To understand where the smell comes from, we need to understand the relationship between the Hydrogen Sulphide and Sulphates which are a combination of sulphur and oxygen and are a part of the naturally occurring minerals in some soil and rock formations that contain groundwater. The Sulphates dissolve over time and are released into the groundwater.
Anaerobic bacteria live in oxygen-deficient environments such as in deep wells. Iron bacteria and sulphur-reducing bacteria both thrive in these types of surroundings provided they have an energy supply. The Sulphur-reducing bacteria, which use sulphur as an energy source, are the primary producers of large quantities of hydrogen sulphide. These bacteria chemically change natural sulphates in water to hydrogen sulphide. The Iron bacteria, live on Iron and Manganese and produce a brownish slime if their intake is Iron and a blackish slime if it’s Manganese, this slime has a tendency to block up pipes.
When there is a smell from the water at the cold kitchen tap it is almost always caused by Hydrogen Sulphide which is recognised by a rotten-egg smell. This happens very often when there is Iron in the water for the reasons mentioned above. These bacteria are not harmful to health in any way, but they certainly can cause a lot of problems.
It is also possible to get Hydrogen Sulphide in water where there is no Iron and no bacteria. Water sometimes dissolves Hydrogen Sulphide into itself as sulphur-containing organic matter decomposes and the water will then permeate up through the rocks creating a sulphur spring (spa water) as it reaches the surface.
Hydrogen Sulphide is very difficult to analyse in a laboratory because the minute the water exits the tap it begins to release the gas into the air, more is released on transit to the laboratory and when the bottle is opened there, the Hydrogen Sulphide is again released. Because of this, it is always better to have the water analysed at your own kitchen, when there is a rotten egg smell from the water.
Hydrogen Sulphide filters are built to suit the level found in your water. In some very mild cases a simple carbon filter may suffice. When the Hydrogen Sulphide levels are higher a larger more complex system would be needed.