Acid deposition and acidification of waters in South Africa: status and prognoses given future climate change

Acid deposition can lead to acidification of freshwaters and damage to fish and other biota. Norway has suffered severely from acidification due to emissions of sulphur and nitrogen pollutants in Europe, with long-range transport through the atmosphere and deposition in remote natural areas. South Africa is currently facing potentially similar challenges. The main source of electricity is coal fired power plants, emitting sulphur and nitrogen, and with an increasing energy demand, new power plants are built. Data on wet and dry deposition in South Africa are sparse, but indicate comparable levels to those shown to be causing acidification of European soils and waters. Studies of acidification effects on ecosystems are even more scarce. The main objective of this project was thus to provide an overview of acid deposition levels and acidification status of streams in different regions of South Africa, through collecting new data. Secondary objectives were to calculate critical loads (the maximum amount of acid deposition a given ecosystem can tolerate without adverse effects on the biota) and potential exceedances of these, and to model the effects of the combined threat posed by climate change and acid deposition in the coming decades. Three study regions were selected, the Eastern Highveld, the Waterberg and South West Cape. In each region a deposition monitoring station was set up, and stream sampling was conducted for stream water chemical analysis and analysis of the benthic invertebrate communities. The study regions were selected to represent a gradient in deposition levels. The monitoring part of the project is finalised and data analysis is ongoing. Rain chemistry data show that pollution levels are highest in the Eastern Highveld and lowest in South West Cape. The deposition estimates will quantify the total acidification load to the systems. Preliminary data analysis of stream chemistry data indicate a range in sensitivity to freshwater acidification. In the most highly polluted Highveld region, none of the streams are acidified, but a few can be considered at risk. The reason for the low level of acidification is that the soils have high capacity to buffer the acid deposition, meaning that the soils are acidified rather than the water. In the intermediate polluted Waterberg region there are some acidified streams, and the majority of the remaining streams are at risk. Many of the South West Cape streams have a unique water chemistry, with high sea salt inputs and high humic content, giving strong natural acidity. The high humic content seems to be related to the fynbos vegetation special to this region. The project is pioneer work in many respects, as data is collected from new areas. The relationship between benthic invertebrates and water chemistry in areas with no other pressures than acidification has also been little studied. Such relationships are needed to establish critical levels of acidification for biota. The examination of invertebrates has also revealed several previously undescribed species. The project findings are thus of high scientific interest. However, the outcomes are also highly relevant to environmental management and policy makers. The identification of probable acidification impacts in the Waterberg, close to a major new power plant, indicates the urgency of measures to incorporate ecosystem sensitivity into air quality and emissions policy formulation. Likewise, sensitive systems in nationally important conservation areas (Marakele and Garden Route National Parks) indicate the importance of new data being collected on ecosystem sensitivity and biodiversity impacts.

South Africa has both areas with high deposition of acidifying compounds and acid-sensitive terrain with vulnerable stream ecosystems. Future new sources of air pollution and future climate change pose a risk to such ecosystems. We will choose three regio ns in South Africa in which acid sensitive surface waters are likely to occur (or have been previously shown to occur). The regions will be chosen to represent present-day low, moderate, and high acid deposition loads. In each of the regions we will set u p a wet-deposition collector for precipitation and dry deposition samplers, using the equipment and protocols of the Atmospheric Chemistry Monitoring Network in Africa (IDAF Programme). We will collect streamwater samples from about 20-30 sites in each re gion, for major component chemistry and benthic invertebrates. Based on these new data we will calculate critical loads for each of the streams and exceedence under present-day acid deposition. We will use the well-established SSWC (steady-state water che mistry) and FAB (first-order acidity balance) methods (Henriksen and Posch 2001). Finally we will apply MAGIC, a dynamic process-oriented acidification model (Cosby et al. 1985, Cosby et al. 2001), to two sites to simulate acidification under future scen arios of acid deposition and climate change. This will be the first systematic study of freshwater acidification in South Africa, and the first projections of future status. The results will be sought published in international peer-reviewed scientific jo urnals. The project will involve young researchers and women scientists, contribute to education of students at all levels, and will produce outputs relevant for policy makers and the general public.