And, of course, the depth of the sediment layer is only one factor influencing H2S production and toxicity. H2S toxicity is a function of pH, with toxicity decreasing as pH increases. It is also a function of temperature, with toxicity increasing as temperature rises.
Redox also plays a role: Redox potential drops through the sequence of electron acceptors, as O2 is the acceptor at
400-600 mV. Nitrate becomes an acceptor at 250 mV, manganese at 225 mV, iron between +100 and -100 mV, and sulfides at -100 to -200 mV. Carbon, or CO2, will become the terminal electron acceptor below -200 mV.
The effective lifetimes for hydrogen sulfide have been estimated to be 0.23 and 2.3 days.
And H2S is subject to additional microbial action-
"Several species of soil, aquatic, and marine microorganisms oxidize hydrogen sulfide to elemental sulfur, and its half-time in these environments usually ranges from 1 hour to several hours "(Jørgensen 1982).
@Meyer Jordan ... these EXACT statements are also talked about in the Randy-Holmes Farley hyperlink (
http://reefkeeping.com/issues/2005-12/rhf/) shared in my previous post about this.... the article is incredibly well researched...
Article also talks about pH, temperature, and actual LC50 (ppb) toxicity levels for a variety of freshwater organisms.
The typical freshwater Western Mosquitofish (also generically called Gambusia) many people may have in their ponds is basically immune due to its HUGE tolerance of hydrogen sulfide up to 750,000ppb while the typical goldfish tolerance is only down (although still a higher tolerance) around 58.4ppb and rosy red minnow is 57.3ppb...
For freshwater... If you smell hydrogen sulfide, then this means the concentration is already at around 0.029ppb to 300ppb... If your "sniffer" is good enough, able to easily smell it at 1ppb levels, then you can smell the sulfur before the hydrogen sulfide actually reaches potentially toxic levels.. which at this time would be good to lay down an air diffusser stone in the general area for a few minutes... doesn't take long for the hydrogen sulfide to release out of the water.
...all there has to be is anoxic or anaerobic conditions along with an abundance of sulfate and organic matter and water temperature at around or above 77*F.. .. For
maximal (that is 100% production ability) of hydrogen sulfide creation, just 1~2 cm depth is necessary in these conditions.... Hydrogen sulfide production is significantly slowed down when water temperature falls below 77*F.
...if water's pH is 7.0... then 50% of the sulfide is the toxic unionized H2S (hydrogen sulfide) and 50% is the "less toxic" ionized HS- ...
ionized HS- converts back to the toxic unionized H2S when pH lowers... kind of like ammonia/ammonium, except at opposite ends of the pH... High pH ammonia is more toxic, hydrogen sulfide less toxic... Lower pH ammonium less toxic, hydrogen sulfide more toxic...
This is all talked about in that very well research referenced article previously hyperlinked...
Quote from article... "Exactly what processes take place at what depths in sediments depends on many factors, such as the nature of the sediments themselves (size distribution and chemical makeup), the amount of organic material being deposited and the temperature. In one study of the sediments below seawater fish farms, the hydrogen sulfide level in the sediments appears to vary with the season, and peaks as high as 70,000 ppb in the Fall.9 Another study showed that even though sulfate reduction took place maximally at 1-2 cm depth, free hydrogen sulfide was present only below a depth of 6-7 cm, above which Fe++ was available for precipitation as iron sulfides.10"
.... this is why I wrote the statement below...
I don't know about that... I would need to read a reference that verifies "only 4~6 inches and fairly compact" and "takes a prolonged period of time" for significant H2S to occur so to become harmful to fish...
... all I am saying... don't go digging around your pond's organic material unless there is good water circulation and/or aeration and/or oxidation occurring while you are digging...
if spraying down your stream.. no worries... the toxic hydrogen sulfide will leave the water since the stream and/or waterfall will actively aerate the water to remove the hydrogen sulfide out of the water...
If ya want to go digging around in the pond.... lay a couple good air difussers/stones near the area where you are digging just for a few minutes after done digging... no worries..
If your pH is around 8.3~8.5 or higher, then no worries at all about hydrogen sulfide toxicity...
I really suggest folk reading that article...
It is NOT "doom and gloom", but also does NOT tell you to simply ignore it... This is an educated caution approach to hydrogen sulfide.
