This chapter has three principal objectives: (1) to summarize the present chemical composition of North American surface waters and point out any discernible trends with time; (2) to review chemical and biochemical principles and processes that control natural water composition, and the ways in which these may be involved in attaining the particular chemical compositions and trends that we can observe; and (3) to point out some of the more important factors that must be considered in collecting surface-water-quality data.

This discussion is concerned principally with inorganic chemistry and geochemistry. However, biochemical processes in river and lake water influence their chemical composition extensively, and specific effects are pointed out. Aquatic biology is discussed in another chapter. The physical processes occurring in lakes also have very important effects on water chemistry; these aspects of limnology are covered more fully in a separate chapter.

Data on which this chapter is based relate mostly to waters of Canada, the United States, and Mexico. For most of Central America, very little water-quality information is available. Tables 1 through 3 contain analytical data for river waters that illustrate some of the principles discussed, and summarize major features of the chemical composition of North American surface waters.

To a certain extent, at least, one would expect the average concentrations of dissolved elements in surface fresh water to reflect the relative abundance of the elements in the crustal rocks exposed at and near the land surface.