7.1.1 Volatilization Processes

condition. Since ponded conditions can remain over dredged material in a CDF

for considerable periods, the ponded condition is likely the most critical for most

sites.

Because chemicals must enter the water phase before they can volatilize from

dredged material, the tendency of a chemical to volatilize from dredged material

can be generally related to the Henry's constant. Henry's constant is the

equilibrium distribution of a volatile chemical between air and water if true

equilibrium solutions exist in both phases (Thibodeaux 1979). Henry's constant

and, therefore, volatilization tendency depend on aqueous solubility, vapor

pressure, and molecular weight. Chemicals with high Henry's constant will tend

to volatilize while chemicals with low Henry's constant will tend to dissolve in

water. Henry's constant is directly proportional to vapor pressure and inversely

proportional to aqueous solubility. The actual direction of chemical movement

across the air-water interface depends on chemical concentrations in aqueous and

air phases and Henry's constant. The transfer rate (desorption for transfer to water

and volatilization for transfer to air) depends on wind-induced turbulence at the

air-water interface.

Contaminant transport from in situ dredged material to air is a relatively slow

process because most contaminants should first be released to the water phase

prior to reaching the air. Thibodeaux (1989) discusses volatilization of organic

chemicals during dredging and disposal and identifies four locales or conditions in

which volatilization may occur:

1. Dredging site, disposal site, and other water areas where suspended solids

are elevated, usually during active operations.

2. Quiescent, ponded CDF with a low-suspended solids concentration after

disposal is completed and prior to dewatering.

3. Dredged material exposed directly to air during transport and disposal and

during dewatering after disposal is completed.

4. Dredged material covered with vegetation and crust.

Figure 7.1 illustrates these conditions. Conditions 1 and 3 above are of the

most concern for volatilization in CDFs, and, therefore, the volatile loss analyses

presented in this manual are limited to the conditions of ponded water overlying

dredged material and exposed dredged material solids (USEPA 1996).

7-2

Chapter 7

Guidance for Evaluation of Volatile Emissions