If your well water smells like rotten eggs, you are smelling hydrogen sulfide gas, and it comes from one of three places: the well itself, sulfate-reducing bacteria in your water heater reacting with the magnesium anode rod, or sulfur and iron bacteria growing in the plumbing. Two questions sort out which one you have. Does the smell come from the hot tap only? Then it is the water heater, and the fix is an anode swap and a tank flush, not a filter. Does it come from the cold tap too? Then it is the well or the plumbing, and you are looking at an oxidizing filter or ozone system ahead of the softener.
Everything else in this article hangs on those two questions. I run the free in-home water tests for Midwest Water Solutions out of our shop just east of DeWitt, and the rotten-egg call is the one where I most often tell a homeowner they do not need what they were about to buy.
The two-question test
Run the cold water at the kitchen sink for a full minute and smell it. Run the hot water at the same sink and smell it. Then check a toilet tank and an outside spigot.
- Hot only, cold clean: water heater. Skip to the next section.
- Hot and cold, every tap, all the time: the well. The sulfide is coming up with the groundwater.
- Hot and cold, but strongest first thing in the morning or after you have been away, fading after a minute of running: bacteria in the plumbing, pressure tank or well casing. Lift the toilet tank lid and look for gray, black or reddish-brown slime.
- Only at one fixture: a dead leg of pipe, a fouled aerator or a drain trap. Pour a glass, carry it into another room and smell it there before you blame the well.
Five minutes of sniffing decides whether the answer costs less than a tank of gas or is a whole-house system.
The water-heater case: hot only, not a well problem
Every standard tank water heater carries a sacrificial anode rod, usually magnesium, that corrodes so the steel tank does not. As it corrodes it releases hydrogen. Sulfate-reducing bacteria, harmless and common in groundwater, thrive in warm tank water and use that hydrogen to convert the natural sulfate in your water into hydrogen sulfide. Cold taps stay clean because the bacteria need the heat.
The wells around here feed this reaction well. DeWitt's deep Jordan wells run 80 to 90 ppm sulfate and Clinton's about 57 ppm in their published data, and rural dolomite wells carry sulfate too. Turn the heater down to save money, let a guest bath sit unused for a month, and the colony has everything it needs.
The fix, in order: flush the tank to clear sediment and the bacterial film; sanitize it, most often with a few pints of ordinary 3 percent hydrogen peroxide per 40 gallons left to sit for a couple of hours (a chlorine solution also works); then replace the magnesium rod with an aluminum-zinc anode or a powered anode, neither of which feeds the reaction. National price guides put an aluminum-zinc rod at roughly $20 to $60 for the part plus a plumber's visit, and powered anodes at a few hundred dollars for the part alone.
Pulling the anode out and leaving it out is the shortcut you will read about online. It does stop the smell. It also lets the tank rust from the inside and usually voids the heater's warranty, so it only makes sense on a heater that is already old. And if the smell returns within a few weeks of a proper anode swap, bacteria are being reintroduced from the well, and you are back to the well case.
The well case: all taps, all the time
When cold water carries the odor, the sulfide is in the groundwater before it reaches the house. Sulfide forms where water sits in rock or sand with little dissolved oxygen and some sulfate or organic matter for bacteria to work on, and that describes a lot of the wells between DeWitt and the Mississippi.
The Silurian dolomite that most drilled wells on the Clinton County uplands tap runs low in oxygen, which is why iron stays dissolved until it hits air and why Eldridge's raw well water carries ammonia the city has to aerate out. The same chemistry that keeps iron and manganese in solution keeps sulfide in solution. On the Illinois side, the shallow sand-and-gravel wells in the Mississippi bottoms around Fulton draw water that the City of Fulton's own raw Well 4 shows at 1.3 to 1.5 mg/L iron and about 0.6 mg/L manganese, plus arsenic above the federal limit, all of which the city's iron-filter plant removes before it reaches town. A private well on the same sands has no plant in front of it. The deep Jordan sandstone wells are a different aquifer with different baggage (sodium, radium), and sulfide there varies well to well; there is no useful county-wide number, which is exactly why we test at the tap.
One practical note: hydrogen sulfide gasses off quickly, so a bottle mailed to a lab often reads lower than what you smell. Sulfide is best checked on site, and our free in-home test does that along with iron, manganese, hardness, pH and TDS, because those results decide which system you need.
The bacteria case: slime in the toilet tank
Iron bacteria and sulfur bacteria are not a health threat, but they build slime colonies on everything the water touches: well casing, pump, pressure tank, toilet tank, softener resin. Iron bacteria leave rusty or reddish-brown gel; sulfur bacteria leave gray, black or white slime and the stronger odor. Both make water smell worse after it sits, which is the morning-only pattern.
Shock chlorination of the well is the standard first step: a measured dose of unscented bleach down the casing, circulated through the house lines, held overnight and flushed. The Whiteside County and Clinton County health departments publish the procedure, and it is the same routine they recommend after a well has been under floodwater, which the river-bottom acreages outside Fulton's levee have seen in 1965, 1993, 2001, 2019 and 2023. Have a coliform test run afterward; Clinton and Scott County residents get that free through their county health departments, while Whiteside County charges about $35.
Why it comes back: chlorine kills what it reaches, and the bacteria live in the aquifer, the gravel pack and the corners of the pressure tank, so one shock usually buys months, not years. When a well needs shocking every season, the honest fix is continuous oxidation, and this is where ozone earns its keep over a plain air-injection filter, because ozone kills the bacteria rather than just oxidizing the sulfide they produce.
Is it dangerous?
At the concentrations typical of household wells, hydrogen sulfide is an aesthetic problem. There is no federal drinking-water limit for it, and people smell it at a fraction of a part per million, far below any level that matters to your health. Three reasons to fix it anyway:
- It corrodes plumbing. Sulfide water attacks copper, brass and steel, blackens silverware and stains fixtures dark.
- It wrecks softeners. Sulfide and bacterial slime foul ion-exchange resin, and the iron that rides along coats the beads, which is why a softener on an untreated sulfide well loses capacity year by year.
- It is a hint. A sulfide well is a low-oxygen well, and low-oxygen wells here usually carry iron and manganese and sometimes bacteria. A sudden new odor after a flood, a pump replacement or well work is a reason to test for coliform before drinking the water.
What actually removes it, and why a softener does not
Every real fix works the same way: oxidize the dissolved gas into solid sulfur particles, then filter the particles out. The options differ in how hard they oxidize and how much upkeep they need.
Air-injection oxidizing filter. A single tank with a pocket of air at the top and a bed of catalytic media. Water passes through the air, sulfide and dissolved iron convert to solids, the media catches them, and the unit backwashes to drain on a schedule. No chemicals, minimal maintenance, and it handles low to moderate sulfide and several parts per million of iron at once. This is the standard tool on our iron filter installs, and for the ordinary "some iron, a bit of smell" well on the Clinton County uplands it is usually all that is needed. National cost guides put an installed whole-house air-injection iron and sulfur filter at roughly $1,500 to $3,500 depending on flow rate and media.
Ozone injection. Same principle with a far stronger oxidizer. An ozone generator injects gas into the water ahead of a contact tank and filter; ozone converts sulfide and iron faster and more completely than air, and it kills iron and sulfur bacteria on contact, which air cannot do. It is the right call when the odor is strong, iron is heavy, or slime keeps coming back after shocking. It costs more up front (national ranges run roughly $2,000 to $5,000 installed) and the generator will eventually need service, but there are no chemicals to buy. Our ozone filter page walks through when it beats an air-injection unit.
Chlorination with retention and carbon. A feed pump doses chlorine, a retention tank gives it contact time, and a carbon filter strips the chlorine and the oxidized sulfur. It handles the highest sulfide levels and bacteria, but it means buying chlorine, maintaining a pump and a bulkier install.
Catalytic carbon alone. Works on faint odors and is cheap, but the carbon exhausts quickly once sulfide is more than a trace, so it becomes a filter-change habit rather than a fix.
What none of these are is a softener. A softener trades calcium and magnesium for sodium; hydrogen sulfide is a gas, not a mineral, and it passes straight through. The right order for a rural well here is oxidation first, softener second, reverse osmosis at the kitchen sink if the test finds nitrate, arsenic, radium or sodium worth removing. And if the two-question test pointed at the water heater, do not buy any of this. Change the anode.
Why it is worse in summer
Search interest for this problem peaks in summer, and the reasons are physical. Warm basements and warmer incoming water speed up the bacteria in the heater and the plumbing. Summer demand pulls wells harder and can drop the water level into lower-oxygen zones. Lake cabins and second homes get opened after months of stagnation. And a hot shower releases dissolved gas faster than a cold tap, so the same water smells stronger in July than in January. The well did not change; the conditions that make sulfide noticeable all line up at once.
If you are on the Fulton side of the river
Fulton is about 16 road miles and a 27-minute drive from our shop, straight out US-30 through Clinton and over the Gateway Bridge, and it is the one part of our territory where the well belt drinks from river sand rather than dolomite. Inside the city limits the municipal iron-filter plant handles the raw iron, manganese and arsenic, so an in-town rotten-egg complaint is nearly always the water-heater case. Outside the levee, on the Cattail Road and Penrose Road acreages and in Fulton Township toward Albany, private wells sit in the same sands the city treats, with no plant in front of them and no free county testing program to fall back on. Our Fulton page covers the local picture in more detail.
Wherever you are between Maquoketa and the Quad Cities, start with the two questions. If they do not settle it, our free in-home water test will, before you spend anything. We are insured, we sell, install and service all brands, and we schedule Monday through Friday, 8 to 5. Call (563) 221-9009 or request a free estimate, and if the answer turns out to be a $40 anode rod, we will tell you that too.