I have just had a batch of Riesling go all H2S on me, and I can't
figure out why. Any help would be appreciated -

I used 150 lbs of ripe grapes, with just a touch of botrytis (no fuzzy
grapes, though), from a local vineyard, who also grow for local
commercial wineries. They claim that their pest control process only
uses Sulfur prior to bud-break, 3 applications total. I got the
grapes on a Friday, crushed them that afternoon, and let them sit on
their skins overnight (14 hours total). 15 gallons of must, measuring
20 brix, .62 TA, 3.1 pH (that last might be off, I'm having trouble
with my pH meter :-)). No sugar or acid adjustment. Added 15 campden
tablets, and mixed as well as I could. Pressed in the morning,
getting 6 gal free run, and 4.5 gal pressed juice, keeping them
separate. Brought up each to 50ppm sulfite.

Started the yeast later that day, using two strains from Morebeer.com
(QA23 and RHST), neither one being Montrachet, nor implicated in H2S
production before that I know of. The RHST started better than the
QA23, such that I stole a little of the float of the RHST to boost the
QA23. They ran for about a day and a half without problems, then
suddenly instead of a brown yeast cap, I suddenly had a white cap and
a heavy rotten egg smell. It's at 16 Brix, and the sulfite level went
down to about 25ppm.

Following one of the books I have, I've now bubbly-racked them both
twice, and there is virtually no sludge on either of them. The smell
is still there, and strong. If you blow the smell away, the juice
tastes good. I'm afraid that it's probably a loss at this point.
sigh...

However, any ideas on what to do to avoid this in the future? I'd be
glad to answer any questions anyone has if it'll help figure out the
problem.