A sewage or musty smell at a tap is alarming and it is frequently not the water at all. The most common cause is bacterial growth in the drain, which releases odour as water disturbs it and reaches your nose at the same moment the tap runs. Separating that from a genuine contamination problem takes one simple test and it matters a great deal.
The glass test that separates drain from water
Fill a glass from the tap, then take it to another room, ideally outside, and smell it there. If the water in the glass smells fine away from the sink, the odour was coming from the drain rather than the water: bacteria growing in the tailpiece and overflow release gas as running water disturbs them. If the glass still smells in another room, the odour is in the water and the investigation continues at the well.
Cleaning a smelly drain
The growth lives in the tailpiece, the overflow channel of a basin and the disposer if there is one, all of which are wet, warm and full of organic material. Flushing with hot water, cleaning the overflow with a brush and a suitable cleaner, and cleaning the disposer per its instructions usually clears it. The overflow is the part most often missed, and it is a common reason a smell returns after a bathroom basin has apparently been cleaned thoroughly.
If the water itself smells
Distinguish it from the rotten egg smell of hydrogen sulfide, which is sulfur and has its own causes. A genuinely sewage like or musty smell in the water suggests either bacterial growth in the plumbing or well, or septic influence reaching the source. The second is serious: it belongs with a coliform test, a nitrate test and an inspection of the well's construction, since septic influence usually means surface water or effluent is reaching the casing or the aquifer near it.
What to check at the well
A cracked or short casing, a missing or damaged sanitary cap, a well pit that floods, and ground sloping towards the wellhead rather than away all let contamination in. Distance and elevation relative to a septic field matter. The EPA notes private wells are not federally regulated under the Safe Drinking Water Act, and a USGS study of 2,100 private wells found about one in five carried at least one contaminant above a human-health benchmark. Test before treating. Our well filtration guide covers what follows a result.
Questions people ask about why does my well water smell like sewage
How do I tell if it is the drain rather than the water?
Fill a glass and smell it in another room. If it smells fine away from the sink, the odour was from the drain.
What causes a smelly drain?
Bacterial growth in the tailpiece, the basin overflow and any disposer. The overflow is the part most often missed when cleaning.
Is a sewage smell the same as rotten eggs?
No. Rotten egg smell is hydrogen sulfide, which is a sulfur problem. A musty or sewage smell suggests growth or septic influence.
What should I test for?
Coliform bacteria and nitrate, alongside an inspection of the casing, cap and the ground around the wellhead.