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.     

A four-year validation study of an assessment center in a financial corporation

Ratings on 18 assessment dimensions formulated independently by assessors (psychologists and high-level managers), consensus ratings on the same dimensions, and overall assessment ratings were collected for 329 assessees. Two hundred and seventy-four of the assessees were promoted based on their scores obtained in the assessment process. For 240 of these we succeeded in obtaining ratings on two criterion measures over a period of four years. These criterion data were provided by their superiors who were unaware of the scores obtained in the assessment process. Multiple regression analyses were run on the data from all the assessment ratings and criterion measures. The results demonstrated a long-term validity for the assessment center. However, we were not able to discern a consistent pattern of statistically significant differences between the predictive validities of high-level managers and psychologists as assessors. Possibly, the type of criterion measure moderates the predictive validity of managers' and psychologists' ratings. Further, theoretical and practical implications of the findings are discussed.     

Two-dimensional distribution of microbial activity and flow patterns within naturally fractured chalk

The two-dimensional distribution of flow patterns and their dynamic change due to microbial activity were investigated in naturally fractured chalk cores. Long-term biodegradation experiments were conducted in two cores ( approximately 20 cm diameter, 31 and 44 cm long), intersected by a natural fracture. 2,4,6-tribromophenol (TBP) was used as a model contaminant and as the sole carbon source for aerobic microbial activity. The transmissivity of the fractures was continuously reduced due to biomass accumulation in the fracture concurrent with TBP biodegradation. From multi-tracer experiments conducted prior to and following the microbial activity, it was found that biomass accumulation causes redistribution of the preferential flow channels. Zones of slow flow near the fracture inlet were clogged, thus further diverting the flow through zones of fast flow, which were also partially clogged. Quantitative evaluation of biodegradation and bacterial counts supported the results of the multi-tracer tests, indicating that most of the bacterial activity occurs close to the inlet. The changing flow patterns, which control the nutrient supply, resulted in variations in the concentrations of the chemical constituents (TBP, bromide and oxygen), used as indicators of biodegradation.     

Why is latex usually white and only sometimes yellow,orange or red? Simultaneous visual and chemical plant defense

Latex is a toxic and sticky defense substance produced by about 20,000 plant species and secreted following wounding. It defends the wounded plants by both chemical and physical components. A striking fact about latex is that the majority of plants produce it white, although yellow, orange, and red latex can sometimes be found. Theoretically, it is possible that there is no function or importance to latex’s color, but it seems unlikely. It is also possible that there are chemical constraints that dictate the production of white latex, but the various colors indicate that this is not a valid explanation. However, since white latex evolved independently many times in various plant taxa, there should have been a strong selection for latex being white, a possibility that has never been addressed, to my knowledge. I propose that latex is generally white because white is the best solution for visual aposematism under the lighted conditions within and under plant canopies, on the background of typical leaf and bark color, even towards color-blind animals. Yellow, orange, and red latex types should naturally also be considered as aposematic because these are typical colors of toxic and aposematic organisms. In addition, latex-exuding plants simultaneously use a chemically based, non-visual olfactory aposematism because wounding perse also causes the emission of non-latex defensive volatiles and because latex contains various but little-studied defensive volatiles. The volatile defensive aspect of latex exudation may also operate against nocturnal or blind herbivores.     

Single umbilical artery stenosis associated with intrauterine fetal death post-transfusion

Single umbilical artery is among the most common funicular vascular anomalies. In contrast, umbilical artery stenosis is rare, and has only been reported in three-vessel cords. We describe a case of single umbilical artery stenosis in a fetus with no associated malformations. Intrauterine fetal death occurred at 28weeks' gestation following cordocentesis and intravascular transfusion for Rhesus alloimmunization. Single umbilical artery stenosis may place the fetus at increased risk, particularly in cases requiring interventions involving cord manipulation. Copyright © 2002 John Wiley & Sons, Ltd.     

Evaluation of soil flushing potential for clean-up of desert soil contaminated by industrial wastewater

The flushing potential of a desert loess soil contaminated by the flame retardant Tetrabromobisphenol A (TBBPA), chloride (Cl(-)) and bromide (Br(-)) was studied in undisturbed laboratory column experiments (20 cm diameter, 45 cm long) and a small field plot (2 x 2 m). While the soluble inorganic ions (Cl(-) and Br(-)) were efficiently flushed from the soil profile after less than three pore volumes (PV) of water, about 50% of the initial amount of TBBPA in the soil was also flushed, despite its hydrophobic nature. TBBPA leaching was made possible due to a significant increase in the pH of the soil solution from 7.5 to 9, which increased TBBPA aqueous solubility. The remaining TBBPA mass in the soil was not mobilized from its initial location in the topsoil due to the decrease in pH at this horizon. In situ soil flushing demonstrated that this method is a feasible treatment for reducing soil contamination at this site.