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(1999)Journal ArticleThis paper examines the history of selenium pollution in Lake Macquarie, NSW, Australia, and three factors that may affect the redistribution and remobilisation of particle bound selenium: changes in redox state, bioturbation, and bioaccumulation by macrobenthos and bacteria. Sediment cores were taken from Nords Wharf, a relatively unpolluted area, and from Mannering Bay near the Vales Point coal-fired power station. The age profile at the unpolluted site seems to indicate that mild selenium pollution has been occurring for over 100 years, however, some mixing of the sediments has occurred. At the polluted site, the age profile indicated that major contamination has occurred in the last 30 years, due to an ash dam associated with nearby electric power generation facilities. The contamination chronology suggests that remobilisation and reduction processes have affected the selenium profile. Changing the redox state of Lake Macquarie sediment results in a release of selenium under oxidising conditions and immobilisation under reducing conditions. The sediment-bound selenium was associated with the operationally defined `organic/sulfide' fraction under reducing conditions, and as the redox potential increases this moves into the `exchangeable' and `iron/manganese oxyhydroxide' phases to a limited extent. Bioturbation by the animals Marphysa sanguinea and Spisula trigonella caused increases in the redox potential and pore water selenium concentrations in surfcial sediments relative to unbioturbated controls. Both animals accumulated significantly more selenium when exposed to contaminated sediment than when exposed to uncontaminated control sediments. Selenium concentrations in molluscs from Mannering Bay were all significantly higher than those collected from Nords Wharf. Most of the selenium in the mollusc tissues was found to be associated with the protein fraction. Selenium isolated from hydrolysed muscle tissue was not present as selenate or selenite but as selenomethionine and an unidentified compound. Seven types of bacteria were isolated from Lake Macquarie sediment. All seven isolates were able to transform selenite quantitatively to elemental selenium as evidenced by a red precipitate and identified by X-ray diffraction. Six isolates grew on media containing selenate but no elemental selenium was formed. Mass balances showed that for three isolates total selenium was conserved, selenate decreased and selenium (0; II-) increased indicating the production of non-volatile organic selenium compounds. For two isolates both total selenium and selenate decreased with no increase in selenium (0; II-), therefore, loss of selenium occurred from the media. Selenium is immobile in anoxic reduced sediments but may become available to benthos and fish as a consequence of sediment oxidation associated with bioturbation leading to bioaccumulation and transformation by macrobenthos and bacteria. These mechanisms can be invoked as possible transport pathways to explain the presence of selenium above background concentrations in preindustrial sediments, but further work dating the sediments in which elevated concentrations of selenium are found is needed to confirm this hypothesis.
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(1999)Journal ArticleMeasurements of selenium in sediments and benthic infauna of Lake Macquarie, an estuary on the east coast of Australia, indicate that sediments are a significant source of selenium in the lake's food web. Analysis of surficial sediment samples indicated higher selenium con- concentrations near what are believed to be the main industrial sources of selenium to the lake: a smelter and a power station. Sediment cores taken from sediments in Mannering Bay, near a power station at Vales Point, contained an average of 12 times more selenium in surficial sections than sediment cores from Nord's Wharf, a part of the lake remote from direct inputs of selenium. The highest selenium concentration found in Mannering Bay sediments (17.2 mg/g) was 69 times the apparent background concentration at Nord's Wharf (0.25 mg/g). Pore water concentrations in Mannering Bay were also high, up to 5 mg/l compared to those at Nord's Wharf which were below detection limits (0.2 mg/l). Selenium concentrations in muscle tissues of three benthic-feeding fish species (Mugil cephalus, Platycephalus fuscus, Acanthopagrus australis) were significantly correlated ( p < 0:05) with surficial sediment selenium concentra- tion. Selenium concentrations in polychaetes and molluscs of Mannering Bay were up to 58 times higher than those from Nord's Wharf. Two benthic organisms, the eunicid polychaete Marphysa sanguinea and the bivalve mollusc Spisula trigonella, were maintained at different densities in selenium-spiked sediments. Both animals accumulated selenium from the spiked sediment, confirming that bioaccumulation from contaminated sediments occurs. Collectively, these data suggest that benthic food webs are important sources of selenium to the fish of Lake Macquarie.
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(1999)ThesisThis thesis studies the estimation of terrain elevations using interferometry synthetic aperture radar (InSAR). The procedure uses the difference of phase measurements of the received radar signals at two antennas separated slightly in the cross-track direction, to determine the elevations more accurately than stereo SAR mapping, which uses SAR intensity images. The SAR phases are more sensitive to terrain elevation variations than SAR intensities. It is also possible to perform the same measurements with only one antenna by deriving two images of the scene on two separate passes (repeat-pass interferometry). An interferometric image (interferogram) is formed by multiplying pixels from the reference image with the complex conjugate magnitudes of pixels from the second registered image. This thesis reviews the basic principles of SAR imaging. These are followed by a discussion of principles of InSAR and its application. The stages in the processing to determine height information using InSAR are also addressed. The parameters which affect the accuracy of elevations determined by InSAR are then discussed, including the correlation of pixels of corresponding areas on two images, and the effects of different parameters on the correlation. The mathematical expressions needed to calculate the height error budget are presented. A model is developed to calculate absolute terrain elevations incorporating ground control data. The computation procedure is based on stereo radargrammetric mapping of overlapping SAR images, incorporating expressions for elevations based on the fringe information in InSAR. The model develops a simultaneous least squares adjustment of all the measurements by radargrammetry and interferometric SAR, together with ground control, using condition and observation equations. In this case the basic measurements include range, unwrapped phase, and the ground coordinates of control points. The purpose of the least squares adjustment is to determine the most probable solution for the ground coordinates of points identified in the image. The algorithm has been tested with a pair of ERS-1 and ERS-2 images in tandem mode over Sydney. The algorithm shows good performance in achieving SAR elevation mapping, as compared with contours on an orthophoto map. Moreover, the model simultaneously, determined planimetry positions of the points. The errors are highly affected by quality of the phase correlation of the points, since it was observed that there is a very high correlation between the position and height accuracy and the level of the phase coherency.
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