Your water's taste and smell tell you exactly what is in it. Chlorine taste? Municipal disinfection -- safe but unpleasant. Rotten egg smell? Hydrogen sulfide from the water heater anode rod, sulfate-reducing bacteria, or the supply itself. Metallic? Dissolved iron, copper, or manganese from corroding pipes or mineral content. Musty or earthy? Algae or organics in the source water. Chemical or solvent smell? Test immediately. First diagnostic step: does it affect only hot water, only cold, or both? Hot only points to the water heater. Both means it is in the supply.