Sources of sulfate and hydrogen sulfide in drinking water
Sulfates are a combination of sulfur and oxygen and are a part of naturally occurring minerals in some soil and rock formations that contain groundwater. The mineral dissolves over time and is released into groundwater. Sulfate minerals can cause scale buildup in water pipes similar to other minerals and may be associated with a bitter taste in water that can have a laxative effect on humans and young livestock. Sulfate can make cleaning clothes difficult.
Hydrogen sulfide 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 also can enter surface water through springs, although it quickly escapes 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, which use sulfur as an energy source, are the primary producers of large quantities of hydrogen sulfide. These bacteria chemically change natural sulfates in water to hydrogen sulfide. Sulfur-reducing bacteria live in oxygen-deficient environments such as deep wells, plumbing systems, water softeners, and water heaters. These bacteria can flourish on the hot water side of a water distribution system. Hydrogen sulfide gas produces an offensive “rotten egg” or “sulfur water” odor and taste in the water. In some cases, the odor may be noticeable only when the water is initially turned on or when hot water is run. Heat forces the gas into the air, which may cause the odor to be especially offensive in a shower. A nuisance associated with hydrogen sulfide includes its corrosiveness to metals such as iron, steel, copper and brass. It can tarnish silverware and discolor copper and brass utensils. Hydrogen sulfide also 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, and the appearance and taste of cooked foods can be affected.
Occasionally, a hot water heater is a source of hydrogen sulfide odor. The magnesium corrosion control rod present in many hot water heaters can chemically reduce naturally occurring sulfates to hydrogen sulfide.
A problem that can result from sulfate in water is sulfur-oxidizing bacteria. These nonpathogenic (not health-threatening) bacteria convert sulfide into sulfate, producing a dark 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. Sulfur-oxidizing bacteria are less common than sulfur-reducing bacteria.
Potential health effects of sulfate and hydrogen sulfide in drinking water
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. Usually it is not a health risk at concentrations present in household water. Atmospheric hydrogen sulfide concentrations can be elevated when water with hydrogen sulfide is released into confined areas.
Sulfur-oxidizing bacteria pose no known human health risk. Sulfur-reducing bacteria pose no known health risk.
Testing for sulfate and hydrogen sulfide in drinking water
The quality of water supplied by public water systems is regulated by the U.S. Environmental Protection Agency (EPA). Sulfate is classified under the Secondary Maximum Contaminant Level standards, which are based on aesthetic factors such as taste, odor, and staining properties of water, rather than health effects. The standard in drinking water for sulfate is 250 milligrams per liter (mg/l), sometimes expressed as 250 parts per million (ppm). Secondary standards and guidelines and are not enforced.
Hydrogen sulfide is not regulated by the EPA. A concentration high enough to be a drinking water health hazard also makes the water unpalatable. The odor of water with as little as 0.5 ppm of hydrogen sulfide concentration is detectable by most people. Concentrations less than 1 ppm give the water a “musty” or “swampy” odor. A 1-2 ppm hydrogen sulfide concentration gives water a “rotten egg” odor and makes the water very offensive.
Consumers of private drinking water can have water tested for sulfate through laboratory analysis. The rotten-egg odor of hydrogen sulfide gas generally makes testing unnecessary. In addition, the gas readily dissipates when water is exposed to the atmosphere.
Options for sulfate and hydrogen sulfide in drinking water
Secondary standards for drinking water contaminants are established as guides to manage aesthetic properties of water. Drinking water suppliers are not required by federal law to meet these secondary standards. If sulfate levels in drinking water approach or exceed the standard, some public water suppliers voluntarily reduce or remove sulfate from the water.
If excessive sulfate or hydrogen sulfide is present in private drinking water, consumers can obtain an alternate water supply or use some type of treatment to remove the impurity.
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. Another alternate source is bottled water that can be purchased in stores or direct from bottling companies. This alternative might be considered when the primary concern is water for food preparation and drinking. Several methods of removing sulfate from water are available. The treatment method selected depends on many factors including the level of sulfate in the water, the amount of iron and manganese in the water, and if bacterial contamination also must be treated. The best option also depends on how much treated water is needed.
Hydrogen sulfide formation may be reduced in some instances. Performing a shock chlorination procedure may reduce, but does not eliminate, the sulfur reducing bacteria.
If hydrogen sulfide odor is associated primarily with the hot water system, a hot water heater modification may reduce the odor. Replacing the water heater’s magnesium corrosion control rod with one made of aluminum or another metal may improve the situation.