Peter asked: Are you saying "unavailable" metabolically, or simply
(primarily) physically? Also, to which "organisms" (e.g., to
bottom/ooze-dwelling invertebrates,microbes, or ...?

"Unavailable" in the context of eco-risk usually means chemically. That
is, the substance is chemically inactive or inert and cannot pass through
skin, gill, intestine, etc. If it cannot be assimilated it is considered
"bio-unavailable." The route of delivery and the chemical state are both
very important. For example, you could drink a pint of metallic mercury
and, aside from that heavy feeling, it would cause no toxicity. If you
were to breathe a few micrograms of mercury they would pass the lung/blood
barrier and be delivered to your central nervous system. ("Mad hatters"
were suffering from mercury poisoning via this route.) If the mercury you
ingested were methylated ;i.e., as methymercury in fish, you could become
seriously ill with Minimata Disease, since methylmercury can pass through
the intestine/blood barrier.

In the case of lead we usually don't see high concentrations in benthic
organisms, including invertebrates. Microbes can and do methylate some
elements but I'm not aware that lead is one of them.

Incidently, the concentrations of lead as well as other trace metals is
decreasing in Puget Sound sediments as a result of source control.

Herb Curl
Seattle WA

h.curl at hazmat.noaa.gov "You may be only young once but you can be
Hazmat/NOAA, 7600 Sand Pt. Wy., NE immature the rest of your life."
Seattle, WA 98115-0070
(206) 526-6272