Step 5: Describe release mechanisms

Step 5: Describe release mechanisms

This step will describe mechanisms which may release contaminants from the

dredged material management area and allow them to contact ecological or human

receptors. Such mechanisms may include disturbance of the sediment, bioturbation,

dissolution, resuspension, diffusion through engineered barriers, or advection. It is

important to remember that the mechanisms are considered only if they result in a

release which brings contaminants into contact with potential receptors. The

product of this step is a narrative which describes potential release mechanisms

associated with the management option under consideration.

Example 5: Description of Potential Release Mechanisms

During this dredged material management operation, there are several potential release mechanisms

which could result in exposure to COCs. Once the material has reached the management area, sediment

can become suspended in the water during placement. The area is a low-to-moderate energy environment,

has a seasonal thermocline (indicating little surface-to-bottom mixing during summer), and is generally

depositional. There is some potential for resuspension of the sediments and advection through wave or

storm action and during winter with the breakdown of the seasonal thermocline. There is also potential

for diffusion from pore water and advection offsite. These mechanisms could bring the potential COCs

into contact with receptors.

Step 6: Describe potential routes of exposure

The simple existence of a release mechanism which may transport a contaminant

to a receptor will not result in a complete exposure pathway unless there is some

route by which the receptor contacts the contaminant. These routes may include

dermal contact, ingestion, absorption across the gills, or inhalation. The conceptual

model should specify the likely route or routes of exposure for each receptor

separately.

Step 7: Describe complete exposure routes

The last step is to decide whether there is a complete exposure pathway between

a contaminant and a receptor. The conceptual model should describe each complete

pathway in detail including the source of the contaminant, the release mechanism,

the route of exposures and the potential receptors. A complete exposure pathway is

a combination of physical, chemical, or biological mechanisms which may transport

a contaminant from a source, such as sediment, to an ecological receptor, such as a

commercial fish species or an endangered aquatic bird, or to a human receptor, such

as a recreational fisherman or someone consuming commercial fish, from an area

under the influence of a dredged material management activity.

Whether a pathway is complete depends on:

a. The presence of a particular receptor.

Chapter 2 Problem Formulation