Bruce

Let us imagine that your friend collects just 1% of all the weights cast
in the SF area. That means that the total is in the region of 1.5
tonnes. I don't have a hydrological map of SF bay handy, but I imagine
we are talking about at least 100 km3 of water, probably a lot more,
i.e. 10^11 tonnes. Just for the sake of argument, let us assume the lead
from those weights all dissolves instantaneously into the bay. Result: a
very small fraction of ppb, nearly ppt. However, it will be even less a)
because the water in the bay is not static because it is being
constantly replaced as water flows into the bay from land sources
(including treated waste water) and by tidal flushing; b) lead will not
dissolve instantaneously ; c) the solubility of lead in sea water is
lower than in pure water (cf lead artefacts recovered from 3000 year old
shipwrecks in the Mediterranean). Even if my original hypothetical
estimations are way out, there is plenty of margin to show that there is
absolutely no risk of SF Bay becoming sterile because of this source. In
reality, the geology of the region is probably such that the number of
tonnes of lead washed down in alluvium during the rains will be many
times all the lead dropped off wheels in the whole of California.

Let's keep a sense of proportion.

However, there are ways of mitigating your hypothetical problem: a)
force tyre manufacturers to make their products so that less wheel
balancing is required; b) force weight manufacturers to redesign the
clips, so that the weights don't come off; c) replace the lead by tin
(see http://www.itri.co.uk/wweights.htm ) and d) have more cyclists
collect discarded weights.

Brian

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