The rotten egg or sulfur smell is caused by naturally occurring sulfide ions in the City's source water. When water sits in the main lines for long periods of time without movement, the sulfide can form hydrogen sulfide gas. The gas remains entrained in the water until it leaves the customer's water taps.
If you smell a rotten egg or sulfur smell from your cold water, call the Las Cruces Utilities Water Production Section at 575-528-3547 so they can send an operator to make sure that you have an ideal level of residual chlorine, and if not, flush the lines. If you smell a rotten egg or sulfur smell from your hot water, you may need to flush your hot water heater and raise the temperature setting slightly to prevent proliferation of bacteria that feed on the iron in the water.