C0 = initial concentration, ppm C = concentration at time t, ppm

The time needed for half of the concentration to react, half-life, can be calculated if k is known with use of the following equation:

0.693 k

where t1/2 is equal to half-life.

Table 9.12.3 lists the biodegradation rates for many pesticides; the biodegradation rates for other organic chemicals are in Dragun (1988b). However, note that the estimate of biodegradation rates of organic chemicals may not be accurate. Biodegradation rates can be affected by many factors such as pH, temperature, water content, carbon content, clay content, oxygen, nutrients, microbial population, acclimation, and concentration. Most of these factors are interrelated. For example, the pH can affect both the availability of a substrate as well as the composition of the microbial community.

After the degradation rate constant k is estimated, the behavior of a specific compound can be modeled with use of a form of the advection-dispersion equation, Equation 9.13(1), that includes a first-order degradation term.

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