Sulfate vs hydrogen sulfide

Best for

Anyone whose lab report mentions sulfate or sulfide, or who is trying to work out whether a bitter taste and a rotten-egg smell are the same problem.

Wrong fit

If your only symptom is a rotten-egg smell in hot water, it's the water heater, and the sulfate level matters much less.

Tradeoff

Sulfate is easy to test, but removing it from all the water in the house takes whole-house ion exchange; hydrogen sulfide is harder to test accurately but is treated with more common equipment.

Two different things

People say “sulfur water” for both, but sulfate and hydrogen sulfide behave very differently.

Sources: EPA, Virginia Cooperative Extension, UGA Extension, Penn State Extension, Indiana Department of Health.

The Indiana Department of Health notes that sulfate and sulfide can both produce a rotten-egg smell and bitter taste in practice, which is why people mix them up. Sulfate itself doesn’t smell. It smells when part of it has been turned into hydrogen sulfide.

Where each one comes from

Sulfate is everywhere in water. Connecticut’s health department and the Indiana Department of Health both say it comes from minerals in soil and rock that dissolve into groundwater. EPA’s 2003 review adds human sources, including burning sulfur-containing fuels, household wastes such as detergents, industrial discharges and some fertilizers, and notes that nearly all surface and groundwater sites sampled by USGS had detectable sulfate. Penn State Extension mentions sulfate dissolved from bedrock or from nearby coal mining. Levels high enough to matter are less common: EPA reported that only about 1.8 percent of groundwater sites in the USGS data exceeded 500 mg/L.

Hydrogen sulfide comes from decaying organic matter, from certain rock such as shale and sandstone or deposits near coal, oil or peat, and above all from bacteria that consume sulfate, according to Connecticut and Indiana. It forms in low-oxygen water, so it’s mainly a groundwater problem. Connecticut also notes that sulfur-oxidizing bacteria can work the other way and raise sulfate levels.

How sulfate turns into hydrogen sulfide

Two processes do it.

Bacteria. Sulfate-reducing bacteria live in low-oxygen water in aquifers, wells, pressure tanks and plumbing. UGA Extension explains that they convert sulfate into hydrogen sulfide, and that wells with poor wellhead protection can let surface water carry in organic matter that feeds them.

The water heater. UGA says magnesium anode rods in water heaters can convert sulfate to hydrogen sulfide, and that if this is the only source, only the hot water will smell. Penn State Extension lists high sulfate as a possible cause of a rotten-egg odor for this reason.

So a well can have plenty of sulfate and no hydrogen sulfide in the cold water, then smell strongly from the hot taps. The fix there is the water heater, not sulfate removal.

What sulfate level is a problem

Sources give different numbers because they measure different things.

EPA’s 2003 review describes sulfate’s laxative effect as temporary, with rapid recovery, and lists the sensitive groups as formula-fed infants, older people or invalids using powdered nutritional supplements, and visitors not used to high-sulfate water. Virginia Cooperative Extension says most people and livestock adjust over time, and that infants with diarrhea may become dehydrated.

If you’re mixing infant formula with well water that tests above 250 mg/L, talk to your state health department or pediatrician, and consider bottled water for formula in the meantime. For more on the health side, see is sulfur water safe.

Livestock, gardens and appliances

Sulfate matters for animals as well as people. UGA Extension says sulfate above 250 ppm is especially hard on infants and young animals, and Virginia Cooperative Extension notes that most people and livestock adjust to it over time. If you water young livestock from the well and sulfate is high, ask your county extension office about limits for that species.

Both compounds affect equipment. Sulfate contributes to scale, like other dissolved minerals, according to Virginia Cooperative Extension. Hydrogen sulfide corrodes metal: Connecticut says it attacks iron, steel, copper and brass, and Virginia says it can clog softener resin. In a softened home, UGA notes the softener can speed up the water heater anode’s breakdown, which feeds the sulfate-to-sulfide reaction in the tank.

Testing for each

Sulfate is easy. UGA Extension says it’s measured in standard lab water tests; just confirm your package includes it. Virginia Cooperative Extension says it requires a lab test and to follow the lab’s collection instructions.

Hydrogen sulfide is harder because the gas escapes from the sample. Labs use bottles with a preservative, and UGA says refrigerated samples generally must reach the lab within 48 hours. Details are on how to test for hydrogen sulfide.

Ask for both at the same time, along with iron, manganese and pH. If the H2S sample was properly preserved, a high sulfate result with a low H2S result suggests the smell, if you have one, is being made after the water leaves the aquifer: in the well, the plumbing or the heater.

Treatment is different

Virginia Cooperative Extension recommends sulfate treatment above 250 mg/L or when it causes a problem, and says distillation and RO suit drinking and cooking water. UGA Extension and Connecticut’s health department both say point-of-use distillation or RO is usually enough, with whole-house ion exchange for very high levels.

For hydrogen sulfide, the treatment-by-level table shows which method fits which concentration.

Which do you have?

• A smell, mainly in the shower or hot water: probably H2S from the water heater. Check the cold water.
• A smell at every tap, hot and cold: H2S in the groundwater, or bacteria in the plumbing. Test H2S.
• No smell, but a bitter taste or new visitors getting an upset stomach: possibly sulfate. Test sulfate.
• A lab report with high sulfate and a smell from the hot taps: both, linked through the water heater.

The free smell diagnosis tool on the homepage walks through the first two, and why water smells like rotten eggs covers each cause in detail.