Hydrogen Sulfide layers in water

[Chris H]

I just looked up Hydrogen Sulfide in the Merck Index and this blurb seems most appropriate as reasons we see what we see.  Temperature is Celsius
 
One gram H2S dissolves in 187 ml water at 10°, in 242 ml water at 20°, in 314 ml water at 30°; in 94.3 ml abs alcohol at 20°; in 48.5 ml ether at 20°. Sol in glycerol, gasoline, kerosene, carbon disulfide, crude oil. Water solns of H2S are not stable, absorbed oxygen causes the formation of elemental sulfur, and the solns become turbid rapidly.

From this I would gather that whatever is producing the gas, is likely at the bottom, as is the cold water where the gas is more soluble in the water.  Once it is in the water, the elemental sulfur crashes out and forms dark water.   I’m guessing here, but I think turnover of lakes, depending on conditions would be a likely cause for the change in position of this layer, or a portion of it, in the water column.

Additionally, Tami’s described symptoms are listed with the overexposure symptoms in the Merck Index.   This stuff isn’t very nice, so don’t swim in it if you have the choice.

Caution: Highly toxic irritant and chemical asphyxiant; overexposure can be fatal. Insidious poison, since sense of smell may be fatigued and fail to give warning of high concns. Direct contact with gas may cause irritation of eyes and respiratory tract resulting in keratoconjunctivitis, photophobia, lacrimation, corneal opacity; rhinitis, laryngitis, cough, bronchopneumonia. Direct contact with solution may cause skin irritation, erythema. Potential symptoms of overexposure by inhalation include salivation, GI disturbances; giddiness, headache, vertigo, confusion, unconsciousness; tachypnea, tachycardia, sweating, fatigue. Exposure to very high vapor concentrations may result in systemic intoxication leading to paralysis of respiratory center of brain, apnea and sudden collapse. See Clinical Toxicology of Commercial Products, R. E. Gosselin et al., Eds. (Williams & Wilkins, Baltimore, 5th ed., 1984) Section III, pp 198-202; NIOSH Pocket Guide to Chemical Hazards (DHHS/NIOSH 97-140, 1997) p 170.