Introduction

Continental freshwaters are critical to terrestrial life, being the only reliable source of drinking water, and in some cases the medium for life itself. Water is thus also a vital human resource and its quality and quantity are key determinants of human development. Freshwater arrives on the continental land surface from precipitation (rain and snow), mostly derived from evaporation of seawater, and is found in ice-caps, lakes, rivers and groundwater (see Section 1.3.2). The polar ice-caps are long-term storage reservoirs of predominantly pure precipitation. The other reservoirs contain water that is usually much altered from the original precipitation by interactions with mineral weathering and biological processes. The important processes that chemically alter water vary for ground-waters, rivers and lakes. For instance, photosynthetic processes play a major role in regulating lakewater chemistry, but have little direct role in groundwater where there is no light. Similarly, individual rivers, lakes and even aquifers may have quite distinct chemistries depending on the relative importance of the processes that act within them. Of course these three reservoirs are physically interconnected and in general we treat them together in this chapter, although emphasizing clear differences where necessary. Although the chemical composition of rivers, lakes and groundwater varies widely, it is governed predominantly by three factors: element chemistry, weathering regimes and biological processes. In addition, human perturbations may have a major effect on some freshwater systems.

Table 5.1 Comparison of the major cation composition of average upper continental crust (from Wedepohl 1995) and average riverwater (from Berner & Berner 1987); except aluminium and iron from Broecker and Peng (1982).

Upper continental crust (mg kg-1)

Riverwater (mg kg-1)

Al

77.4

0.05

Fe

30.9

0.04

Ca

29.4

13.4

Na

25.7

5.2

K

28.6

1.3

Mg

13.5

3.4

0 0

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