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Sulfur is a common element and essential nutrient found in several chemical forms in nature. Sulfate and hydrogen sulfide are two of these forms. They are not a particular health hazard, but can cause problems with odor, taste, or use of a domestic water supply.
Sulfates (SO4-2) are combinations of sulfur and oxygen and are part of naturally occurring minerals. Sulfate minerals can be found in aquifer formations that contain ground water. Sulfates are released over time into the ground water as these minerals dissolve.
Hydrogen sulfide (H2S) gas also occurs naturally in some groundwater. It is formed from decomposing underground deposits of organic matter, such as decaying plant material. It is found in deep or shallow wells and can enter surface water through springs. It does quickly escape to the atmosphere. Hydrogen sulfide often is present in wells drilled in shale or sandstone, or near coal or peat deposits or oil fields.
Sulfur-reducing bacteria are a group of microbes which use sulfur as an energy source. These bacteria chemically change natural sulfates in water to hydrogen sulfide. If populations develop adequately, they can produce large quantities of hydrogen sulfide.
Sulfur-reducing bacteria live in oxygen-deficient environments such as deep wells, plumbing systems, water softeners, and water heaters. They are not usually found in free-flowing water.
They are typically part of a microbial community with other organisms, growing deep within biofilms or slimes where anaerobic (oxygen-free) conditions may exist. These bacteria can flourish on the hot water side of a water distribution system.
Sulfur-oxidizing bacteria are nonpathogenic (not health-threatening) bacteria that convert sulfide into sulfate, producing a dark slime. Sulfur-oxidizing bacteria are less common than sulfur-reducing bacteria.
Sulfate may have a laxative effect that can lead to dehydration and is of special concern for infants. With time, most individuals will become acclimated to the sulfate and the symptoms disappear.
Hydrogen sulfide is flammable and poisonous. It is usually not a health risk at the low concentrations typically present in household water, unless it is concentrated in confined areas.
Sulfur-oxidizing bacteria pose no known human health risk. Sulfur-reducing bacteria pose no known health risk, but can cause staining, blackening of equipment from sulfides, slime formation, fouling, and the start of corrosive processes.
Dissolved sulfate can cause scale buildup in water pipes similar to other minerals. Sulfates can give water a bitter taste and can also have a laxative effect on humans and young livestock. Sulfate can make cleaning clothes difficult.
Sulfur-reducing bacteria may form slimes and films that can interfere with operation of plumbing systems. These bacteria can produce large quantities of hydrogen sulfide if populations develop to high levels.
Hydrogen sulfide gas produces an offensive “rotten egg” or “sulfur water” odor and taste in the water. The odor may be noticeable only when the water is initially turned on or when hot water is run in some cases. Heat forces the gas into the air, which may cause the odor to be especially offensive in a shower.
Hydrogen sulfide can be corrosive to metals such as iron, steel, copper and brass. It can tarnish silverware and discolor copper and brass utensils.
Hydrogen sulfide can cause yellow or black stains on kitchen and bathroom fixtures. Coffee, tea and other beverages made with water containing hydrogen sulfide may be discolored. The appearance and taste of cooked foods can be affected when using water with hydrogen sulfide.
A hot water heater can occasionally be a source of odor. The magnesium corrosion control rod present in many hot water heaters can chemically reduce naturally occurring sulfates to hydrogen sulfide.
Sulfur-oxidizing bacteria can produce slime that can clog plumbing and/or stain clothing. Blackening of water or dark slime coating the inside of toilet tanks may indicate a sulfur-oxidizing bacteria problem.
Sulfate is classified under the Secondary Maximum Contaminant Level (SMCL) standards, which are not based on health effects, but rather than on aesthetic factors such as taste, odor, and staining properties.
The sulfate SMCL in drinking water is 250 milligrams per liter1 (mg/l SO4). See Table 1 and rankings. There are no enforcement actions if a public water supply exceeds the SMCL, but it may not be a desirable situation.
Hydrogen sulfide is not regulated. A concentration high enough to be a drinking water health hazard also makes the water unpalatable. Most people can detect an odor in water with as little as 0.5 mg/L H2S.
Hydrogen sulfide concentrations less than 1 mg/L give the water a “musty” or “swampy” odor. Concentrations of 1 to 2 mg/L H2S gives water a “rotten egg” odor, making the water very offensive and unpalatable.
Sulfate concentrations are determined by approved drinking water methods, like ion chromatography.
The rotten-egg odor of hydrogen sulfide gas generally makes testing unnecessary. The H2S gas readily dissipates when water is exposed to the atmosphere so any testing would have to be done on-site.
The relative populations of sulfate-reducing bacteria can affect management and treatment choices. Population numbers are expressed as the number of “colony forming units per milliliter (CFU/ml)”. Biological Activity Reaction Test (BART) biodetectors are used to identify the relative presence and populations of sulfate-reducing bacteria. See Table 2 and rankings.
If sulfate levels in drinking water approach or exceed the SMCL of 250 mg/L SO4, some will voluntarily reduce or remove sulfate from the water supply.
If excessive sulfate or hydrogen sulfide is present, consumers can choose obtain an alternate water supply or use some type of treatment.
It may be possible to obtain a satisfactory alternate water supply by drilling a new well in a different location or a deeper well in a different aquifer.
Purchasing bottled water is an option when the primary concern is water for food preparation and drinking. Water with high sulfate or hydrogen sulfide can be used for other purposes, although it may be an undesirable alternative.
Several methods of removing sulfate from water are available. The appropriate treatment method depends on many factors including the sulfate level, the iron and manganese levels, and if bacterial contamination must also be treated. The best option also depends on how much treated water is needed for domestic use.
Options for treating small quantities of water with sulfate include distillation and reverse osmosis. The most common method of treating large quantities of water is ion exchange.
Hydrogen sulfide formation may be reduced in some instances. Performing a shock chlorination procedure may reduce, but does not eliminate, the sulfur reducing bacteria.
Modifying the hot water heater may reduce the hydrogen sulfide odor. Replacing the water heater’s magnesium corrosion control rod with one made of aluminum or another metal may improve the situation.
Hydrogen sulfide may be removed with activated carbon filters, oxidizing filters, or chemical oxidation and filtration, but this depends on the concentration.
USDA Extension. 2019. Drinking Water Contaminant – Sulfur, hydrogen sulfide. accessed 06July2022. https://drinking-water.extension.org/drinking-water-contaminant-sulfur-hydrogen-sulfide/
Bradshaw & Powell. 2000. Bull. MF-912. Understanding Your Water Test Report. Kansas State Univ. Coop. Ext. Svc., Manhattan KS 12 pg.