Odor - Sulfur (Hydrogen Sulfide) in your water

Hydrogen sulfide is a gas that causes an obnoxious rotten egg odor in your water. Hydrogen sulfide is caused by rotting and decaying organic matter in the underground aquifer. Hydrogen sulfide ( H2S ) will ruin your silverware, eat holes in copper used in many appliances, and generally stink up your whole house.
It is common in all parts of the world, and as a rule, the deeper the well, the more likely it will contain this gas.

Also, the level in the water will change during the coarse of the year ( the dry seasons tend to have higher levels as the water table drops ), and the more water pumped at one time ( things like lawn watering, or even back to back loads of laundry ) will cause the level of hydrogen sulfide to increase.

And the LEVEL ( measured in ppm ) is what determines the type and size of filtration required to remove the odor 100% ( as removing only 99% still results in a bad odor from the water ).

The two methods for removal are Oxidization ( aeration, chlorination, and various oxidizing Media in a tank ), and Absorption ( by activated carbon ), or a combination of the two.

The proper size equipment, levels of chemicals, and frequency of cleaning all depend on the amount of H2S in the water.
So, the first thing you need to do, before purchasing or adjusting equipment for removal is to get an accurate test of the level in the water.

This is best done onsite after running the source water for about 5 minutes. Professional water treatment dealers have the test kits to accurately measure the levels in your water source, and will often test for free ( of course they will want to sell you some equipment too ), or kits can be purchased online ( $ 50 - $ 100 is the common price range for a professional kit ).