5

The Chemistry of

Continental Waters

5.1 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 composi-

tion 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.