A number of factors can cause tap water to smell bad, including concentrations of metal from the pipes used to deliver it, organisms living in the water, chemicals used to remove organisms living in the water, and environmental contaminants. As a general rule, tap water which smells bad is probably safe to drink if it comes from a municipal treatment plant, as the water is tested on a regular basis. If the tap water smells extremely unpleasant or comes from a well, have the department of public health test it for potentially harmful contamination. If you notice a sudden change in the scent, taste, or appearance of your tap water, notify your municipal water source immediately, as it may be an indicator of a serious problem.

In many cities, municipal tap water simply tastes bad because of the treatment process used, the pipes it travels through, and the source of the water. Running the water for a few seconds may dissipate the taste, or you can filter the tap water through a pitcher or tap filtration system to alter the taste. City water is tested on a regular basis and test results are available: if you are curious about what is in your water, you can ask the water company for recent test results. In some areas, the city may recommend that you filter water, especially if you live in an older home with questionable pipes.

Two common sources of bad taste are sulfur and chlorine. Sulfur can be naturally occurring in many areas, and sometimes peaks during certain times of the year. Sulfur accounts for the rotten egg smell that tap water can acquire, along with a bitter taste. Chlorine comes from chlorination, the most universally used water purification method. Sometimes excess amounts are used, or you end up with a high concentration of chlorine in your pipes: run the water for a few moments, and the smell and taste should go away.

Another common source of odd smells and tastes in tap water is metals from the pipes used to carry your tap water. Copper, iron, and lead have all been found in tap water, and lend unique smells and tastes. Usually copper and iron appear in low concentrations and will not be harmful, although they can discolor your water. Rusty brown water is a result of high amounts of iron, and can indicate that your pipes may need to be replaced. Lead is a more serious concern, because it can have negative impacts on human health in even small amounts; if you suspect that your water contains lead due to contaminated pipes, get it tested immediately and refrain from drinking it.

Algae blooms in your water source can also cause an odd smell. Although water treatment removes the algae and other organisms, smell and odor compounds may linger which cause the water to taste or smell “off” to your nose. You can usually eliminate these compounds by filtering the tap water, but they are not usually harmful.

Finally, seepage can cause tap water to smell bad, and can also be seriously harmful. Seepage occurs when environmental contaminants get into the water supply. Sewage does not usually seep into to tap water, since sewer lines should always be laid below water lines, but other environmental contaminants such as PCBs, mercury, lead, arsenic, and other heavy metals held in the soil can enter the water supply. If you live in an area with a high concentration of these contaminants, follow the advice of government officials when it comes to water safety to prevent serious illness.