7.1.2 Condition 1 - Submerged Dredged Material

Figure 7-1. Illustration of locales or conditions for volatile emissions from CDFs

7.1.2 Condition 1 - Submerged Dredged Material

Dredged material slurries pumped to primary settling facilities or CDFs

undergo sedimentation, resulting in a thickened deposit of settled material

overlain by ponded water containing varying concentrations of suspended solids.

Thus, the submerged dredged material condition is characterized by water

containing contaminated suspended solids and a thickened bottom deposit of

dredged material. The volatilization pathway in this case involves desorption

from the contaminated suspended solids followed by transport through the air--

water interface.

The deposited dredged material is not part of the pathway because suspended

solids control dissolved contaminant concentrations, and it is the dissolved

chemicals that volatilize. While deposited dredged material can contribute to

dissolved contaminant concentrations, the contribution from deposited material is

not important until the suspended solids concentration becomes negligible. In a

primary settling facility, there is a continuous flux of suspended solids through the

water column while dredged material is being disposed. Diffusion from bottom

deposits is, therefore, unimportant relative to desorption from suspended solids in

controlling dissolved contaminant concentrations in primary settling facilities.

7.1.3 Condition 3 - Exposed Dredged Material

This volatilization condition is characterized by dredged material that is

exposed directly to air and void of vegetation or other cover. Exposed dredged

material is probably the largest of the four volatilization conditions as a source of

volatile emissions (Thibodeaux 1989). Dredged material begins evaporative

drying and volatile chemical emission as soon as it is exposed to air. Initially,

gas-side resistance affects the chemical emission rate. The top microlayer quickly

becomes depleted of volatile chemicals (and water); so that, continuing losses of

volatile chemicals come from the pore spaces within the dredged material. At this

point, the emission process is transient and changes from being gas-side resistance

controlled to dredged material-side vapor diffusion controlled. Exposed dredged

7-3

Chapter 7

Guidance for Evaluation of Volatile Emissions