3.4.2 Characteristics of Contaminants of Concern

Toxicological importance

Persistence in the environment

Propensity to bioaccumulate from sediments/soil matrices, which is

controlled primarily by the following chemical properties of the

constituents:

Hydrophobicity - Literally, "fear of water"; the property of neutral

(i.e., uncharged), organic molecules that causes them to associate with

surfaces or organic solvents rather than to be in aqueous solution.

The presence of a neutral surface such as an uncharged organic

molecule causes water molecules to become structured around the

intruding entity. This structuring is energetically unfavorable, and the

neutral organic molecule tends to be partitioned to a less energetic

phase, if one is available. In an operational sense, hydrophobicity is

the reverse of aqueous solubility. The octanol/water partition

coefficient(Kow, log Kow, or log P) is a measure of hydrophobicity.

The tendency for organic chemicals to bioaccumulate is related to

their hydrophobicity. Bioaccumulation factors increase with

increasing hydrophobicity up to a log Kow of about 6.00. At

hydrophobicities greater than about log Kow = 6.00, bioaccumulation

factors tend not to increase due, most likely, to reduced

bioavailability.

Aqueous Solubility - Chemicals such as acids, bases, and salts that

speciate (dissociate) as charged entities tend to be water-soluble and

those that do not speciate (neutral and nonpolar organic compounds)

tend to be insoluble, or nearly so. Solubility favors rapid uptake of

chemicals by organisms but at the same time favors rapid elimination,

with the result that soluble chemicals generally do not bioaccumulate

to a great extent. The soluble free ions of certain heavy metals are

exceptional in that they bind with tissues and thus are actively

bioaccumulated by organisms.

Stability - For chemicals to bioaccumulate, they must be stable,

conservative, and resistant to degradation (although some

contaminants degrade to other contaminants that may bioaccumulate).

Organic compounds with structures that protect them from the

catalytic action of enzymes or from nonenzymatic hydrolysis tend to

bioaccumulate. Phosphate ester pesticides do not bioaccumulate

because they are easily hydrolyzed. Unsubstituted polycyclic

aromatic hydrocarbons (PAH) can be broken down by oxidative

metabolism and subsequent conjugation with polar molecules. The

presence of electron-withdrawing substituents tends to stabilize an

organic molecule. Chlorines, for example, are bulky, highly

electronegative atoms that tend to protect the nucleus of an organic

molecule against chemical attack. Chlorinated organic compounds

tend to bioaccumulate to high levels in animals because organisms

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Chapter 3

Initial Evaluations