What the smell actually is
Hydrogen sulfide is a dissolved gas, and your nose is a better detector for it than most test equipment. People pick it up at concentrations well under one part per million, so a well can smell like a boiled egg sandwich and still come back from the lab at a couple tenths of a milligram per liter. That mismatch throws people. The smell is real and the small number is real too.
Three things put it in your water. The first is sulfate reducing bacteria, which live without oxygen and turn the sulfate that's already dissolved in your water into sulfide gas. They're harmless to drink and they're everywhere, and they set up shop in a well casing, in low spots in the plumbing, in a pressure tank that never fully turns over, and above all in a water heater. The second is the rock itself. Water sitting in an oxygen free fracture 400 feet down picks up sulfide from sulfur bearing minerals with no help from biology. The third, on shallower wells, is organic matter breaking down where surface water reaches the well.
There are side tells worth noticing. Hydrogen sulfide tarnishes silverware black, darkens brass and copper fittings, leaves black flecks in an aerator screen, and over years it eats pinholes in copper pipe. If you're seeing black staining along with the smell, that's the same chemistry.
Hot only, both, or cold only
Do this before you spend a dollar. Go to a bathroom you don't use much, run the cold for thirty seconds, fill a glass, and step outside to smell it, because your nose goes blind to a smell you've been standing in. Then do the same with the hot. Then repeat at an outside spigot if you have one that runs off the raw water before any treatment gear.
Hot water only is the water heater, almost every time. The tank is warm, it's dark, water sits in it for hours, and it has a magnesium rod in it that hands the bacteria exactly what they need. Fixing the heater fixes the house.
Both hot and cold means the gas is arriving in the water, not being made in the tank. The heat just drives more of it out of solution, which is why the hot side smells stronger even when the source is the well. Cold only, with clean hot water, is unusual and points at something between the well and the fixtures: a stagnant branch line, a rarely used bathroom, or a treatment tank.
One more comparison is worth making. If the raw water at the wellhead or an outside hydrant is clean and the water inside the house smells, your treatment equipment is the incubator. A carbon tank or a softener that's never been sanitized is a warm, dark bed full of surface area, and it will grow a colony of its own.
The water heater and the anode rod
Every glass lined water heater has a sacrificial anode rod hanging down inside it. Its whole job is to corrode so the steel tank doesn't. Most factory rods are magnesium, which is the most electrochemically active choice and the best protection for the tank. It's also the problem. As magnesium gives up electrons it releases hydrogen, and sulfate reducing bacteria use that hydrogen to convert sulfate into hydrogen sulfide. A well with mildly sulfurous water and a magnesium rod is a gas generator running at 120 degrees.
The fix is to change what's in the tank. An aluminum or aluminum zinc alloy rod is far less reactive and quiets most cases down for the price of a hardware store part. A powered anode, which is a titanium rod with a small transformer, produces no hydrogen at all and lasts the life of the heater. They cost more than a standard rod and are the answer for a heater that keeps coming back. What you should not do is pull the rod and leave the hole plugged. The tank rusts through in a few years and you've voided the warranty.
Change the rod and sanitize the tank in the same visit. Flush the tank out, then run it at 160 degrees for a few hours to kill what's living in it, then flush again and put it back to 120. Be careful with that, because 160 degree water scalds in seconds, so warn everybody in the house and don't leave it there. Raising the temperature alone buys you a few weeks. Raising it and changing the rod is what actually holds.
If the heater is past ten or twelve years old, put the anode money toward a new one and specify the rod you want when you buy it.
Is it dangerous to drink?
At the concentrations that make a house smell, no. Hydrogen sulfide in drinking water is regulated as an aesthetic problem, not a health one, and the water tastes so bad long before it reaches a harmful dose that nobody drinks enough to be hurt by it. Plenty of families in these hills have lived on faintly sulfurous water for decades.
That said, treat it as a signal rather than a shrug. Something in your well changed or something has been growing in it, and the same conditions that let bacteria set up can come with other findings. A strong new smell is a good reason to run a full panel: total coliform, nitrate, iron, manganese, sulfate, pH and the standard state list. The smell is free information and the test tells you whether it's the only thing going on.
The real hazard is the gas in a confined space, not the water in a glass. Hydrogen sulfide is heavier than air, it collects in pits and it deadens your sense of smell at high concentration, so the warning goes away right as it gets dangerous. Never climb into a well pit, a sump pit, a cistern or an unvented crawlspace that smells of rotten eggs. If a basement smells strongly enough to notice from the stairs, open it up and ventilate before you work down there.
Shock chlorination, and why the smell comes back
Shocking a well means putting a measured dose of chlorine down the casing, circulating it through every fixture until you can smell chlorine at each one, letting it sit twelve to twenty four hours, and then flushing the system out to a place where the chlorinated water won't kill a lawn or a septic field. Done right it kills the colony living in the casing, the drop pipe and the house plumbing. Homeowners do it themselves, and having it done is an affordable service call.
It works, and then a lot of the time it comes back. Two reasons. The bacteria don't only live in the casing, they live out in the water bearing fractures feeding the well, and chlorine poured down a borehole doesn't reach in there. Or the sulfide was geologic from the start, in which case chlorination was never going to matter. Rule of thumb: if the smell returns within days, you have a continuous source and you need treatment. If it stays gone for a season or more, shocking it again once a year is a reasonable way to live.
There's one case where you should do almost nothing. A house that sat empty, a lake place closed up for the winter, a listing that sat vacant through a slow spring, will smell strongly for the first few days back. Water stood still in the well, the tank, the heater and the lines for months, and the bacteria had no interruptions. Run every fixture hard for twenty minutes, flush the water heater, and give it two or three days of normal use. If the smell fades on its own, don't buy equipment. That was stagnation, not a water problem.
The treatment ladder, and testing before you buy
For a light case, under about 1 mg/L of sulfide with no iron in the water, simple aeration handles it. Bring the water into contact with air, let the gas come out of solution, vent it outside. A catalytic carbon filter also works at low levels and costs the least to install, but the bed loads up and you replace media on a schedule, so it's a light duty answer that gets expensive if you push it.
The middle of the ladder is an air injection oxidizing filter, and it's the workhorse in this part of the state. One tank holds a pocket of compressed air above a bed of catalytic media. Water passes through the air pocket, the sulfide oxidizes, and the media catches it. The tank backwashes itself to drain overnight. No chemicals to buy or mix. It handles a few milligrams per liter of sulfide and, more to the point around here, it takes iron and manganese out in the same pass. Installed, it is a real piece of equipment, sized to your water and your plumbing.
The top of the ladder is chlorine injection: a small feed pump paced to the well pump, a contact tank sized to give the chlorine enough time to work, and a backwashing filter or carbon tank after it to take out the chlorine and the solids it created. That's the most expensive option here, and it needs solution mixed and a filter serviced. It's what you use for high sulfide, for an active bacterial colony that keeps reseeding, and for water carrying heavy iron and manganese along with the smell.
Which rung you belong on is a numbers question, not a guess. Sizing depends on the sulfide concentration, on iron and manganese levels, on pH, because oxidation slows down in acidic water and manganese in particular needs pH on the high side of neutral to come out at all, and on your well's yield, because a backwashing filter needs a certain flow to lift its bed and a three gallon per minute fractured rock well can't always give it. Test the raw water at the wellhead first. Every system we've been called to replace after two years was sized off a smell instead of a number. Call 973-853-1550 and we'll pull a sample and tell you which rung you're actually on.