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.     

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.     

Observations of H2O2 and PAN in a rural atmosphere

Measurements of gaseous H2O2 and peroxyacetyl nitrate (PAN) concentrations in air are given for a site in rural southern England over an approximately 4-year period. In both cases the data show both diurnal and seasonal patterns. Temperature and wind direction had clear influences on the recorded concentrations of both species. There was an indication of increasing H2O2 concentrations with time. The use of a two-dimensional global model confirmed that this would be expected to occur alongside growth in ozone formation. It is suggested that, during photochemical episodes, the sequential build-up of ozone, PAN and H2O2 could be important in influencing the ability of vegetation to resist the effects of aggressive pollutants.     

Cadmium availability to the cyanobacterium Synechocystis aquatilis in solutions containing chloride

Availability of cadmium to Synechocystis aquatilis (estimated by 109Cd sorption and cadmium toxicity-14C method) in solutions containing cadmium and complexing (KCl) or non-complexing (KNO3) salts, in the range of 0-0.5 m was investigated. Both cadmium surface adsorption and transport into the cells were lower in solutions containing cadmium chloride complexes (CdCl+, CdCl2, CdCl3-) than in those containing cadmium in the form of Cd2+. Also, cadmium toxicity in solutions of higher KCl concentrations, in which CdCl+ and CdCl2 forms predominated, was significantly limited.     

The effect of temperature and salinity on copper body-burden and copper toxicity to Hediste (Nereis) diversicolor

Temperature from 12 to 22°C and salinity from 30.5 to 7.6 increased accumulation of copper in Hediste diversicolor. Copper accumulated ranged from 85.83 to 217.14 g g^-1. Sediments reduced accumulation of copper under temperature-salinity combinations. Accumulated copper ranged from 90.19 to 153.26 g g^-1.However, mortality of the worms was not solely dependent upon copper body-burden. It ranged from 34 to 45% and from 38 to 80% in the presence of sediment. A combination of osmoregulatory and thermal stresses increased the toxic effect of copper to the worms.     

The effects of salinity,temperature and sediment on the toxicity of copper to juvenile Hediste (Nereis) diversicolor (O. F. Muller)

Without sediment, increasing salinity (7.3 to 29.2) and increasing temperature (12 to 22 °C) reduced the toxicity of copper to juvenile Hediste diversicolor. The LC₅₀ values ranged from 357 gL^-1 in 7.3 to 513 g L^-1 in 29.2 at 12°C and from 247 to 500 g L^-1 at 22°C. In deionized water all the juvenile were dead in all solutions to which copper was added (100 to 600 g L^-1). Dead worms were swollen and everted their pharynxs. In higher doses of copper (500 to 600 g L^-1) the worms were abnormal in behaviour in all salinities (0 to 29.2). The ability to swim or crawl was disturbed.With sediments increasing temperature and increasing salinity increased the toxicity of copper to the worms. The LC₅₀ values ranged from 3200 to 4100 g L^-1 at 22°C. The response of the juvenile to copper was antagonistic to increasing temperature and salinity and synergistic to increasing salinity and increasing temperature without and with the sediment respectively.     

Hunger, poverty and food aid in Sub-Saharan Africa: Retrospect and potential

Frustrations and failures will continue to mount if we do not immediately summon the courage to revise the ways we think and take action-as well as maintaining essential services to support life and health … Saving hundreds of thousands … who are at risk of dying from malnutrition or infection is an immediate imperative. But it must be only one stage in the progress toward other activities, and one element in the truly comprehensive approach… The main intent of this paper is to explore aspects of the nature and evolution of poverty in Sub-Saharan Africa with special reference to food and hunger and their interaction with macro-economic policy. An attempt is made to outline the overall context within which food aid needs to be used in order to have a greater developmental impact. Following an Introductory Section I, Section II provides a sketch map of how recession and lagging food production - with cyclical weather crises superimposed - affect the human condition of poor people. Section III reviews die interaction between "standard" IMF stabilization and World Bank structural adjustment strategies and poverty/hunger. Improvements in the design of stabilization and adjustment programmes, and of the inter-relationship with them of emergency programmes, can be identified. A number turn on the broadened and more innovative or catalytic use of food aid. Section IV reviews aspects of facing a continuing series of emergencies and of designing life support programmes to facilitate rehabilitation of the households directly affected as well as of the national economies. From this base it explores a series of elements in achieving renewed development. The concluding Section seeks to explore the strengths, limitations and potentials of food aid in the context set by the previous sections. The standard criticisms of food aid appear to be overstated and/or to relate to particular modalities or approaches rather than to anything intrinsic. A number of criteria for improving the effectiveness of food aid - especially in respect to rehabilitation, recovery and renewed development - are set out.