Transport and Reaction in Rivers

Figure 18. Orinoco River tributary Li isotope data (Huh et al. 2001). Tie lines connect samples pairs of dissolved (open symbol) and suspended load (filled symbol) for tributaries draining the Andes (circle) and those draining cratonic area (triangle). Isotopic compositions of suspended loads were similar in both cases, but the dissolved loads of Andes drainages were uniformly isotopically heavier. This difference was attributed not to lithology in the headwaters, but to transport- versus reaction-limited weathering conditions.

The input of terrestrial DOM via rivers and aeolian transport was discussed in Chapter 23.3. Riverine concentrations of DOC range from 2 to 20mgC/L. In contrast, little or no terrestrial DOM is detectable in seawater, leading to the cmrent consensus that most is removed close to its point of input. In some estuaries, removal is associated with flocculation reactions promoted by the large increase in ionic strength that occms when river water mixes with seawater. In other estuaries, DOC exhibits conservative behavior, leaving marine chemists with a mystery as to how and where DOC is removed. 

Combustion. The incomplete combustion of fuel of all sorts (petroleum, methane, coal, refuse, etc.) can produce PCAH by free radical reactions in the flame zone (8). Emission of these compounds into the air can occur both from mobile and stationary sources and is usually associated with soot production. PCAH from combustion sources could reach the Charles River mostly by way of rainwater which both scrubs them from the air and transports already precipitated PCAH (adsorbed on soot) by way of terrestrial runoff. 

Many of the physical characteristics of the atmosphere, such as wind, temperature, cloud cover, humidity, and precipitation, are easily perceived. Sometimes, chemicals in the atmosphere also can be observed, as in smoke plumes and smog, and their physical transport tracked downwind just as downstream transport of substances in a river can be measured. Other atmospheric processes are less apparent to the unaided observer, however, occurring either on the microscopic scale of a chemical reaction, or on a global scale, or at high altitudes. Such processes may be detected only by instrumentation on satellites or some high-altitude aircraft. 

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