Water hardness refers to the mineral content of water, commonly calcium and magnesium. Washington, D.C.'s water is moderately hard, but may vary throughout the city and by seasons of the year. Hardness usually peaks during the warmer months and is lower during the winter months. When using dishwashers, you may notice a slight increase in spotting on glassware or white residue on kitchenware and showers in warmer months. The hardness of the city's tap water is typically around 70 to 120 parts per million or 3 to 9 grains per gallon.