Problems you can see

Why tap water smells like bleach, sulfur or sewage

Smell is the most useful sense you have for tap water, because different causes produce recognizably different smells.

Start by locating the smell

Before identifying what the smell is, establish where it comes from. Three questions do most of the work.

1. Does it follow the glass? Fill a glass, move to another room, and smell it. If the smell disappears, it is coming from the drain, not the water.
2. Hot tap, cold tap, or both? Hot only points at the water heater. Both points at the supply.
3. One faucet or all of them? One faucet suggests a fixture or a branch of your plumbing. All of them suggests the incoming supply.

Bleach or a swimming pool

This is chlorine or chloramine, added deliberately to keep water free of pathogens all the way through the distribution system. It is not a fault. Levels are usually higher near the treatment plant and in cooler months, and utilities sometimes switch temporarily from chloramine to free chlorine to clean the system, which makes the smell more noticeable for a few weeks.

The federal limit for both disinfectants is 4.0 mg/L, and most systems run well below it. Refrigerating water in an open jug for a few hours reduces the smell of free chlorine considerably. Chloramine is more stable and does not dissipate the same way; removing it needs catalytic carbon. We cover the trade-off in chlorine and chloramine in tap water.

Rotten eggs or sulfur

Hydrogen sulfide. Where it comes from depends on which tap.

• Hot water only. Almost always the water heater. The magnesium anode rod that protects the tank from corrosion can react with sulfate-reducing bacteria to produce hydrogen sulfide. A plumber can replace it with an aluminum or zinc rod.
• Both taps, on a private well. Common and usually geological. Groundwater moving through certain formations picks up sulfur compounds naturally.
• Both taps, on municipal water. Less common and worth reporting, since it can indicate stagnation somewhere in the distribution system.

Hydrogen sulfide is unpleasant long before it is hazardous. At the concentrations that make water undrinkable it is a nuisance rather than a health risk.

Musty, earthy or like a lake

Two compounds, geosmin and 2-methylisoborneol, produced by algae and bacteria in surface water reservoirs. The human nose detects them at a few parts per trillion, which is why a quantity far too small to matter is immediately obvious.

They are not harmful and they are not removed by conventional treatment, because removing something at that concentration is disproportionate to the problem. Utilities on surface water often see complaints in late summer when algal activity peaks. A carbon filter removes the smell easily.

Sewage

Usually not the water. Organic matter builds up in the drain and in the overflow opening of a sink, and the bacteria growing there produce gases that you smell as the water disturbs them. The glass test confirms it in seconds.

If the smell does follow the glass, stop drinking the water and contact your utility. A genuine sewage smell in the supply is rare and warrants immediate investigation.

Metallic

Iron, copper, manganese or zinc dissolving from plumbing, sometimes with low pH accelerating it. Iron and manganese are aesthetic problems rather than health ones, and often come with discoloration.

The one to take seriously is the context rather than the taste. Water aggressive enough to dissolve copper from your pipes is water that can dissolve lead from solder and older service lines, and lead has no taste at all. In housing built before 1986, a persistent metallic taste is a reason to test at the tap rather than to buy a filter.