Chemical classification of river water

If one graphs the relative proportions of principle anions and cations in the world's surface waters against TDS, a curve with two arms emerges (Figure 4.4). Gibbs (1970) interpreted this as evidence that three major mechanisms control surface water chemistry. At the left of the "boomerang" lie systems where the rocks and soils of river basins are the predominant source of their dissolved materials. Relief, climate, and age and hardness of rocks determine the positions of rivers within this grouping. Proceeding along the lower arm to the right of the figure we encounter waters that are lower in ions and whose chemical composition most closely resembles the rain. These are mainly the tropical rivers of Africa and South America with their sources in highly leached areas of low relief. In these humid regions, precipitation with a composition similar to seawater is of major importance as a source of ions. Average river water concentrations are < 30mgL~1. Proceeding along the upper arm to the right, we encounter systems with high concentrations of dissolved ions and again a relative predominance of Na+ and Cl". These are rivers of hot, arid regions, and the combined influence of evaporation and precipitation of CaCO3 from solution accounts for their chemistry. Total dissolved salts exceed 1,000 mg L"1, and can be as high as 6-7g L"1, as in Kazakhstan. Thus three major mechanisms -atmospheric precipitation, dissolution of rocks and the evaporation-crystallization process - are considered to account for the principal trends of dissolved ions in the world's surface waters. Other factors, including relief, vegetation, and the composition of rocks and soils then can be invoked to explain differences within these major groupings.

Critics of this scheme question the interpretation of control at the ends of the boomerang. The Rio Negro's chemistry is equally a consequence of its long history of intense weathering and a basin of mainly silicious rocks (Stallard and Edmond 1983). Similarly, saline rivers might be strongly influenced by near-surface halite deposits (Kilham 1990). These arguments downplay the roles of precipitation and evaporation, and suggest that local geology is of primary importance in determining river chemistry over all extremes. Regardless, most of the world's rivers are closer to the middle than the ends of this diagram, are low in Na+/(Na+ + Ca2+), and are dominated by Ca2+ and HCO" from carbonate dissolution. This is in accord with the view that the weathering of sedimentary rocks provides most of the dissolved ions in most of the world's major rivers (Berner and Berner 1987).

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