The principles of air–water mass transfer are often difficult to apply in field measurements and thus also in field predictions. The reasons are that the environment is generally large, and the boundary conditions are not well established. In addition, field measurements cannot be controlled as well as laboratory measurements, are much more expensive, and often are not repeatable.

Table 9.1 lists some of the theoretical relationships from Chapter 8, for example, and the difficulties in applying these relationships to field situations. Eventually, application to the field comes down to a creative use of laboratory and field measurements, with a good understanding of the results that theory has given us and to make sure that we do not violate some of the basic principles of the theoretical relationships.

The value of β has not been attempted in the field to date. So, how do we determine KL for field applications? The determination of dynamic roughness, z0, has also been difficult for water surfaces. The primary method to measure KL and KG is to disturb the equilibrium of a chemical and measure the concentration as it returns toward either equilibrium or a steady state. Variations on this theme will be the topic of this chapter.

Gas Transfer in Rivers

Measurement of Gas Transfer Coefficient

Rivers are generally considered as a plug flow reactor with dispersion. Determination of the dispersion coefficient for rivers was covered in Chapter 6, and determination of the gas transfer coefficient is a slight addition to that process.