From: jhc@world.std.com (John HC)
Newsgroups: rec.scuba
Subject: Divers' Environmental Survey Freshwater Bulletin
Date: 23 Dec 1992 07:33:51 -0600
Organization: UTexas Mail-to-News Gateway
NNTP-Posting-Host: cs.utexas.edu

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
Newsgroups: rec.scuba