>I don't want to pretend to be a chemist

>Maybe someone who really knows the chemistry could comment on this.

Maybe one will, maybe one won't, but I'll take a stab at it: the rate of
mixing which occurs under the substrate is probably very low unless it
is being constantly disturbed.

So, while the chemical reaction you are referring to is undoubtably
taking place, the lead-contaminated result is tucked underneath
the substrate rather than being mixed uniformally throughout
the pool.

I know nothing of lead pickup by aquatic plants, but assume the
folks studying the phenomena of lead poisoning in waterfowl have
looked at this, as it is an obvious concern since this is what
most of the ducky-poos are eating. Aquatic plants are less dependent
on their root system for nutrient pickup (indeed, in many the roots
are really just anchors) than those on land so it might not be as bad
as you'd think.

Also, low-level contamination in general can have different effects
on different species. For instance, relatively low levels of lead
can cause problems with mental development in humans, but I don't know
if this is true in ducks, which after all aren't as bright as us in the
best of circumstances. I believe lead poisoning is cumulative, and
ducks do reach adulthood very quickly, which could lessen effects.

This phenomena is often true of low-level contamination. Such
contamination often leads to long-term health problems, i.e.
cancer, heart disease, etc which simply aren't issues with
wildlife as the average lifespan is simply too short for such
diseases, even if the rate of occurance is artificially raised,
to have much effect on overall mortality rates.

So problems which crop up immediately are of much more concern.
In the case of lead shot, it is death by poisoning in a matter
of days to weeks. In the case of selenium poisoning, developmental
problems that increase mortality immediately. In the case of
DDT, crushed eggs and the obvious problems.

On the other hand, ducks could probably smoke four packs a day
without significantly shortening their life expectancy.

- Don Baccus, Portland OR <donb at rational.com>