River dissolved constituents

Table 10-12 Number of times river constituents have passed through the ocean in 10 years assuming present annual worldwide river discharge, mean dissolved constituent concentration of rivers and ocean, and ocean volume of 1.37 x 10 L (amounts in lO kg)...

The first step in accounting for the dissolved species in river water is subtraction of the amounts of dissolved materials in precipitation. This step is particularly important in balancing NaCl, the most important component of sea aerosol washed out of the atmosphere by precipitation. After the precipitation component of river water is removed, the remainder represents dissolved constituents derived from rocks, and in the case of HC03", in part from the atmosphere ... 

Meybeck, in a recent (1987) comprehensive article on the global sources of dissolved constituents in world s rivers, arrived at a balance more similar to the one presented here. He presented a compilation of various authors estimates of the sources, given here in Table 9.10. There is seemingly good agreement between the works of Holland (1978), Wollast and Mackenzie (1983 this book), Berner et al. (1983), and Meybeck s latest work (1987). Table 9.10 provides the reader with some idea of the history of source estimates for the dissolved materials of rivers and their range of values. 

Table 6.2. Average concentrations (mg/L) of the main dissolved constituents in major rivers...

The main mechanisms for delivery of dissolved constituents to the ocean are river inflow and atmospheric input. Formation of authigenic minerals (those minerals that form in situ) is the ultimate sink (Fig. 2.1) for these constituents. Authigenesis primarily involves precipitation of plant and animal shells, chemical reactions in sediments, and high-temperature reactions at hydrothermal regions. We begin with a brief review of the chemical reactions influencing the dissolved ion concentrations of rivers, and end with an attempt to balance the river sources with plausible sinks for the major seawater ions. 

The ratio of the inventory (total amormt) of an element in the sea to its inflow (or outflow) rate at steady state (rmchanging total mass) is a measme of its reactivity. These two terms combine to form the residence time of the dissolved constituent with respect to river inflow, tr ... 

The major dissolved constituents of the oceans are ions that do not form very insoluble compounds. Large amounts of many common elements such as Ca and Si are carried into the sea in soluble form by rivers, but many are precipitated either by inorganic or by biological processes (see... 

The focal areas of this book have relatively minor relationships to the primary formation of the Earth s crust which has caused a certain distribution of the chemical elements. They mainly deal with products of the alteration of the crust in geologic processes. We can presently still observe the weathering of solid rocks, the erosion of mountain ridges, and the transport of eroded materials as suspended and dissolved constituents in river and rain water, in ice and wind. In-situ weathering forms soils, and soils are the basis of food production for human nutrition. Therefore, soUs need special protection against the impact of toxic substances (see Part I, Chapters 4 and 5). 

Movement of a soluble chemical throughout a water body such as a lake or river is governed by thermal, gravitational, or wind-induced convection currents that set up laminar, or nearly frictionless, flows, and also by turbulent effects caused by inhomogeneities at the boundaries of the aqueous phase. In a river, for example, convective flows transport solutes in a nearly uniform, constant-velocity manner near the center of the stream due to the mass motion of the current, but the friction between the water and the bottom also sets up eddies that move parcels of water about in more randomized and less precisely describable patterns where the instantaneous velocity of the fluid fluctuates rapidly over a relatively short spatial distance. The dissolved constituents of the water parcel move with them in a process called eddy diffusion, or eddy dispersion. Horizontal eddy diffusion is often many times faster than vertical diffusion, so that chemicals spread sideways from a point of discharge much faster than perpendicular to it (Thomas, 1990). In a temperature- and density-stratified water body such as a lake or the ocean, movement of water parcels and their associated solutes will be restricted by currents confined to the stratified layers, and rates of exchange of materials between the layers will be slow. 

The rivers and streams of the world discharge, on average, 37 x 10 m year of water, carrying 4.6 x lO g year of dissolved constituents and... 

In the biosphere, rain that is not lost back to the atmosphere by evaporation from the ground or from trees may pass deep underground, only to emerge at a much later date (Table 2.27) in a river or lake. Water coming into contact with rocks (and derived soils) reacts with primary minerals contained in them. The minerals dissolve to varying extents, and some of the dissolved constituents react with one another to form new, secondary minerals. Dissolution is mainly controlled by the water acidity provided from plant mineralization (humic acids), atmospheric carbonic acid and acid rain . The overall process is called chemical weathering (see Chapter 2.2.2.5, Eqs. 2.62 and 2.62 Berner and Berner 1996). 

Table II summarizes analytical data for dissolved inorganic matter in a number of natural water sources (J3, 9, J 9, 20, 21). Because of the interaction of rainwater with soil and surface minerals, waters in lakes, rivers and shallow wells (<50m) are quite different and vary considerably from one location to another. Nevertheless, the table gives a useful picture of how the composition of natural water changes in the sequence rain ->- surface water deep bedrock water in a granitic environment. Changes with depth may be considerable as illustrated by the Stripa mine studies (22) and other recent surveys (23). Typical changes are an increase in pH and decrease in total carbonate (coupled), a decrease in 02 and Eh (coupled), and an increase in dissolved inorganic constituents. The total salt concentration can vary by a factor of 10-100 with depth in the same borehole as a consequence of the presence of strata with relict sea water. Pockets with such water seem to be common in Scandinavian granite at >100 m depth.

The most stable minerals are often physically eroded before they have a chance to chemically decompose. Minerals that decompose contribute to the dissolved load in rivers, and their solid chemical-weathering products contribute to the secondary minerals in the solid load. The secondary minerals and the more stable primary minerals are the most important constituents of clastic sedimentary rocks. Consequently, the secondary minerals of one cycle of erosion are... 

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