If your well water smells like rotten eggs, you’re smelling hydrogen sulfide gas. It’s one of the most common well water complaints in San Diego County, especially in East County and inland areas where private wells are more common. The smell is unpleasant, but the fix is usually straightforward once you figure out where the gas is coming from.

What hydrogen sulfide is

Hydrogen sulfide, written H2S, is a gas that dissolves into water and gives off that distinct rotten egg or sulfur odor. Even a small amount is enough for your nose to catch it, because people can detect it at very low concentrations.

It shows up in well water two main ways. Some groundwater carries hydrogen sulfide naturally, picked up as water moves through rock and soil that contains sulfur compounds. Other times the gas is produced by sulfate-reducing bacteria, which are harmless to most healthy people but feed on sulfur in the water or in the well and give off H2S as a byproduct.

For most households the smell is a nuisance and an aesthetic issue rather than a health emergency. Still, it’s worth testing your well, because a sulfur smell can travel alongside other well water problems worth knowing about.

First, find where the smell is coming from

Before you treat anything, narrow down the source. This one check saves a lot of wasted money.

If the smell is at every tap, hot and cold, the hydrogen sulfide is coming from the well or the source water itself. That’s a whole-house treatment problem.

If the smell is only in the hot water, the well probably isn’t the culprit. The reaction is usually happening inside your water heater. The magnesium anode rod in many heaters can react with sulfur in the water and generate hydrogen sulfide right there in the tank. In that case the fix is at the water heater, often swapping to a different anode rod, not treating the whole house.

If the smell comes and goes or is worse after the water sits, that leans toward bacterial activity in the well or plumbing.

Sorting this out first tells you whether you’re solving a source-water problem or a water-heater problem.

Treating a bacterial cause

If sulfate-reducing bacteria are behind the smell, shock chlorination is the usual first step. This means disinfecting the well and plumbing with a chlorine solution to knock back the bacteria. It’s a well-established well maintenance procedure, and for some homes it resolves the smell for a good stretch of time.

Shock chlorination is not always permanent, though. If conditions in the well favor the bacteria, they can return, and you may need ongoing treatment on top of periodic disinfection.

Treating ongoing hydrogen sulfide

When the H2S is persistent, whether it’s natural or bacterial, ongoing treatment handles it at the point where water enters the house.

Low levels can often be reduced with catalytic or activated carbon filtration, similar to how a whole-house carbon filter handles taste and odor. Carbon has a limited capacity for sulfide, so this works best when concentrations are on the lower end.

Higher levels usually call for oxidation followed by filtration. The idea is to convert the dissolved gas into a solid particle you can then filter out. Oxidation methods include air injection (aeration), or dosing an oxidizer like chlorine or hydrogen peroxide, after which a filter removes the resulting particles. Air injection systems are popular for well water because they don’t add chemicals.

The right method depends on how much hydrogen sulfide you have and whether iron or manganese is present too, since those often travel together in well water and change the treatment approach.

Test before you buy a system

Hydrogen sulfide rarely shows up alone. Iron, manganese, sulfate, and bacteria commonly appear in the same well, and the combination determines what treatment you actually need. A guess here is expensive, because a system sized for the wrong problem won’t fix the smell.

A comprehensive well water test tells you what’s in the water and at what levels. That’s what sizes the system correctly. Our free in-home water test is a starting point, and for a private well we’ll talk through the full panel that a well needs beyond a basic city-water screen.

If you want the broader picture on treating a private well, our guide to well water treatment in San Diego walks through the common contaminants and how a multi-stage system handles them. If you’re also seeing discoloration, rusty water in San Diego covers the iron side of the problem.

The bottom line

A rotten egg smell means hydrogen sulfide, and the smart move is to find the source before spending on treatment. Check whether it’s every tap or just the hot water. If it’s the whole house, test the well, then match the treatment to what the test finds, carbon for low levels, oxidation and filtration for higher ones, and shock chlorination when bacteria are involved.

To get your well water tested and matched to the right system, call Filter Pros San Diego at (858) 400-6508 or schedule a free in-home water test. We serve well owners across San Diego County.

Related: Whole-house water filtration | Well water treatment in San Diego