Transport, Mobility, and Partition of Pollutants

The substances in Table 8.4 are not normally found alone in the environment but rather in simple or complex mixtures. These mixtures may be associated with the release, storage, or transport of chemicals in surface or groundwater, waste-treatment systems, soils, or sediments. Although many of these chemicals are either consumed or destroyed, a significant portion find their way into the air, waters and soils far away from their original discharge sites. 

The first three together contribute around two-thirds of the total toxic releases. Air emissions alone involve more than 3 x 105 tons per year. In the same vein, approximately 106 tons of petroleum are released annually into the oceans due to natural seeps, or extraction-transportation-consumption processes. 

The environmental effects of pollutants and wastes have traditionally been confronted with actions at the immediate vicinity level. A typical example involves the emissions resulting from fossil fuel burning in industrial and power plants, where taller stacks were built in the past to disperse pollutants in the air (mainly sulfur oxides) in a more efficient way. However, it is now recognized that these oxides can have important effects even in regions far away from the emission source due to environmental transport phenomena. Consequently, increasing the height of the stacks did not solve the problem. 

The pollutants thus transported include fertilizers, pesticides, oil, grease, other organic and inorganic industrial chemicals, sediments, salts, acid mine drainage, nutrients, radionuclides, microorganisms, and noxious gases. 

From the previous discussion it is clear that pollutants may be transported away from their sources by physical, chemical, or biological means, or a combination thereof. They may also accumulate in various media. Assessment of their chemical fate requires knowledge of many factors including those 

---

Sorption/desorption is the key property for estimating the mobility of organic pollutants in solid phases. There is a real need to predict such mobility at different aqueous-solid phase interfaces. Solid phase sorption influences the extent of pollutant volatilization from the solid phase surface, its lateral or vertical transport, and biotic or abiotic processes (e.g., biodegradation, bioavailability, hydrolysis, and photolysis). For instance, transport through a soil phase includes several processes such as bulk flow, dispersive flow, diffusion through macropores, and molecular diffusion. The transport rate of an organic pollutant depends mainly on the partitioning between the vapor, liquid, and solid phase of an aqueous-solid phase system. 

A given pollutant may penetrate in soil down to a specific depth, and therefore transport calculations need individual depth data. Owing to mass transport restrictions, residence times of many pollutants in soils are (unfortunately) much longer than those in the gas or liquid phases. In addition, partitioning effects in soils can be dramatic a case in point is the concentration effect that occurs with uranium, which sometimes reaches levels up to 104 times higher than its concentration in water with which the soil is in equilibrium. Biota plays a key role in the transport and mobilization of pollutants from soil, because for example, many of them bioaccumulate in vegetation and cattle (see Section 9.2). 

---