Hydrogen sulfide (H2S) is a dissolved gas that sometimes is found in well water. It is not usually a health risk, but even a tiny amount (0.05 mg/L) can cause the water to smell like rotten eggs, discolor bathroom and kitchen fixtures, corrode metals such as iron, steel, copper, brass, and silver, make food taste and look different, and foul the resin bed of an ion-exchange water softener. Very high concentrations of hydrogen sulfide in drinking water can cause nausea and other illnesses.
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Ascertain that the hydrogen sulfide is in the well water and not being produced in your water heater by turning on a cold water faucet to determine if the rotten egg smell is present. If it is, the problem is the well water (or, less likely, in the water distribution system). Sulfur-reducing bacteria are usually responsible for producing hydrogen sulfide. The bacteria use the sulfur in decaying plants, rocks, and soil for their energy source and as a by-product create hydrogen sulfide. These bacteria thrive in oxygen-deficient environments such as deep wells. While faucet-specific filters can remove low concentrations of hydrogen sulfide from your well water before you use it, installing a whole-house treatment is the recommended way to go.
Install a chlorinator between the ingress pipe from the well and the holding tank. A reaction between the chlorine released into the water and the hydrogen sulfide will create a tasteless, odorless, yellow particle that can be removed by a filter. Other chemicals including hydrogen peroxide and potassium permanganate can be added to the water to oxidize and remove the hydrogen sulfide. Filtration can remove the particles from the water.
Eliminate hydrogen sulfide by having a professional install an aeration system. Aeration systems spray water into a ventilated storage tank. Since hydrogen sulfide is a gas, creating a fine mist or spray will separate it from the water. The hydrogen sulfide can then be eliminated as a gas through the ventilation system. Chlorination may still be required. Aeration is usually not the first choice of homeowners because the system is so expensive to install, but it can be a last resort if nothing else works.
Consider a system-wide filter. An activated carbon filter will remove small amounts of hydrogen sulfide. The filter absorbs the gas and will need to be replaced periodically. An oxidizing filter is another option. This kind of filter uses sand coated with manganese oxide to convert the hydrogen sulfur gas to hydrogen sulfide particles, which are then trapped in the filter.