Why Does My Water Smell Like Rotten Eggs?

Why well water smells like rotten eggs

Why does my water smell like rotten eggs? In most homes the answer is hydrogen sulfide, a gas that Penn State Extension says gives water a distinctive "rotten egg" odor. The University of Georgia (UGA) Extension notes that its common names include stink gas and sewer gas. Underground, the gas stays trapped in the water; once the water is pumped to the surface, the gas escapes, which is why a sulfur smell in well water is so easy to notice.

Where the gas comes from decides what to do about it. These are the sources the extension services name:

• Bacteria in the groundwater. Penn State Extension says hydrogen sulfide occurs naturally in groundwater through sulfur-reducing bacteria, which feed on small amounts of sulfur and thrive in the low-oxygen conditions of wells and plumbing. It is most common in wells drilled into acidic bedrock such as shale and sandstone. UGA adds that it can occur in both deep and shallow wells.
• The water heater. When only the hot water smells, Penn State Extension says the water heater may be the source: the magnesium rod fitted to protect the heater from corrosion can chemically reduce sulfates to hydrogen sulfide.
• Treatment equipment. In rare cases, Penn State Extension says, adding equipment such as a water softener gives sulfur-reducing bacteria a place to grow. UGA says a treatment system that brings a smell that was not there before usually points to sulfate-reducing bacteria inside it, which leave a black slime in ion exchange units.
• Sulfate. Sulfate itself is odorless, UGA says, and it dissolves into groundwater from natural minerals. It is the raw material the bacteria and the heater rod turn into hydrogen sulfide, and Penn State Extension lists a rotten-egg odor among the possible symptoms of sulfate, along with a bitter, medicinal taste.
• A drain, not the water. Penn State Extension notes that an odor at one sink may be caused by bacteria in the drain rather than by the water.

The strength of the smell gives a rough idea of the amount. UGA says most people can smell hydrogen sulfide at as little as 0.5 parts per million^[3], that water at 0.5 to 1 part per million^[3] smells musty or swampy, and that above 1 part per million^[3] it smells like rotten eggs and corrodes plumbing. A smell cannot give you a number, though. Only a test can.

Check the water heater and each tap first

Before you order a test, find out where the smell shows up. It narrows down both the cause and the test you need.

Timing matters too. UGA says the odor may be noticeable only when the water is first turned on or when hot water is running, and that heat forces the gas into the air, so the smell can be strongest in the shower.

A new or sudden change is a reason to test. CDC lists a change in your water's taste, color or smell among the reasons to have a well tested, and the Minnesota Department of Health (MDH) recommends testing for coliform bacteria any time the water changes in taste, odor or appearance.

The tests that confirm it

The box below opens the test finder. Tell it you noticed a rotten-egg smell and it adds hydrogen sulfide and sulfate to your plan. Test for hydrogen sulfide with care. Penn State Extension says the test must be done in the home, or the sample chemically stabilized before it goes to a lab, and suggests asking state-accredited labs whether they test for it. UGA says most labs provide special sample bottles that contain the stabilizing chemical. Because that chemical often interferes with other tests, a separate sample is needed for them, and UGA says a refrigerated sample usually has to reach the lab within 48 hours^[3] of collection. Sulfate, UGA says, is easily measured in a lab, so make sure any test package you choose includes it.

Is it harmful? Health, nuisance and a sewage smell

Mostly a nuisance. Penn State Extension calls hydrogen sulfide in water an aesthetic concern that gives water a disagreeable taste and odor. The gas is poisonous and flammable, but the human nose detects it well before it causes health concerns, and there is no drinking water standard for it because it makes water undrinkable long before it reaches harmful concentrations. UGA says drinking water with high levels can cause nausea and stomach pain, yet that it is highly unlikely anyone could drink a harmful dose, because the taste and odor make the water unpalatable first. Both extension services say the sulfur- and sulfate-reducing bacteria themselves do not harm human health. A water test cannot diagnose anything; if you feel unwell, talk to a health professional.

EPA's yardstick. EPA's secondary standards list a "rotten-egg", musty or chemical smell under odor, and set a secondary standard for sulfate of 250 mg/L^[5]. EPA does not enforce these standards. They are guidelines for taste, color and odor, and EPA says the contaminants they cover are not considered to present a risk to human health at those levels.

Damage to plumbing. Penn State Extension says hydrogen sulfide can corrode the metals in a plumbing system and leave yellow or black greasy stains on fixtures or inside pipes. UGA calls it extremely corrosive to iron, steel, copper and brass.

No smell is not a clean result. MDH says you cannot taste, see or smell most contaminants, and that testing is the only way to detect most of them.

When well water smells like sewage

A sewage smell is often the same gas: UGA lists sewer gas among the common names of hydrogen sulfide. Two other causes are worth ruling out, and both point to a bacteria test.

• Iron bacteria. MDH lists sewage among the odors iron bacteria can cause, along with swampy, oily, cucumber, rotten vegetation and musty smells, and says they may be more noticeable after the water has not been used for a while. Iron bacteria are not known to cause disease, MDH says, but they can create conditions where disease-causing organisms grow, so it advises testing the water for nitrate and coliform bacteria.
• Sewage pollution. In rare cases, UGA says, a hydrogen sulfide odor comes from sewage pollution, particularly in poorly constructed or shallow wells near leaky sewer lines or septic drainfields. Sewage can also bring coliform bacteria and nitrate into the water, so if you suspect it, UGA says to test for both. Start with a water bacteria test and a nitrate water test.

How to get rid of the smell once a test confirms the cause

Fix the cause the test confirms, not the smell. Penn State Extension says results from a state-accredited laboratory should be used to confirm the cause and its severity, and that testing may turn up other problems the obvious symptom hides. Our guide to how to read your lab report explains the numbers.

"No treatment needed" is a valid outcome. If the smell came from one drain, Penn State Extension says disinfecting the drain removes it, and the water itself needs nothing. Hydrogen sulfide has no health-based standard, so treating a faint smell you can live with is a choice about taste, not a health requirement.

Hot water only: the water heater rod. Penn State Extension says removing the heater's magnesium rod can often stop the smell, but that removal could increase corrosion, shorten the heater's life and will likely void the manufacturer's warranty; replacing it with an aluminum rod should eliminate the odor while keeping the corrosion protection. UGA says to replace it with an aluminum or zinc rod, preferably by a licensed plumber, and to use only aluminum if the water is kept above 150°F^[3]. This is a job to call a plumber for.

Hot and cold water: treatment matched to the measured level. Penn State Extension says the most efficient option depends mainly on the hydrogen sulfide concentration, and that most systems treat all the water entering the home. The options it describes:

• Activated carbon filtration, for small concentrations.
• Aeration, which is not normally used at high concentrations.
• Oxidizing filters such as manganese greensand, for moderate concentrations.
• Continuous chlorination or potassium permanganate followed by filtration, which suit high concentrations.
• Shock chlorination of the well to kill the sulfur-reducing bacteria. Penn State Extension says that in most cases the bacteria and the odor return quickly afterward.

Before you buy, Penn State Extension advises comparing units and prices from several dealers, understanding each unit's maintenance, and getting a written warranty.

A sewage smell or iron bacteria. Act on the bacteria and nitrate results first. For iron bacteria, MDH says treatment can be difficult and only partly effective, and suggests a licensed well contractor or water treatment professional to find the best approach.