Multi-element geochemical analysis of snow samples: Use of discriminant analysis in interpreting the results.

This paper records the concentrations of major and trace elements determined from snow samples collected during a comprehensive survey undertaken in the Scottish Highlands during the winter and spring period of 1987. The configuration of calculated back-trajectories allowed the samples to be categorized into one of five geographical sectors. Discriminant analysis was used to check the validity of these calculations, to isolate potentially deviant samples, and to predict the possible source of one sample whose back-trajectory could not be computed with confidence. Limitations of the statistical method are discussed, but we conclude that the technique justifies more use by environmental scientists involved in the evaluation of data.

Comparison of episodic acidification in Canada, Europe and the United States.

Episodic acidification is practically a ubiquitous process in streams and drainage lakes in Canada, Europe and the United States. Depressions of pH are often smaller in systems with low pre-episode pH levels. Studies on European surface waters have reported episodes most frequently with minimum pH levels below 4.5. In Canada and the United States, studies have also reported a number of systems that have had minimum pH levels below 4.5. In all areas, change in water flowpath during hydrological events is a major determinant of episode characteristics. Episodic acidification is also controlled by a combination of other natural and anthropogenic factors. Base cation decreases are an important contributor to episodes in circumneutral streams and lakes. Sulphate pulses are generally important contributors to episodic acidification in Europe and Canada. Nitrate pulses are generally more important to episodic acidification in the Northeast United States. Increases in organic acids contribute to episodes in some streams in all areas. The sea-salt effect is important in near-coastal streams and lakes. In Canada, Europe and the United States, acidic deposition has increased the severity (minimum pH reached) of episodes in some streams and lakes.

Modelling episodic acidification of surface waters: the state of science.

Field studies of chemical changes in surface waters associated with rainfall and snowmelt events have provided evidence of episodic acidification of lakes and streams in Europe and North America. Modelling these chemical changes is particularly challenging because of the variability associated with hydrological transport and chemical transformation processes in catchments. This paper provides a review of mathematical models that have been applied to the problem of episodic acidification. Several empirical approaches, including regression models, mixing models and time series models, support a strong hydrological interpretation of episodic acidification. Regional application of several models has suggested that acidic episodes (in which the acid neutralizing capacity becomes negative) are relatively common in surface waters in several regions of the US that receive acid deposition. Results from physically based models have suggested a lack of understanding of hydrological flowpaths, hydraulic residence times and biogeochemical reactions, particularly those involving aluminum. The ability to better predict episodic chemical responses of surface waters is thus dependent upon elucidation of these and other physical and chemical processes.

Towards an assessment of the influence of climate on wet acidic deposition in Europe.

The EMEP precipitation composition network is used to examine relationships between non-marine SO4(2-), NO3-, NH4+, H+ concentrations and precipitation amount and a local zonal pressure index (an index of the atmospheric circulation). The pattern of the relationships changes across Europe with the zonal pressure gradient explaining more of the variance in ion concentrations in the west, and precipitation amount explaining relatively more of the variance in the east. There is some predictive capability for precipitation composition in the zonal pressure gradient for restricted regions in Europe; R2 values are up to 40% on a daily basis but in some seasons/months attain >60%. The zonal pressure gradient is an index which appears to include pertinent information on transport and wet removal. Preliminary analysis indicates that this approach can be useful in assessing the contributions of changing atmospheric circulation to time-trends of wet acidic deposition in an area stretching from the UK over the North Sea to Denmark. The zonal pressure gradient is known to have varied on time-scales of decades, and the simple index may be one appropriate approach to assessing future deposition patterns from future climate projections.

A field study of the generation of nitrate in a hill cap cloud.

A field experiment to investigate the formation of nitrate as an airstream passes through a hill cap cloud has been performed at the UMIST field station on Great Dun Fell. It has been shown that the aerosol nitrate concentration increased by about 0.5 microg m(-3) as the airstream passed through the cloud during the night. At sunrise the nitrate production disappeared. It is suggested that the most likely mechanism for this nitrate production was due to the solution of N2O5 and NO3 formed from the reaction of NO2 with O3. These higher oxides build up overnight in the absence of short wave radiation to photolyse them. Other possible mechanisms of nitrate production are also discussed.

The trace element composition of stream- and melt-waters at times of the spring-thaw in the Scottish Highlands.

Fresh snow and streamwater samples were collected on a daily basis throughout the winter and spring periods of 1984 and 1985 at a remote, upland catchment located within the Cairngorm Mountains, Scotland. Laboratory based partial-melt experiments undertaken on the snow samples demonstrated that both fractionation and preferential elution of trace-elements occur during melting, with the concentrations being 1.3 to 5.4 times greater than in the first 10 percent meltwater fractions than in the bulk snow (Abrahamset al., in press). At the onset of snowpack melting, the ions may be mobilised and redistributed within the snow profile, concentrating at depths from where they may be quickly removed during the early spring run-offf. The raised major- (Ca, Mg, Na, Cl, NO3 and SO4) and trace-element (Al, Cd, Cu, Fe, Mn and Pb) concentrations recorded in the streamwaters during the "acid-flush" episodes at the time of the first major periods of snow-melt, reflect both the meltwater composition and the influence of the catchment soils (Abrahamset al., submitted for publication). Differences in streamwater chemistry during the two periods of snow-melt which were studies can probably be related to the fact that snow-melt occurs under a variety of circumstances with significant variations in the sequence of precipitation, melt-events, temperature and snowcover occurring from year to year, even in the same catchment. The high concentrations of Al (up to 330 µg L(-1)) in the sireamwaters at the time of snow-melt, probably reflect leaching of this element from the soil. These elevated concentrations, in combination with other streamwater parameters, may prove toxic to aquatic life-forms at this time of year.

The effect of polluted and leached snow melt waters on the soil bacterial community-quantitative response.

The effect of polluted snow melt waters on the number of soil bacteria was determined using soil cores extracted from an upland catchment in the Cairngorm Mountains, Scotland. Total numbers of viable heterotrophic bacteria and bacterial denitrifiers were determined using plate and MPN counts. Separate soil cores were treated with simulated melt waters representative of either the composition of the first melt fraction from polluted or leached snowpacks. The number of bacteria in the Ah soil horizon (Hodgson, 1974) treated with polluted snow melt (PSM) water decreased significantly by 28-fold, but increased by 11-fold in the BC horizon. Denitrifier numbers decreased by 8-fold in the Ah horizon, but increased by over 2-fold lower down the profile. Overall the bacterial community exposed to simulated leached snow melt (LSM) waters showed little change in the Ah horizon. In the BC horizon (Hodgson, 1974), total viable bacterial numbers decreased by 20-fold, but denitrifiers numbers were unaffected.

Leukocyte-depleted blood. A comparison of filtration techniques.

Febrile transfusion reactions due to leukoagglutinins are commonly seen in multitransfused patients. It has been suggested that reduction of the number of leukocytes per transfusion to 0.5 x 10(9) or less would prevent nonhemolytic febrile transfusion reactions in the majority of patients. Therefore, we have compared the ability of several filters to reduce the leukocyte content of stored whole blood drawn into citrate-phosphate-dextrose. The leukocyte absorption filters, Imugard IG500 and Erypur, produced 98.3 +/- 2.4 and 98.3 +/- 1.7 percent leukocyte depletion, respectively. The leukocyte adherence filter, Leuko-Pak, produced a depletion of 54.6 +/- 14.7 percent. The microaggregate filters, Biotest MF10B Microfiltration set, Ultipor Blood Transfusion Filter SQ40S, and Travenol 20 micron High Capacity Transfusion Filter, produced 37.5 +/- 10.8, 13.3 +/- 11.2, and 44.5 +/- 11.3 percent leukocyte depletion, respectively. It would appear that of the filters tested, the Imugard IG500 and the Erypur are the only filters which can invariably reduce the absolute number of leukocytes in a unit of stored whole blood to below 0.5 x 10(9).

Leucocyte-depleted blood: a comparison of cell-washing techniques.

Non-haemolytic febrile transfusion reactions are common in multi-transfused patients. It is generally accepted that the majority of these reactions occur when more than 0.5 x 10(9) leucocytes are transfused. Values equal to or lower than this threshold, which we have called for convenience the critical antigenic load for leucocytes (CALL), can be achieved by decreasing the leucocyte content in one unit of whole stored blood by about 80%. We have compared the efficiency of leucocyte depletion of whole stored blood, using fixed-speed centrifugation (Haemonetics Model 15 and Model 115 cell washer), variable-speed centrifugation (Dideco Progress 90 Cell Separator), and serial centrifugation (IBM 2991 Blood Cell Processor. Fixed-speed centrifugation was least effective in depletion leucocytes; a reduction of 25 and 60% was achieved using the Haemonetics Model 15 and Model 115, respectively. Variable speed and serial centrifugation produced more satisfactory results with leucocyte depletions of 82 and 89% using the Dideco Progress 90 Cell Separator and the IBM 2991 Blood Cell Processor, respectively. Platelet depletion of over 90% was achieved with all the cell washers, except the Haemonetics Model 15. Red cell losses varied from 3 to 30%. It seems unlikely that consistently high (over 90%) leucocyte depletion can be obtained using cell-washing techniques without associated high red cell losses.

Anaesthesia for injection of bleeding oesophageal varices.

Patients with haemorrhage from oesophageal varices associated with portal hypertension are poor risks for anaesthesia and surgery. One method of controlling such haemorrhage is injection of the oesophageal varices (sclero-therapy) via an oesophagoscope. Careful preoperative preparation and use of the Sengstaken-Blakemore tube in combination with the anaesthetic technique of intermittent Althesin and suxamethonium with artificial ventilation with nitrous oxide and oxygen enables sclerotherapy to be carried out successfully.