pesticides (SIGMAOCPs). A sample from Freeport Harbor, Texas, contained 4230 ng g-1 SIGMAPAHs 322 ng g-1 SIGMAPCBs, and 49.6 ng g-1 SIGMAOCPs. Tributyltin exceeded 100 ng g-1 in only four samples, all of which were from stations in Corpus Christi Bay or Galveston Bay in Texas. The detection of a suite of organophosphate pesticides was very rare and did not exceed 15 ng g-1. Sediments from the tidally influenced section of the Mississippi River in Louisiana contained low to moderate levels of all classes of organic compounds. The most contaminated sites were in urban estuaries (e.g., Corpus Christi, Galveston, and Pensacola (Florida bays), underscoring the need to concentrate future monitoring and assessment efforts at the regional and local levelhttp://gbic.tamug.edu/request.ht

Organic and organometallic compounds in estuarine sediments from the Gulf of Mexico (1993-1994)

700-709Sediment samples from 281 estuarine sites in the Gulf of Mexico were collected in 1993-1994 and analyzed for several classes of organic and organometallic compounds as part of the Environmental Monitoring and Assessment Program of the United States Environmental Protection Agency. Polynuclear aromatic hydrocarbons (PAHs) were the contaminant class found most frequently and in the highest concentrations; the sum of 24 congeners (SIGMAPAHs) ranged from < 5 ng g-1 to 15,500 ng g-1 (dry wt basis). A low percentage of samples (3.9%) exceeded 2000 ng g-1 SIGMAPAHs, and only six samples (2.1%) exceeded 4000 ng g-1, a level above which adverse biological effects may be expected to occur. Less than 4% of sediments exceeded 20 ng g-1 for the sum of 20 polychlorinated biphenyls (SIGMAPCBs) and only four samples (1.4%) exceeded 20 ng g-1 for the sum of several organochlorine pesticides (SIGMAOCPs). A sample from Freeport Harbor, Texas, contained 4230 ng g-1 SIGMAPAHs 322 ng g-1 SIGMAPCBs, and 49.6 ng g-1 SIGMAOCPs. Tributyltin exceeded 100 ng g-1 in only four samples, all of which were from stations in Corpus Christi Bay or Galveston Bay in Texas. The detection of a suite of organophosphate pesticides was very rare and did not exceed 15 ng g-1. Sediments from the tidally influenced section of the Mississippi River in Louisiana contained low to moderate levels of all classes of organic compounds. The most contaminated sites were in urban estuaries (e.g., Corpus Christi, Galveston, and Pensacola (Florida bays), underscoring the need to concentrate future monitoring and assessment efforts at the regional and local levelhttp://gbic.tamug.edu/request.ht