This article, written by the American Ground Water Trust was originally published in THE AMERICAN WELL OWNER, 2002, Number 2]


Dissolved sulfate is a normal constituent of most ground water. Its presence may go unnoticed if the water remains oxygenated and if sulfur bacteria are not present. In some cases, however, high sulfate concentrations may have a laxative effect, especially if a person changes from their usual water source to another with a higher sulfate content, such as may occur during a vacation.

Sulfur occurs in some ground water as a dissolved sulfate (SO42-) or hydrogen sulfide gas (H2S). Concentrations of these constituents range widely from aquifer to aquifer depending on the rock types, the possible presence of contaminants, the presence of bacteria species that may interact with the water and the length of time that the water is has been underground. Concentrations generally tend to increase with greater contact time.

Most water well issues related to sulfur revolve around the presence of dissolved hydrogen sulfide. Hydrogen sulfide is a flammable, colorless gas that is soluble in water. Although hydrogen sulfide gas can be lethal when concentrated in the breathing zone, the levels found in ground water are typically too low to create any risk right out of the tap. However, if the gas is able to collect in a low area, such as a well pit, it can build up to dangerous levels by displacing the less dense oxygen in that area.

The United States Environmental Protection Agency (EPA) does not consider consuming hydrogen sulfide gas dissolved in ground water as a risk to human health. However, hydrogen sulfide can be a nuisance because its presence creates a “rotten egg” odor and if used for cooking can change the taste of foods. It also is corrosive and may deteriorate plumbing and discolor fixtures with a black residue.

Treatment methods for hydrogen sulfide will vary according to its concentration. When the concentration is less than one ppm (milligram per liter [mg/L]), activated carbon filtration will adsorb the hydrogen sulfide from the water and reduce the unpleasant taste. For concentrations up to two ppm, adding air to the water (aeration treatment) will reduce the dissolved hydrogen sulfide concentration by releasing it to the surrounding air. This process must be vented to a remote location (perhaps above the roof line) so household living areas are not affected by the odor. It may be necessary to “polish” the aerated water by using an activated carbon filter to further remove any remaining hydrogen sulfide taste or odor.

When concentrations exceed two ppm, it is commonly necessary to condition the water with oxidation treatments such as chlorination, potassium permanganate or ozone. The systems are similar to those used to remove iron and manganese from household drinking water. However, hydrogen sulfide oxidation processes need more oxidant to be effective. Treatment systems for hydrogen sulfide will require a longer contact time (perhaps 20 minutes) with chlorination systems, more frequent regeneration cycles (greensand resin backwashed with potassium permanganate) or the generation of more ozone with ozonation systems.

Because the oxidation process creates solid particles, the backwash cycles must be powerful enough to push the precipitated materials out of the filter. (Activated carbon filters cannot be regenerated in this way and must be replaced periodically). To increase the efficiency of hydrogen sulfide removal when concentrations are high, it is sometimes necessary to combine two or more of these methods.

Sometimes the rotten egg odor is created by sulfur bacteria that use the dissolved sulfate as an energy (food) source. The bacteria chemically change the sulfate to hydrogen sulfide gas. The bacteria prefer the hot water side of the household plumbing system but may be found in the cold water pipes too. Before installing treatment equipment, it is advisable to try to kill the sulfur bacteria with chlorine disinfection (The chlorine disinfection method is described on the Trust website and should include treating the hot water tank). If this shock treatment does not work then a continuous chlorination system may be required.

Sulfur bacteria are not detected by basic bacteria tests used by commercial and local health department laboratories to find coliform bacteria. Specific tests to identify sulfur bacteria are relatively expensive and time consuming. It may be more effective to simply disinfect the water system and observe the result rather than test for the sulfur bacteria.

Finally, a third situation may create a “rotten egg” type odor. Some older hot water tanks use magnesium anode rods to prevent corrosion. These rods sometimes react with the water to produce an odor that resembles hydrogen sulfide. If the odor you detect is only from the hot water and disinfection has not worked, call the manufacturer of your hot water tank to inquire about the type of anode rod in your heater and how the rod can be replaced with a less reactive rod.

[© American Ground Water Trust. This article may be reprinted for non-commercial educational purposes provided it is used in its entirety and that reference is made to its source as an article in THE AMERICAN WELL OWNER, 2002, Number 2]