Flow characteristics of a gravity current induced by differential cooling in a small lake

Extensive field data of a density current driven by the differences in the cooling rates between the two lobes of a small lake are presented. These data illustrate the fact that this gravitational motion affects the whole system and dominates its lakewide circulation. Moreover, data are used to evaluate the entrainment into the density current and also to discuss the regime of the flow from a scaling analysis of the horizontal momentum equations. This analysis is specifically applied to the central area of the lake, where the density current flows from one lobe to the other. Results of this study show that in the longitudinal direction there is a balance between the pressure gradient and the turbulent viscous term. Further, it is found that geostrophy correctly describes the transversal pattern of the passage of the gravity current from the northern lobe to the southern, where it flows confined to the western shoreline.     

The ratio between chrysophycean cysts and diatoms in temperate,mountain lakes: some recommendations for its use in paleolimnology

The ratio between chrysophycean cysts and diatom valves (CD ratio) in lake sediments has been suggested as a useful indicator of changing trophic state conditions in oligotrophic lakes. Other environmental factors, however, may influence the CD ratio because chrysophycean cysts usually reflect conditions in the planktonic environment and diatoms reflect benthic conditions. We investigated the CD ratio in 76 mountain lakes in the Pyrenees to determine the environmental drivers that influence the ratio and assess its value for paleoenvironmental inference. The lakes surveyed included a broad range with respect to bedrock type, altitude and surface area, characteristics that cover much of the variability that can be found in cold, oligotrophic mountain lakes. Lake depth and Ca²⁺ concentration explain most of the variation in the CD ratio. Trophic state factors (e.g. total phosphorus, TP) play a secondary role. As a predictor, CD ratio performs primarily as a lake depth indicator. The predictive models can be improved if trophic state (i.e. TP) and chemical conditions (Ca²⁺) are known or can be estimated independently. Use of the CD ratio for inferring Ca²⁺ oscillations only makes sense in lakes with Ca²⁺ <200 µeq/L or in those that oscillate below and above this threshold through time. Other interpretations of the CD ratio (e.g. lake trophic state changes, ice-cover duration) make sense if complementary paleolimnological evidence indicates that neither water depth nor Ca²⁺ concentration changed significantly. Indeed, paleolimnological interpretation of the CD ratio requires considering the particular characteristics of the lake and may vary depending on the temporal scale considered. This study provides some guidelines for evaluating critically the use of the CD ratio.     

Influence of chemistry on the flux-gradient relationships for the NO-O3-NO2 system

It is shown that K-theory has to be modified for chemical systems that react with time scales similar to the turbulence time scale. In such systems, the value of the exchange coefficient depends not only on the turbulence parameters, but also on the chemical reaction rates. As an example, the NO-O₃-NO₂ chemical system is studied. Using second-moment equations, new flux-gradient relationships for the neutral atmospheric surface layer are obtained which depend on the time scale ratios of turbulence ( _t ) and chemical reactions ( _ch), i.e., reactive K-theory. Within the framework of this reactive K-theory, the flux of a chemical species is both a function of the concentration gradients of the three chemical species involved and of the ratio of the time scale of turbulence to the time scale of chemistry. In the special case of slow chemistry ( _{t ch}) inert K-theory is applicable.The reactive exchange coefficients are implemented in a surface-layer model that calculates the flux and concentration profiles of the three chemical species. The results of the calculations of the effective exchange coefficients are different for reactive K-theory and inert K-theory; the differences are largest for nitric oxide, but smaller for ozone and nitrogen dioxide.     

Identification of groundwater quality trends in a chalk aquifer threatened by intensive agriculture in Belgium

The European Union (EU) has adopted directives requiring that Member States take measures to reach a “good” chemical status of water resources by the year 2015 (Water Framework Directive: WFD). In order to achieve the environmental objectives for groundwater, the identification and reversal of significant upward trends in pollutant concentrations are required. A very detailed dataset available for the Hesbaye chalk aquifer in Belgium is used to evaluate tools and to propose efficient methodologies for identifying and quantifying nitrate trends in groundwater. Results indicate that the parametric linear regression and the non-parametric Mann-Kendall tests are robust; however, the latter test seems more adequate as it does not require verification of the normality of the dataset and it provides calculated nitrate trends very comparable to those obtained using linear regression. From a hydrogeological point of view, results highlight a general upward trend in the whole groundwater basin. The extrapolation of the trend analysis results indicates that measures have to be taken urgently in order to avoid further major degradation of groundwater quality within the next 10–70 years. However, a good groundwater quality status cannot be expected in the Hesbaye aquifer for the 2015 EU WFD deadline.     

Preuves de la discordance de l'Ordovicien supérieur dans la zone axiale des Pyrénées : exemple du dôme de la Garonne (Espagne,France)

The geological mapping of the Garonne Dome had already shown an angular unconformity between the Upper Ordovician and the underlying series. This paper presents data from a nearby location, Planelh deth Pas Estret, southeast of the Garonne Dome, where the unconformity can be seen at outcrop scale. The unconformity angles amount to up to 20°. This unconformity is interpreted as a result of an extensional event of pre-Upper Ordovician age. To cite this article: J. Garc??a-Sansegundo et al., C. R. Geoscience 336 (2004).     

Free allocation rules in the EU emissions trading system: what does the empirical literature show?

This paper analyses the rules for free allocation in the EU Emissions Trading System (EU ETS). The analysis draws on the empirical evidence emerging from two literature strands. One group of studies sheds light on the following questions: how efficient are free allocation rules in minimizing the risk of carbon leakage? Have they become more efficient over the trading periods? What are the technical limits to making them more efficient? Further: is firm behaviour affected by allowance allocation? Did specific provisions induce strategic behaviour with unintended effects? Studies from the second group estimate sectoral pass-through rates for the costs imposed by the EU ETS. Taking cost pass-through into account is necessary for properly targeting free allocation. The difficulty of accurately quantifying sectoral differences in cost pass-through ability, especially in manufacturing sectors (due to limited data availability and market heterogeneity), is the main hindrance to achieving further efficiency in allowance allocation. The new rules defined in the reform for Phase IV (2021–2030) nevertheless make some progress in this direction.

Key policy insights

• The difficulty of accurately quantifying sectoral differences in cost pass-through ability is the main hindrance to efficient free allocation in minimizing carbon leakage risk.

• In Phase IV (2021–2030), carbon leakage risk will be assessed more accurately thanks to: a) carbon intensity and trade intensity considered together through a combined indicator; b) possible use of more disaggregated data, and c) possible consideration of complementary qualitative assessments of abatement potential, market characteristics and profit margins.

• It is expected that benchmarked allocation introduced in Phase III (2013–2020) has induced additional emission abatement, but there is still a lack of empirical evidence.

     

Integrated modelling of Polycyclic Aromatic Hydrocarbons in the marine environment: Coupling of hydrodynamic, fate and transport, bioaccumulation and planktonic food-web models

Spatio-temporal variability of pollutants in the environment is a complex phenomenon that requires a combined approach for its analysis. Whereas data on measured levels of contaminants in various environmental compartments is essential, it is not always possible to monitor at the necessary frequency and with the adequate spatial sampling distribution to capture this variability. Therefore a modelling approach able to complement experimental data and close the gaps in the monitoring programs is useful for assessing the contaminant dynamics occurring at different time scales. In this work a 1D water column fate model has been developed and tested for Polycyclic Aromatic Hydrocarbons (PAHs). The model has been coupled with a simple ecological model that includes a bioaccumulation module. Afterwards, the model has been used to study the temporal variability of contaminant concentrations as well as the fluxes between compartments. The results evidence the complex coupling between spatio-temporal scales and its influence on environmental concentration levels.