Imaging of water flow in porous media by magnetic resonance imaging microscopy

Magnetic resonance imaging (MRI) was used to study the flow of water in a column 14 mm in diameter packed with glass heads. The sample was fully saturated and water was pumped through the column using a peristaltic pump, at flow rates of 125 and 250 mL h(-1). This corresponds to mean velocities of 0.5 and 1 mm s(-1), given a porosity of 0.46 m3 m(-3). Nuclear magnetic resonance (NMR) images of the proton density and velocities within a 2-mm slice were taken at a spatial resolution of 0.15 x 0.15 x 2 mm3. At a mean pore water velocity of 1 mm s(-1) we approximated hydrodynamic dispersion using NMR-measured velocity distributions in a 2-mm slice through the sample. Additionally, we conducted a step pulse tracer experiment with chloride through the same column and at identical initial and boundary conditions. We fitted the convection-dispersion equation to the breakthrough curve and compared the column scale dispersion of the tracer experiment with the respective NMR estimate derived at the slice scale.     

Estimating PM2.5 over southern Sweden using space-borne optical measurements

In the present study Bremen aerosol retrieval (BAER) columnar aerosol optical thickness (AOT) data, according to moderate resolution imaging spectroradiometer (MODIS) and medium resolution imaging sensor (MERIS) level 1 calibrated satellite data, have been compared with AOT data obtained with the MODIS and MERIS retrieval algorithms (NASA and ESA, respectively) and by AErosol RObotic NETwork (AERONET). Relatively good agreement is found between these different instruments and algorithms. The R² and relative RMSD were 0.86 and 31% for MODIS when comparing with AERONET and 0.92 and 21% for MERIS. The aerosols investigated were influenced by low relative humidity. During this period, a relatively large range of aerosol loadings were detected; from continental background aerosol to particles emitted from agricultural fires. In this study, empirical relationships between BAER columnar AOT and ground-measured PM_2.5 have been estimated. Linear relationships, with R² values of 0.58 and 0.59, were obtained according to MERIS and MODIS data, respectively. The slopes of the regression of AOT versus PM_2.5 are lower than previous studies, but this could easily be explained by considering the effect of hygroscopic growth. The present AOT–PM_2.5 relationship has been applied on MERIS full resolution data over the urban area of Stockholm and the results have been compared with particle mass concentrations from dispersion model calculations. It seems that the satellite data with the 300 m resolution can resolve the expected increased concentrations due to emissions along the main highways close to the city. Significant uncertainties in the spatial distribution of PM_2.5 across land/ocean boundaries were particularly evident when analyzing the high resolution satellite data.     

Priority toxic pollutants in human urine: Their occurrence and analysis

This survey reviews and discusses the occurrence of priority pesticides and industrial chemicals in human urine. An overview of some recent analytical methodology for determination of selected toxic pollutants and their metabolites as they are found in human urine is also presented.     

Probabilistic indicators of atmospheric transport for regional monitoring and emergency preparedness systems

In this paper, following a methodology developed within the "Arctic Risk" Project of the Nordic Arctic Research Programme, several probabilistic indicators to evaluate the risk site possible impact on the geographical regions, territories, countries, counties, cities, etc., due to atmospheric transport from the risk site region were suggested. These indicators-maximum possible impact zone, maximum reaching distance, and typical transport time-were constructed by applying statistical methods and using a dataset of isentropic trajectories originated over the selected nuclear risk site (Ignalina nuclear power plant, Lithuania) during 1991-1996. For this site, the areas enclosed by isolines of the maximum possible impact zone and maximum reaching distance indicators are equal to 42 x 10(4) and 703 x 10(4) km(2), respectively. The maximum possible impact zone's boundaries are more extended in the southeast sector from the site and include, in particular, Latvia, Lithuania, Belarus, and several western regions of Russia. The maximum reaching distance's boundaries are twice more extended in the eastern direction from the site (reaching the Caspian Sea) compared with the western direction. The typical transport time to reach the southern territories of Sweden and Finland, northern regions of Ukraine, and northeast of Poland is 1 day. During this time, the atmospheric transport could typically occur over the Baltic States, Belarus, and western border regions of Russia, and central aquatoria of the Baltic Sea. Detailed analysis of temporal patterns for these indicators showed importance of the seasonal variability.     

Stakeholder involvement facilitates decision making for UK nuclear accident recovery

The importance of major stakeholders participating in the formulation of strategies for maintaining food safety and agricultural production following a nuclear accident has been successfully demonstrated by the UK 'Agriculture and Food Countermeasures Working Group' (AFCWG). The organisation, membership and terms of reference of the group are described. Details are given of the achievements of the AFCWG and its sub-groups, which include agreeing management options that would be included in a recovery handbook for decision-makers in the UK and tackling the disposal of large volumes of contaminated milk, potentially resulting from a nuclear accident.     

Impact of microbial activity on the hydraulic properties of fractured chalk

The impact of microbial activity on fractured chalk transmissivity was investigated on a laboratory scale. Long-term experiments were conducted on six fractured chalk cores (20 cm diameter, 23-44 cm long) containing a single natural fracture embedded in a porous matrix. Biodegradation experiments were conducted under various conditions, including several substrate and oxygen concentrations and flow rates. 2,4,6-Tribromophenol (TBP) was used as a model contaminant (substrate). TBP biodegradation efficiency depended mainly on the amount of oxygen. However, under constant oxygen concentration at the core inlet, elevating the flow rates increased the removal rate of TBP. Transmissivity reduction was clearly related to TBP removal rate, following an initial slow decline and a further sharp decrease with time. The fracture's transmissivity was reduced by as much as 97% relative to the initial value, with no leveling off of the clogging process. For the most extreme cases, reductions of 262 and 157 microm in the equivalent hydraulic apertures were recorded for fractures with initial apertures of 495 and 207 microm, respectively. The reductions in fracture transmissivity occurred primarily because of clogging by bacterial cells and extracellular polymeric substances (EPS) produced by the bacteria. Most of the biodegradation activity was concentrated near the fracture inlet, where the most suitable biodegradation conditions (nutrients and oxygen) prevailed, suggesting that the clogging had occurred in that vicinity. The clogging must have changed the structure of the fracture void, thereby reducing the active volume participating in flow and transport processes. This phenomenon caused accelerated transport of non-reactive tracers and doubled the fracture's dispersivity under constant flow rates.     

Die Wirkung von Photooxidantien auf Sommerweizen (Triticum aestivum L. Star)

The effect of short-time fumigation with the peroxidic photooxidants ozone, peroxyacetyl nitrate (PAN), and hydrogen peroxide (H₂O₂) on the gas exchange of wheat leaves was investigated. Two types of experiments are to be distinguished:

1. Fumigation is carried out at atmospheric CO₂ partial pressure (350 μbar) and the assimilation and transpiration rates are followed simultaneously.
2. The alteration of CO₂-response curves by fumigation is determined; from this, characteristic photosynthetic parameters can be calculated.

With this method, statements about the reaction of both photosynthesis as a whole and specific reaction sites within the most important photosynthetic processes are possible. Ozone in concentrations of 260–280 ppb causes a primary perturbation of the photosynthetic apparatus, whereby the carboxylation rate is affected to nearly the same extent as the electron-transport rate.PAN (120 ppb) causes a reduction in the assimilation rate, primarily due to the closing of the stomata. H₂O₂ affects the gas exchange only in concentrations of above 1 ppm, whereby the reduction of the assimilation rate represents an almost pure stomatal effect. All three fumigants cause an increase in the respiration rate, which may point to an activation of detoxification and repair mechanisms.