We recently (about 8 months ago) had a 475 foot well dug drilled on our property. We are building a home slowly and are only just putting in a hot water heater this week, so we know it’s not an issue with a water heater. The water has always tasted great, but this past month it is sulphuric. We were thinking that it would pass and that it was just water that had been sitting in the tank, but unfortunately the smell has persisted. Any ideas why our great tasting well water would now smell like this and what to do? It was an incredibly expensive well as they had to dig really deep and then hydro frack.

We have no plumbing in. It’s a well with a pump to the water tank which is housed in our dried in house foundation. We have a potable water drinking hose off the pipe right now. We are doing plumbing this week so the system is very simple right now. As stated, water was always great until recently. Coming out of winter here in VT, but it started before the snow thawed. No gas set up here. We use wood and electric. I understand that many people have sulfuric well water- the issue is that it just happened suddenly, we had months of non sulfuric water.

  • Some more information about your well, tank, filtration system, and the rest of your plumbing system (if any exist yet) would help as well. Commented Apr 11, 2023 at 20:15
  • Where do you live ? are you sure it is not gas leak, any volcano near by
    – Traveler
    Commented Apr 11, 2023 at 20:53
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    It is not uncommon to have the sulfer smell from well water. It depends on where you are and what aquifer you are drawing water from. I have had classmates that were less than a mile apart where one had sulfur the other did not. This was in Ohio.
    – Gil
    Commented Apr 11, 2023 at 21:58
  • 5
    A side note: Please get someone to independently check it smells like sulfur - Covid/long covid has significant smell or taste changes, some of which are bad water smells. It's simple to rule out, and might save you a bunch of money
    – lupe
    Commented Apr 12, 2023 at 9:11
  • 2
    OT: Rotting animal couscous sounds rather nasty.
    – IconDaemon
    Commented Apr 12, 2023 at 16:33

3 Answers 3


Terminology first: Your well was drilled, not dug. No one in Vermont would dig a domestic water well deeper than about 30 feet. You probably have a steel well casing about 6 inches in diameter sticking out of the ground, right? That’s a drilled well.

The odor is from hydrogen sulfide, a gas that is produced by anaerobic sulfur bacteria which metabolize sulfates and iron in your water and release the gas as a waste product. The bacteria themselves are totally harmless to humans. These bacteria generally do not exist at great depths until and unless they are introduced by human activity such as well-drilling or debris falling into a well. Once introduced and allowed to thrive unchecked, these bacteria can eventually spread over months or years for great distances through the water bearing strata to other properties.

You did not initially have rotten-egg water because there were no sulfur bacteria in the aquifer until your well was drilled and the bacteria were introduced to that depth. They took some time to multiply and now you can smell their waste product.

Next issue: did the well driller install a sanitary well cap? This is supposed to let air in and out of the well pipe but prevent bugs and vermin from getting into the well. It should be bolted to the top of the well casing, not just resting on top. Failure to install a sanitary cap is a violation of well drilling regulations in some states.

Question: Did the well driller sanitize the well before leaving the job site? Every state has its own standards. I don’t know Vermont’s regulations, but I’m your neighbor in New Hampshire and it’s a requirement here. The mechanics of drilling a well always introduces bacteria into the water-bearing stratum, or aquifer. It’s unavoidable. The well driller, when they finish the job and just before they cap the well, is supposed to add a measured amount of bleach or another sanitizer, circulate the water for a time, then pump out the well until there is no detectable bleach. A licensed well driller who fails to perform this important step is risking his license because it is in the State’s best interest to prevent bacterial contamination of water bearing strata.

If you can’t get your well driller to remediate this situation, you should consider reporting them to the licensing agency in your state.

You can remediate the sulfur bacteria yourself using bleach. You’ll need to know the diameter of the well pipe, the depth of the well and the depth of the water surface in the well. That will determine how much bleach to use. It will probably be a few gallons. Please look up instructions for sanitizing a well from a reputable source such as a state agency (any state) or a university. Don’t follow instructions from some random guy on the internet like me. (!)

WARNING WARNING: Do not use scented bleach, or no-splash bleach (which contains detergent) or anything but regular bleach.

The instructions will have you pour bleach down the well, then you’ll connect a hose from your water system’s spigot, open the valve and direct the pump’s flow back down the well. You’ll run the water that way for hours. Don’t worry about your pump burning out; continuous running does not harm a well pump.

The instructions may have you run bleach-y water through your plumbing also, before you shut off the hose’s valve and leave everything to sit for a certain recommended time. This time is the chlorine’s “contact time” which is needed to be sure all the beasties are dead. During this time, some of the bleach-y water at the bottom of the well will diffuse into the aquifer. Hopefully, it will kill any sulfur bacteria that have spread into the aquifer from the well.

Finally, the instructions will have you open the hose’s valve again, but direct the water onto the ground, not back into the well. The well should have its sanitary cap reinstalled at this time to keep it free of contaminants. After several hours of pumping, you can check the water and see if the chlorine odor has dissipated. If not, keep pumping until the water is chlorine-free. You could use a chlorine test kit if you wish, but the nose knows.

Remember to flush out your plumbing too. You may get some black water if there are any pipes involved. Just flush till it’s clear and odor free.

You’ll probably have delicious water again, at least for a time. If the sulfur odor comes back, you didn’t kill all the beasties, or they have spread so far that they can’t be killed.

If that happens, your water can be treated to remove the hydrogen sulfide. This involves a non-pressure tank to hold several months’ worth of a weak bleach solution, a metering pump to inject a very small amount of bleach solution into the water stream as you use it, a large pressurized contact tank where the water will spend some time in contact with the chlorine so that the hydrogen sulfide can be oxidized, and then a backflushing activated carbon tank to absorb any excess chlorine. This is a proven system that is low-maintenance and works very well. It will give you good-tasting water again.

But before you use a carbon filter for any purpose, you must test your water for radon. Carbon filters in high-radon water quickly become radioactive and must be treated as hazardous waste. The good news is that radon-in-water removal, if needed, is highly effective. My water has 80 times the state limit before treatment but no detectable radon after treatment. Don’t panic if you have radon.

I hope you have a better understanding of your stinky water now!

EDIT: Ecnerwal has provided a link to Vermont's web page for hydrogen sulfide in well water.

Vermont well sanitizing instructions. (Includes a chlorine calculator.)

Procedure (video)

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    Thank you! This information is extremely helpful and exactly what I was hoping for. And yes- the well was drilled, of course. Edited that now. One more question- if the folks who drilled the well (a very reputable company here) did sanitize the well and they did put the correct cap on (haven’t asked them these questions yet), would it still be a common/reasonable occurrence for this bacteria to have found its way into the water just from drilling? Hopefully we have caught it in time before having to install a whole new chlorination system.
    – RTown
    Commented Apr 12, 2023 at 2:52
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    @RTown Yes, it's a reasonable assumption that the anaerobic bacteria came from drilling the well and from hydrofracking. As the drill bit passes through all the many layers of rock, sediment, gravel, sand, etc., it can pick up bacteria and deliver them to the deepest part of the bore. Then there's the hydrofracking fluid that literally forces liquid pumped from the surface into the fissures in the rock. Bacteria will go along for the ride.
    – MTA
    Commented Apr 12, 2023 at 4:10
  • "...anything but regular bleach", actually, isn't this a job for "pool shock", i.e. higher-concentration chlorine that's usually used to disinfect pools? (It's also used to... disinfect well water.)
    – Matthew
    Commented Apr 12, 2023 at 19:48
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    @Matthew Regular bleach is always available everywhere, it's less hazardous and it works just fine. Vermont recommends measuring the quantity of bleach called for, then diluting it so as to prevent corrosion of the pitless adaptor. Pool shock would likewise need to be diluted, so there's no advantage. All they want is 200 ppm for coliform, a bit higher for sulfur bacteria, and regular bleach achieves that easily.
    – MTA
    Commented Apr 12, 2023 at 19:59
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    @fraxinus They don't eat metal pipes or well casings. They "breathe" sulfates dissolved in water as we breathe oxygen and they produce hydrogen sulfide as a waste product as we produce CO2. They existed on earth before the atmosphere had any oxygen, and they can only thrive in an oxygen-free environment. More info: en.wikipedia.org/wiki/Sulfate-reducing_microorganism
    – MTA
    Commented Apr 13, 2023 at 13:11

It is very uncommon for wells driven deep for the "supply" to change. You get what you initially get for a very long time.

What is far more likely is you got a anaerobic sulfur colony someplace in your plumbing. I've fought those for years at my place (well water). When the pump company came out to treat it, they poured bleach into the well and we turned on all the faucets until we got a bleach smell from each one. We didn't do the hot water bc it didn't seem affected and it would taken forever to both get enough bleach into the WH and later to get rid of it. Cold water was mostly affected.

Anyway , you let it sit for a few hours and then turn on the cold water faucets. If you get discolored water (grey-black), you had a anaerobic sulfur colony. In my case, after the bleach treatment, the water came out nearly black! Gross!

  • Yes but we don’t have any plumbing installed!
    – RTown
    Commented Apr 12, 2023 at 2:28
  • OK, But, the anaerobic sulfur colony could be living in the pipe coming up the well. Commented Apr 12, 2023 at 3:53
  • Yes, apparently so. Have you had to shock your water system/well more than once?
    – RTown
    Commented Apr 12, 2023 at 3:57
  • Yes, several times at first, then I haven't had to do it for several years. Commented Apr 12, 2023 at 3:59

Presumably you mean "rotten egg" smell, i.e. hydrogen sulfide.

Either you have a seasonal change in where the water in your aquifer is being drawn from, or hydrogen sulfide bacteria have grown in your well.

Possibly (hopefully not if your well driller is competent) the seal around the well casing is bad and surface water has infiltrated.

You can attempt to move the bacteria out again via shocking the well with chlorine, but if they are coming along with wherever the water is coming from now, they will repopulate. In that case you may need more continuous treatment to deal with them.


  • Thank you very much. Is it common for the water to shift and draw from a different area?
    – RTown
    Commented Apr 12, 2023 at 1:57
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    Depends on the hydrography of the specific location (which is often "unknown until you put a well into it," practically speaking.) Seasonal variations are not uncommon. Groundwater starts as surface water somewhere, somewhen, and then filters into the ground and travels around.
    – Ecnerwal
    Commented Apr 12, 2023 at 2:07

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