Hydrogen sulfide (H₂S) occurs at objectionable levels in approximately 5-10 percent of water supply wells in New Hampshire.  DES believes that bedrock wells experience the problem more than dug wells.

CHEMICAL/ BIOLOGICAL ORIGINS OF H₂S
H₂S can be formed naturally as a by-product of the decomposition of organic material possibly aided by the presence of sulfur reducing bacteria.  These bacteria are not hazardous to human health.  Organic material is often prevalent in swampy areas and when dissolved in water, breaks down to form many byproducts including H₂S.  H₂S can also be produced by chemical reactions of soil and bedrock minerals containing sulfur.

There are many possible by-products of these chemical reactions.  Where there is sufficient oxygen available, these by-products include water and sulfate; when there is not sufficient oxygen, then different by-products are produced often having unpleasant taste and odor characteristics.  If the water that recharges your bedrock or dug well has little dissolved oxygen, taste or odor problems are possible.  A lack of oxygen is called an anaerobic condition.

HOT WATER TANKS: CORROSION PROTECTION RODS
It has also been reported that H₂S can be produced within a hot water tank by complex chemical/ biological reactions that are related to the use of anticorrosion rods made of magnesium.  These rods are used to prolong the life of the steel liner in a hot water tank.  This possible explanation of the origin of H₂S can be evaluated by comparing the odor level in the hot water to that found in the cold water.  If there is no odor in the cold water,  these corrosion protection rods could support the production of the H₂S odor.  To be very sure there is no odor from the cold water, you may need to agitate and slightly warm the "cold" water in a large flat bottom pan, since at higher temperatures, taste and odor characteristics are much more observable.

HEALTH SIGNIFICANCE
H₂S gas, at the concentrations found in drinking water, is not hazardous to health.  Odor identification is imprecise and thus it is difficult to accurately and consistently characterize all the odor factors in water.   Other odor characterizations (such as medicine, sweet) may indicate other types of chemical contaminants in the water that may poses a health risk.

TREATMENT OPTIONS
H₂S can be easily removed from drinking water.  The most common approaches are listed below.

Replacement ofAnti-Corrosion Rods:    If the odor is only from the hot water system (the odor is not in the cold water taken directly from the well) and if there are magnesium corrosion protection rods,  then an easy solution to the H₂S may be possible.  If the water is not corrosive, these rods may be removed permanently.  Where the water is corrosive DES suggests substituting aluminum protective rods for the magnesium.  Replacement or removal of these rods however, may void the tank warrantee.  Check with your hot water tank dealer.   See fact sheet WD-WSEB-3-4 relative to determining the corrosiveness of a water supply.

Aeration:  In this process, large volumes of air are blown through the water.  The H₂S volatilizes into the air bubbles.  The "used" air is then vented outside the home.  The principal disadvantage of this method is possible bacterial growth in the treated water caused by the use of dirty air.  Aeration is also beneficial in removing radon gas and in raising the water's pH by allowing the "off-gassing" of excess carbon dioxide (CO₂).  Off-gassing of the H₂S will be less complete where the pH of the water is high.

Oxidation:  In this method an oxygen-like chemical (potassium permanganate, chlorine or ozone) is added to the water.  The oxygen-like compounds chemically react with the odor compounds so as to destroy the odor.  One relatively low cost variation on this process uses a venturi nozzle to add small amounts of air to the water.   Air contains approximately 20 percent oxygen.  The water then proceeds to a detention tank that provides both reaction time and also allows for air release for the unused air.

Adsorption:   In this process water is passed through granular activated carbon (AC).  The taste and odor components adsorb (stick) to the interior surfaces of the carbon particles as the water passes through the treatment device.  If intending to use AC, also test the water for radionuclides and read fact sheet WD-WSEB-3-11.

For more information concerning the purchase and layout of water treatment equipment, please refer to the DES fact sheet WD-WSEB-2-5.

HAZARDOUS CONTAMINANTS
Occasionally taste or odors can be caused by more serious contamination.  This contamination could be from industrial solvents or inappropriate waste disposal.  Very expensive laboratory testing is typically required to determine the presence of these contaminants.  Whether there is sufficient justification for such testing would require a site specific evaluation focusing on nearby and uphill land uses.  For other odor and taste problems, see the DES fact sheet WD-WSEB-3-15 entitled, "Taste and Odors in Drinking Water".

TESTING FOR H₂S
The H₂S concentration of water can be determined by the DES Laboratory at a cost of $18 per sample.  A special sample bottle and special preservative are needed.   Please contact the DES Laboratory at 271-3445, 3446 for a sample container and instructions.  Postal regulations prevent us from mailing a H₂S sample container to you.  The identification of odor components in drinking water, other than H₂S, is often very difficult.  Odor identification is very subjective and thus odor identification is generally not processed by the DES Laboratory.

FOR MORE INFORMATION
For more information please call the DES's Water Supply Engineering Bureau at 271-3139.  We would appreciate your comments concerning this fact sheet and your experiences relative to H₂S treatment.   For an overall listing of water supply related fact sheets, please request fact sheet WD-WSEB-15-2.   Drinking water fact sheets are available through the DES web site at: www.des.state.nh.us/ws.htm   then select: fact sheets.  Please check the internet annually for updates to this document. 9/00