A simple method for calculating growth rates of petroleum hydrocarbon plumes

Consumption of aquifer Fe(III) during biodegradation of ground water contaminants may result in expansion of a contaminant plume, changing the outlook for monitored natural attenuation. Data from two research sites contaminated with petroleum hydrocarbons show that toluene and xylenes degrade under methanogenic conditions, but the benzene and ethylbenzene plumes grow as aquifer Fe(III) supplies are depleted. By considering a one-dimensional reaction front in a constant unidirectional flow field, it is possible to derive a simple expression for the growth rate of a benzene plume. The method balances the mass flux of benzene with the Fe(III) content of the aquifer, assuming that the biodegradation reaction is instantaneous. The resulting expression shows that the benzene front migration is retarded relative to the ground water velocity by a factor that depends on the concentrations of hydrocarbon and bioavailable Fe(III). The method provides good agreement with benzene plumes at a crude oil study site in Minnesota and a gasoline site in South Carolina. Compared to the South Carolina site, the Minnesota site has 25% higher benzene flux but eight times the Fe(III), leading to about one-sixth the expansion rate. Although it was developed for benzene, toluene, ethylbenzene, and xylenes, the growth-rate estimation method may have applications to contaminant plumes from other persistent contaminant sources.     

Higher-mode force response in multi-story strongback-braced frames

Strongback-braced frames employ an essentially elastic steel truss, or strongback, that distributes demands more uniformly to delay or prevent story mechanisms. Because inertial forces are no longer limited by the formation of a story mechanism, strongback-braced frames can exhibit large elastic force demands, particularly in the higher modes. This paper characterizes the higher-mode force response of strongback-braced frames. Four-story archetypes were designed using nonlinear dynamic analyses to incorporate higher-mode force demands into the design process. The response of the archetypes was compared with that of reference buckling-restrained braced frames that were allowed to form story mechanisms. The force demands in the strongback were then described using equivalent-static forces to represent the inertial forces induced by the higher modes. Force demands in the strongback arise from a yielding first-mode ‘pivoting’ and elastic higher-mode ‘bending’ response. These higher-mode force demands are elastic, ill-constrained by the strength of the yield mechanism, and depend significantly on the choice of ground motion record used for the analysis. In remaining elastic in the higher modes, the strongback distributes demands more uniformly and mitigates the formation of story mechanisms. Consequently, design and analysis methods for strongback-braced frames need to include estimates for these near-elastic higher-mode force demands.     

Quality and timing of crowd-based water level class observations

Crowd-based hydrological observations can supplement existing monitoring networks and allow data collection in regions where otherwise no data would be available. In the citizen science project CrowdWater, repeated water level observations using a virtual staff gauge approach result in time series of water level classes (WL-classes). To investigate the quality of these observations, we compared the WL-class data with “real” (i.e., measured) water levels from the same stream at a nearby gauging station. We did this for nine locations where citizen scientists reported multiple observations using a smartphone app and at 12 locations where signposts were set up to ask citizens to record observations on a paper form that could be left in a letterbox. The results indicate that the quality of the data collected with the app was better than for the forms. A possible explanation is that for each app location, a single person submitted the vast majority of the observations, whereas at the locations of the forms almost every observation was made by a different person. On average, there were more contributions between May and September than during the other months. Observations were submitted for a range of flow conditions, with a higher fraction of high flow observations for the locations were data were collected with the app. Overall, the results are encouraging for citizen science approaches in hydrology and demonstrate that the smartphone application and the virtual staff gauge are a promising approach for crowd-based water level class observations.     

Effets combinés de la pêche et de l'eutrophisation sur la perche (Perca fluviatilis L.) dans les eaux vaudoises du Léman

Catches of perch have strongly decreased in Lake of Geneva from 1975 to 1981. This decrease was attributed to the impact of overfishing, enhanced by the accelerated growth of perch. Therefore, too many perches were captured before their first reproduction and the stocks of reproducers were strongly reduced. Furthermore, adverse meteorological conditions occurred probably during several periods of reproduction. The accelerated growth of perch was attributed to the combined effects of increasing eutrophication (more food available) and of overfishing (decreased perch density in the shoals).      

The control of the production process of phytoplankton by the physical structure of the aquatic environment with special reference to its optical properties

This tutorial was designed for nonbiologists requiring an introduction to the nature and general timescales of phytoplankton responses to physical forcing in aquatic environments. As such, an effort was made to highlight biological markers which might assist in identifying, measuring and/or validating physical processes controlling the variability in the distribution, abundance, composition and activity of phytoplankton communities. Given the recent advances in environmental optics and remote sensing capabilities, a special emphasis was placed on the nature and utility of phytoplankton optical properties in current bio-optical modelling efforts to predict temporal and spatial variability in phytoplankton productivity and growth.     

Geology of the Victor Kimberlite, Attawapiskat, Northern Ontario, Canada: cross-cutting and nested craters

The pipe shapes, infill and emplacement processes of the Attawapiskat kimberlites, including Victor, contrast with most of the southern African kimberlite pipes. The Attawapiskat kimberlite pipes are formed by an overall two-stage process of (1) pipe excavation without the development of a diatreme (sensu stricto) and (2) subsequent pipe infilling. The Victor kimberlite comprises two adjacent but separate pipes, Victor South and Victor North. The pipes are infilled with two contrasting textural types of kimberlite: pyroclastic and hypabyssal-like kimberlite. Victor South and much of Victor North are composed of pyroclastic spinel carbonate kimberlites, the main features of which are similar: clast-supported, discrete macrocrystal and phenocrystal olivine grains, pyroclastic juvenile lapilli, mantle-derived xenocrysts and minor country rock xenoliths are set in serpentine and carbonate matrices. These partly bedded, juvenile lapilli-bearing olivine tuffs appear to have been formed by subaerial fire-fountaining airfall processes.

The Victor South pipe has a simple bowl-like shape that flares from just below the basal sandstone of the sediments that overlie the basement. The sandstone is a known aquifer, suggesting that the crater excavation process was possibly phreatomagmatic. In contrast, the pipe shape and internal geology of Victor North are more complex. The northwestern part of the pipe is dominated by dark competent rocks, which resemble fresh hypabyssal kimberlite, but have unusual textures and are closely associated with pyroclastic juvenile lapilli tuffs and country rock breccias±volcaniclastic kimberlite. Current evidence suggests that the hypabyssal-like kimberlite is, in fact, not intrusive and that the northwestern part of Victor North represents an early-formed crater infilled with contrasting extrusive kimberlites and associated breccias. The remaining, main part of Victor North consists of two macroscopically similar, but petrographically distinct, pyroclastic kimberlites that have contrasting macrodiamond sample grades. The juvenile lapilli of each pyroclastic kimberlite can be distinguished only microscopically. The nature and relative modal proportion of primary olivine phenocrysts in the juvenile lapilli are different, indicating that they derive from different magma pulses, or phases of kimberlite, and thus represent separate eruptions. The initial excavation of a crater cross-cutting the earlier northwestern crater was followed by emplacement of phase (i), a low-grade olivine phenocryst-rich pyroclastic kimberlite, and the subsequent eruption of phase (ii), a high-grade olivine phenocryst-poor pyroclastic kimberlite, as two separate vents nested within the original phase (i) crater. The second eruption was accompanied by the formation of an intermediate mixed zone with moderate grade. Thus, the final pyroclastic pipe infill of the main part of the Victor North pipe appears to consist of at least three geological/macrodiamond grade zones.

In conclusion, the Victor kimberlite was formed by several eruptive events resulting in adjacent and cross-cutting craters that were infilled with either pyroclastic kimberlite or hypabyssal-like kimberlite, which is now interpreted to be of probable extrusive origin. Within the pyroclastic kimberlites of Victor North, there are two nested vents, a feature seldom documented in kimberlites elsewhere. This study highlights the meaningful role of kimberlite petrography in the evaluation of diamond deposits and provides further insight into kimberlite emplacement and volcanism.     

A Late‐glacial chironomid record from Hawes Water,northwest England

This paper presents the results of a high‐resolution Late‐glacial chironomid stratigraphy from Hawes Water, a small carbonate lake in northern Lancashire. The samples were from a core taken from the terrestrialised margin of the present lake, which represents an intermediate depth between the true littoral and the profundal. The chironomid assemblage showed a high degree of sensitivity to both broad‐scale and short‐term temperature changes. Comparison with an existing proxy temperature record (δ¹⁸O) for the site confirmed the presence of four temperature inversions within the Late‐glacial Interstadial. A mean July air temperature inference model, derived from acid, soft‐water lakes in Norway and Svalbard, was applied to the data. Despite the absence of carbonate lakes within the Norwegian training set, there was a close similarity between trends in estimated July air temperature and the δ¹⁸O trace, with a particularly strong correspondence in the periods of clay deposition. This suggests that this model is highly robust. The inferred maximum Interstadial temperature was 13.4°C, dropping initially to 7.5°C in the Loch Lomond Stadial. Temperatures reach a maximum of nearly 10°C in this period, cool for a short period before rising rapidly to 13.2°C at the start of the Holocene. These temperatures are similar to but slightly higher than those estimated for Whitrig Bog, southeast Scotland, and lower than those inferred from coleopteran‐based models for sites in South Wales. Copyright © 2004 John Wiley & Sons, Ltd.