> If a pesticide has a long half life in the soil, then that means that it
> can't be leaching into the water.  Conversely, if it is leaching into the
> water, then that means that it is not leaving residue in the soil.   Peter,
> you can't have it both ways.

I don't see how that follows.

The half life in soil is the half life in soil and has to do with 
breakdown and has nothing AFAIK to do with leaching.  If a chemical 
leaches out, it has left the soil in question, but is still intact. 
That queers that particular measurement.

The half life in soil reflects (or should) only the breakdown of the 
chemical in some imaginary "typical" soil when it actually stays there.
Often these measurements conveniently omit mention of the metabolites 
which sometimes (not always by any means) be more destructive than the 
original chemical.

Half life is a concept which is applied to many different processes 
somewhat indiscriminately.  Some decay rates are dependent on 
concentrations or the development of biological populations that feed on 
the chemical.  The Deepwater Horizon incident demonstrated that nature 
is complex and has a unpredictable response curve that even the experts 
cannot explain -- or predict.

Moreover, these sorts of numbers that are bandied around without 
qualifying statements are usually averages made up from composites and 
listeners (and often those quoting them) have no idea what the scatter 
is in the underlying data -- or how outliers were treated.

As always, these dumbed-down numbers do more to obfuscate the real 
concerns than illuminate the landscape.  As beekeepers, we are very 
interested in the exceptions.  By virtue? of the process, exceptions or 
anomalies are ignored or discounted.

Leaching is a special case.  If leaching takes place, then that process 
will remove the chemical from the soil, but not affect the half-life, 
just as washing a nuclear waste from a Japanese power plant into the sea 
and the soil does not affect the half life of those byproducts, merely 
to location.

Stan has dealt with question of the the real-world hall life in water. 
but again, I would question the presentation of any one number as being 
particularly meaningful.

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