Acid rain is the result of a process that begins with the release of sulphur dioxide and nitrogen oxides to the atmosphere from a variety of sources including coal and oil-fired power generating plants, metal smelters and automobiles. Once in the atmosphere, sulphur and nitrogen oxides react with moisture to form acids, which may be carried many hundreds of kilometers away from their source before falling to the ground. The deposition of acidifying sulphates and nitrates is collectively referred to as "acid rain", although acids can reach the earth's surface in rain, snow, fog, on particles of dust, or even in gaseous form.

In recent years, industrial emissions of sulphur dioxide have decreased in Ontario, and the amount of acidic sulphates falling on lakes and forests has decreased correspondingly. On the other hand emissions of nitrogen oxides, which are primarily from cars, but much harder to control, have remained relatively constant over time. The damage to many lakes and forests in Eastern Canada over the past three or four decades by acid rain is well known. Lakes and forests located on the Canadian Shield are particularly sensitive to acid inputs, as Canadian Shield soils are typically thin, and have little natural ability to neutralize acids. It was thought that by reducing industrial sulphur emissions, and thereby reducing acid rain, that lakes and soils, which had been previously impacted by acidification would recover, and return to their natural state. However, the recovery of many acidified lakes and soils has not yet occurred, and, as a consequence, there has been considerable discussion about the need for further reductions in sulphur emissions. Ontario Power Generation Inc. (OPG) and the Natural Sciences and Engineering Research Council of Canada (NSERC) are funding a long-term research project at Trent University which I am leading. Much of the work is centered in the acid-sensitive Muskoka-Haliburton region, where a number of impacted lakes, streams and forests have not responded as expected to recent reductions in industrial sulphur emissions. The ultimate goal is to provide a better understanding of the processes involved in lake and forest recovery to acidification, and to predict the effects of recent and projected increases in emissions on acid-sensitive lakes and streams.

A number of specific projects are being carried out as part of the research effort; for example:

n Climate effects: Some of the sulphur that falls as acid rain may be stored in an "inactive" state in soils and wetlands. However, recent studies have shown that accumulated inactive sulphur may be transformed into biologically damaging sulphuric acid following periods of drought. Rainstorms may then transport this newly produced acid to downstream lakes, damaging aquatic organisms. The evidence that droughts are becoming increasingly frequent and that the climate phenomenon known as El Nino produces them has led to an awareness that climate and climate change may be controlling the rate at which our ecosystems recover from acid rain. Much of my research is focused on unraveling the interaction between climate change and acidification, and determining how long recovery of acidified lakes and streams will be delayed.

n Effects on forests: Forest soils contain large quantities of calcium, magnesium and other minerals, collectively called bases, which neutralize acid rain and prevent lakes and rivers from acidifying. These minerals are also plant nutrients, and are essential for tree growth. Continued acid deposition may eventually deplete soils of these minerals which could lead to acidification of streams and lakes and ultimately cause trees growing on acidified soils to die. This research will determine whether soils in the Muskoka-Haliburton region are losing nutrients more rapidly than they can be replaced by the weathering of soil. Modeling techniques will also be used to calculate the amount of acidity in rainfall that forest soils can tolerate without further acidification.

n Dissolved organic carbon: Dissolved organic carbon (DOC) is produced mainly in wetlands, and is a complex mixture of organic materials in various states of decomposition. Lakes and streams surrounded by wetlands often have a tea-like colour due to high DOC concentrations. DOC is an important component of lake water for a number of reasons. For example by giving lake water colour, DOC acts as a "sunscreen" and prevents biologically-damaging UV-B light from penetrating very far beneath the surface. DOC concentrations in acidified lakes, however, are low, for reasons not yet fully understood.

n Mercury: Coal-fired power plants are responsible for at least 20 per cent of mercury emissions in Ontario. High mercury levels have been detected in fish and aquatic birds in Ontario, and mercury has been shown to interfere with reproduction in loons. Part of my research addresses projected increases in mercury emissions, and possible effects on aquatic life.

The studies that are underway focus on how two factors that stress our environment, acid rain and climate change, interact and how this interaction leads to unexpected responses in the natural world. We have, for too long, looked at each environmental problem individually without considering the effects each has on any other.

Peter Dillon is the NSERC Industrial Research Chair and professor of chemistry and environmental and resource studies at Trent University.