Newsgroups: bit.listserv.scuba-l
Date:         Wed, 23 Dec 1992 08:32:00 -0500
From:         John HC <jhc@WORLD.STD.COM>
Subject:      Divers' Environmental Survey Freshwater Bulletin

We all know that diving in contaminated water is dangerous. To do it safely
requires specialized equipment that must be customized for the type of
contamination the diver will encounter. As we discovered in Walden Pond,
apparently clean water may contain a layer of toxicity that most divers
may not expect and certainly won't be equipped for.


This stuff is hydrogen sulfide (H2S). It is a natural byproduct of anaerobic
decomposition. If it occurs in a place undisturbed by water movement, it can
collect and concentrate, creating hydrogen-sulfide soup. If mixed with
oxygenated water, it forms sulfuric acid (H2SO4). This last is not likely to
form in significant quantities in otherwise healthy water where H2S forms
because the normal buffers (salts of carbonic acid) present in freshwater
would prevent significant shifts in Ph.


Since H2S is a byproduct of anaerobic decomposition, it occurs where
there is no oxygen. Since H2S is almost 2 times heavier than H2O,
it always drops to the bottom of the water column, below all
of the oxygenated water.


H2S has two insidious characteristics: it is easily metabolized by human
tissue up to the point of overload, and it causes respiratory paralysis. In
other words, if you inhale it, you won't notice it because your body won't
react negatively until it has absorbed more than it can handle. (H2S resembles
CO in this way.) At that point, your respiratory system goes into paralysis,
and it won't matter how much air you have left in your tank or how well your
regulator or your buddy performs.


This is something to watch out for because every time you exhale through your
regulator, some minuscule amount of the surrounding liquid enters your second
stage through the exhaust ports. This liquid is atomized when the regulator
delivers the next breath, and you inhale it. This is harmless when you are in
relatively clean water, fresh or salt. It can be somewhat less benign if
you're in contaminated water, and absolutely deadly if you spend too much time
in H2S solution.


So how do you recognize it if you ever encounter it?


The H2S solution appears black, and it absorbs light. A UK400 with four fresh
D-cells penetrates only a foot.

If your light is as strong as a UK400, you can look into its beam
from the side and see transparent threads suspended in the darkness. These are
filamentous sulfur bacteria colonies. (Just to be safe, though, please don't
stick around long enough to admire the filamentous bacteria.)


In theory, concentrated H2S solutions form in deep holes such as the Walden
Pond hole, where the H2S is protected from the affects of wind- and
temperature-induced water movement. However, we also encountered it in the
eastern end of Walden at 45 feet, a mere three feet from the bottom of
oxygenated water.


In all my freshwater diving, I have seen the light-absorbing blackness of H2S
in only three places: Torsey Lake in central Maine; Flax Pond in Lynn,
Massachusetts; and Walden Pond. It is not common.


It is, however, real, so please be careful when you encounter black water at
the bottom of a lake or pond. Just get out of it and back up to the green or
blue or brown or gray water above it.


John H-C
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