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This is for Cap and JoAnne, to conclude my part of this cyst settling question,
which I raised in response to Cap's query. This is a long entry, that gets a
wee bit graphic towards the end (no pun intended, really), but after Jim's
reference to moose contributions to lake water, you're all in the right frame
of mind! To bring my understanding up to date, spurred on by JoAnne's citing of
the Berlin, NH water supply reference, I called Dr.Juranek at the Centers for
Disease Control in Atlanta, a man who stays on top of water pathogens and esp.
related to municipal water sources. He did indeed confirm that, in the absence
of complicating factors (see below), giardia cysts will gradually settle, but
he emphatically went on to say that no one knows how much, how soon, under
varying conditions. Those not interested, DELETE here, because for both Cap and
JoAnne's sake, I'm going to include a transcript of our short conversation.
I think you will concur that, even though I now know that cysts settle in
principle, one's cautious approach to water purification should not
change. Here goes:
Dr.J: Cysts will settle, but "what I don't know and what I haven't seen is any
scientific evaluation of that settling rate, under different water conditions.
If the water were cold, if the water were warm, if the water had other mud and
stuff in it, would they go down faster or slower. Some of those variables I
think that would be important for the backpacker - is the water of a certain
temperature or a certain character, how long should you let it sit there to
have fairly good confidence that most of the giardia cysts would have sunk to
the bottom. I haven't seen that sort of data. It may be out there, but I
haven't seen it... When you're looking at river (stream) water, it keeps things
BE: "From what you do know, w/o scientific studies being done, if a hiker were
to take water and let it sit, can one relate (the settling of cysts) at all to
the visual particulate that has settled on the bottom, or is it just too much a
Dr.J: " I don't think you can rely on the visual particulates on the bottom.
The heaviest things fall out first. That's what you're going to see visually...
We're (looking at) municipal drinking water and filtration capabilities as an
indicator of how well the filters are working and whether or not there might be
giardia or crypto in the final water. Those turbidity levels are *way* lower
than the naked eye can discern."
BE: "So, your advice right now for people who are thinking in terms of (cysts)
settling to the bottom would be caution, but what else would you say to them?"
Dr.J: "Well, I'd say that if you didn't have any other way of dealing with it,
that would throw in a *little* more of a barrier. I'm just not aware of the
science yet to give them guidelines to let it settle for 30 minutes, for 2
hours, or a day. I don't know what would be a safety margin and under what
conditions... Those would be the variables I would be concerned with. And
then the other variable would be how do you dip the material out of there
without stirring things up? If you took one dip out, you might be all right,
but that might be enough to re-suspend some particles for the next dip. I just
haven't seen any data but it's true, they [cysts] will settle out like any other
particle... I haven't seen any literature or practical studies looking at
those settling times and different water temperatures and different situations.
Here are some scenarios: What if you had something like a little bit of
chopped up leaves or finely-ground straw in that water, and the giardia cysts
happened to be attached to it - just mechanically juxtaposed to it, by surface
tension with stuff attached to it. Those "pieces of straw" would float
forever. Then the cysts would't go down. You can have debris in there that
might still be suspended, that might have organisms [giardia cysts] there, that
could keep it [cysts] suspended. Those are some of the dynamics that we keep
dealing with. We're trying to do studies even with stool specimens in lake
water. Well, with some stools, very heavy and dense and close to the bottom,
the risk of giardia from that getting back up seems very small, but other ones
are floaters and they get hit by wave action and some of the particulates in
there are going to *stay* floating and does that mean that the giardia and
crypto just stay floating with that a long, long time, or do they eventually
get dislodged and sink to the bottom? All of those dynamics I don't think
anybody has given a lot of attention to. So, if I were a backpacker and
couldn't chemically disinfect and couldn't filter, and this is pretty "muddy"
water, I'd certainly let it settle out, I just don't know how confident I'd
feel in doing it in a 1/2 hour, versus an hour, versus 4 or 5 hours."