Evaluation of the method of collecting suspended sediment from large rivers by discharge-weighted pumping and separation by continuous-flow centrifugation

A method for collecting suspended sediment samples has been developed that pumps a discharge-weighted volume of water from fixed depths at four to 40 locations across a river and separates the suspended sediment in the sample using a continuous-flow centrifuge. The efficacy of the method is evaluated by comparing the particle size distributions of sediment collected by the discharge-weighted pumping method with the particle size distributions of sediment collected by depth integration and separated by gravitational settling. The pumping method was found to undersample the suspended sand sized particles (> 63 μm) but to collect a representative sample of the suspended silt and clay sized particles (< 63 μm). The centrifuge separated the silt and clay sized particles (< 63 μm) into three fractions. Based on the average results of processing 17 samples from the Mississippi River and several of its large tributaries in 1990, about 10% of the silt and clay sized material was trapped in a centrifuge bowl-bottom sealing unit containing the nozzle and consisted of mostly medium and coarse silt from 16 to 63 μm. About 74% was retained on a Teflon liner in the centrifuge bowl and consisted of sizes from 0–1 to 63 μm. About 9% was discharged from the centrifuge in the effluent and was finer than 0–1 μm. About 7% was lost during the processes of removing the wet sediment fractions from the centrifuge, drying and weighing. The success of the discharge-weighted pumping method depends on how homogeneously the silt and clay sized particles (< 63 μm) are distributed in the vertical direction in the river. The degree of homogeneity depends on the composition and degree of aggregation of the suspended sediment particles.     

An overview of toxicant identification in sediments and dredged materials

The identification of toxicants affecting aquatic benthic systems is critical to sound assessment and management of our nation's waterways. Identification of toxicants can be useful in designing effective sediment remediation plans and reasonable options for sediment disposal. Knowledge of which contaminants affect benthic systems allows managers to link pollution to specific dischargers and prevent further release of toxicant(s). In addition, identification of major causes of toxicity in sediments may guide programs such as those developing environmental sediment guidelines and registering pesticides, while knowledge of the causes of toxicity which drive ecological changes such as shifts in benthic community structure would be useful in performing ecological risk assessments. To this end, the US Environmental Protection Agency has developed tools (toxicity identification and evaluation (TIE) methods) that allow investigators to characterize and identify chemicals causing acute toxicity in sediments and dredged materials. To date, most sediment TIEs have been performed on interstitial waters. Preliminary evidence from the use of interstitial water TIEs reveals certain patterns in causes of sediment toxicity. First, among all sediments tested, there is no one predominant cause of toxicity; metals, organics, and ammonia play approximately equal roles in causing toxicity. Second, within a single sediment there are multiple causes of toxicity detected; not just one chemical class is active. Third, the role of ammonia is very prominent in these interstitial waters. Finally, if sediments are divided into marine or freshwater, TIEs perforMed on interstitial waters from freshwater sediments indicate a variety of toxicants in fairly equal proportions, while TIEs performed on interstitial waters from marine sediments have identified only ammonia and organics as toxicants, with metals playing a minor role. Preliminary evidence from whole sediment TIEs indicates that organic compounds play a major role in the toxicity of marine sediments, with almost no evidence for either metal or ammonia toxicity. However, interpretation of these results may be skewed because only a small number of interstitial water (n = 13) and whole sediment (n = 5) TIEs have been completed. These trends may change as more data are collected.     

The clustering of K∼20 galaxies in 17 radio galaxy fields

We investigate the angular correlation function, ο(θ), of the galaxies detected in the 2.1-μm K ' band in 17 fields (101.5 arcmin² in total), each containing a z ∼1.1 radio galaxy. There is a significant detection of galaxy clustering at a limit of K ∼20, with a ο(θ) amplitude similar to that estimated by Carlberg et al. at K =21.5. The ο(θ) amplitudes of these K -limited samples are higher than expected from the faint galaxy clustering in the blue and red passbands, but consistent with a pure luminosity evolution model if clustering is stable (ε=0) and the correlation function of early-type galaxies is steeper than that of spirals.
We do not detect a significant cross-correlation between the radio galaxies and the other galaxies in these fields. The upper limits on the cross-correlation are consistent with a mean clustering environment of Abell class 0 for z ∼1.1 radio galaxies, similar to that observed for radio galaxies at z ∼0.5, but would argue against an Abell class 1 or richer environment. As Abell 0 clustering around the radio galaxies would not significantly increase the ο(θ) amplitude of galaxies in these fields, stable clustering with a steep ξ( r ) for E/S0 galaxies appears to remain the most likely interpretation of the ο(θ) amplitude.
At K ≤20, the number of galaxy–galaxy pairs of 2–3 arcsec separation exceeds the random expectation by a factor of 2.15±0.26. The excess of close pairs is comparable to that previously reported for R -band data, and consistent with a ∼(1+ z )² evolution of the galaxy merger rate.     

The radio properties of radio-quiet quasars

Although radio-quiet quasars (RQQs) constitute ≳ 90 per cent of optically identified quasar samples, their radio properties are only poorly understood. In this paper we present the results of a multi-frequency VLA study of 27 low-redshift RQQs. We detect radio emission from 20 objects, half of which are unresolved (≤ 0.24 arcsec). In cases where significant structure can be resolved, double, triple and linear radio sources on scales of a few kpc are found. The radio emission (typically) has a steep spectrum (α ∼ 0.7, where S  ∝ ν^−α), and high brightness temperatures ( T _B ≥ 10⁵ K) are measured in some of the radio components. The RQQs form a natural extension to the radio luminosity–absolute magnitude distribution of nearby Seyfert 1s. We conclude that a significant fraction of the radio emission in RQQs originates in a compact nuclear source directly associated with the quasar. There are no significant differences between the radio properties of RQQs with elliptical hosts and those in disc galaxies within the current sample.     

Organic carbon isotope ratios of recent sediments from coastal lagoons of the Gulf of Mexico,Mexico

The stable carbon isotope composition sedimentary organic carbon was determined in the sediments of seven coastal lagoons of the Gulf of Mexico, Mexico. For most of the lagoons the δ¹³C values for sediments ranged from ?20.1 to ?23.9%. Anomalously low values, ?26.8 to 29.3%. were determined in sediments of two of the studied lagoons, probably due to the presence of organic carbon from anthropogenic sources, naturally absent in these environments. The δ¹³C values determined in the tissues of oysters collected at the same time in the different lagoons were very similar to those recorded in the sediments.     

Serpentinization: a review

The common alteration assemblage produced by serpentinization of ultramafic rocks is: lizardite, chrysotile, magnetite±brucite±antigorite. Lizardite-chrysotile serpentinites are more common than antigorite; the presence of antigorite indicates that the serpentinite has undergone prograde metamorphism or that the periootite was serpentinized in a higher P,T regime than lizardite and chrysotile. The iron subsitution into serpentine minerals and brucite is a function of temperature at low f_O2, with increased temperature enhancing magnetite formation. The presence of awaruite and native Fe are strong evidence for a locally very reducing environment. Isotopic studies have shown a wide variety of origins for the fluids involved in serpentinization. The increased boron content of serpentinized rocks when compared to boron contents of the parent ultramafic body indicates a possible sea water origin for the fluids. Serpentinization takes place under both constant volume and constant chemical composition conditions. The factors in evaluating the importance of the two processes for an individual serpentinite are: (1) determination of the mineral assemblage and its paragenesis, (2) the structural and tectonic relationship of the ultramafic body to its country rock, (3) fluid access to the rock in duration and amount, and (4) timing of serpentinization - before, during or after emplacement into the crust.     

Efficacy of a multi-metric fish index as an analysis tool for the transitional fish component of the Water Framework Directive

The WFD has introduced an international commitment to assess the ecological status of transitional waters (TWs), within which fish communities are a key biological monitoring component. The Transitional Fish Classification Index (TFCI) outlined in this paper uses 10 ecological measures to analyse fish populations caught from various ecological niches using a variety of gear types within the Thames estuary. These reach and method-specific communities are then compared to a reference population created from a 'healthy' population from TWs of a similar type. The results indicate a progressive downstream increase the quality of fish communities, consistent with previous work; variation between methods can be accounted for by gear selectivity. Overall, the TFCI is an effective communication tool for converting ecological information into an easily understood format for managers, policy makers and the general public.