Water impacts the flavor of your coffee in three primary ways. The first is pretty straightforward: the flavor of your water will have a direct impact on the flavor of your coffee. The flavor of water is determined by the minerals and other matter present. The content of your tap water largely depends on where you live and how the water was treated. For example, calcium, often from limestone, can give water a smooth milky flavor. If you live near an ocean your water may have a slight sulfur smell due to the presence of sulfur producing microbes. Water taken from lakes can taste earthy from leftover plant matter and so on. The key takeaway here is that any off flavors in your water before you brew your coffee are still going to be there when you’re finished making it. Starting with distilled or reverse osmosis water is the best way to ensure you are starting with a clean slate when brewing coffee.
Hardness in water is defined by the amount of calcium and magnesium ions present in the water. While the exact mechanisms by which these ions interact with coffee and our perception of the flavors in coffee are not yet fully understood, the effects are generally agreed upon: too much hardness will produce bitterness and astringency, while too little hardness can produce a weak and tea-like cup.
Not to be confused with “alkaline,” meaning basic on the PH scale, alkalinity is a measure of the amount of carbonate and bicarbonate ions in water and represents its buffering capacity. Alkalinity tends to have a greater overall impact on your cup when compared to hardness and the range that is generally accepted to be desirable is smaller as well. If water has too much alkalinity, the resulting cup will be flat and chalky. Too little and you coffee may come out tasting sharp and too acidic.