Surficial geochemistry, south-central Canadian Shield: implications for environmental assessment

Abstract

An ongoing problem in evaluating the effects of acid rain is distinguishing natural (spatial) variations of the geochemical environment from anthropogenic (temporal) variations. In other words, it is important to determine whether a lake is naturally acid because of its geological setting or whether it has been acidified as a result of dry and wet acid deposition. To provide a model that might acid in answering these questions, the Geological Survey of Canada has carried out a series of sampling programs on the south-central part of the geologically complex Canadian Shield. With these programs we have tried to link the easily observable lithological variations of bedrock with the chemical compositions of overlying glacial deposits with the geochemical compositions of lake sediments and lake waters. Presumably, once these linkages are understood, variations in life systems that inhabit terrestrial and aquatic environments in the areas sampled with be easier to evaluate. The ultimate goal is to provide a base against which observed variations in life systems can be judged as natural or anthropogenic. In addition, sound geological data can be used to refine estimates of SO x and NO x target loadings and to determine areas where acid deposition may mobilize potentially noxious trace metals into hydrologic cycle. The importance of establishing geological and geochemical linkages is illustrated by the fact that significant parts of the Canadian Shield, while formed of granitoid rock, are overlain by calcareous unconsolidated sediments that have been glacially transported long distances from limestone sources. For instance, in areas north of Lake Superior and Eastern Ontario, high concentrations of carbonate minerals in glacial sediments overlying carbonate-poor bedrock result from glacial transport of carbonate minerals tens to hundreds of kilometres from limestones on their up-ice sides. In these same areas, high concentrations of As, Hg and other trace elements in sediment and water can be related to glacial dispersal from bedrock known to host mineralized occurrences or high background concentrations of these elements. Awareness of bedrock composition and of glacial distortion of the chemical signature of bedrock is essential for insuring that a complete and credible database is provided to the legislators and regulatory agencies charged with establishing realistic target loading for acid deposition.