Why does my cold tap water smell like onions not rotten eggs sulfur but and only cold?

There are a few possible reasons why your cold tap water might smell like onions:

- Bacteria: Your water heater could be a breeding ground for bacteria, which can produce a variety of gases, including hydrogen sulfide, which smells like rotten eggs. If this is the case, you may need to clean your water heater.

- Anode rod: The anode rod in your water heater is made of magnesium or aluminum and helps to prevent corrosion. However, as it corrodes, it can release an onion-like smell into the water. If this is the case, you may need to replace the anode rod.

- Galvanized pipes: If your home has galvanized pipes, they may be corroding, which can release zinc into the water. This can cause the water to smell like onions or metallic. If this is the case, you may need to replace the galvanized pipes.

- Well water: If you have a well, the water may be contaminated with bacteria or other organic matter, which can produce a variety of smells, including an onion-like smell. If this is the case, you may need to have your well water tested and treated.

- Water treatment plant: If your water comes from a water treatment plant, it may be treated with a chemical called chlorine, which can react with organic matter in the water to produce a variety of smells, including an onion-like smell. If this is the case, you may need to contact your water treatment plant to inquire about the chemicals they use and their safety.

If you are concerned about the smell of your water, you can have it tested by a qualified water testing laboratory. This will help to identify the source of the smell and determine the best course of action to take.