Today’s dairy cows may consume more than 20 gallons of water per day yet water is often overlooked as a potential cause of herd health issues or reduced milk production. Experience in Pennsylvania has shown that aesthetic pollutants like iron, manganese and hydrogen sulfide, are the most common water-related causes of problems with dairy herds. These pollutants cause tastes or odors that result in reduced water intake and milk production. Other pollutants, such as nitrate or heavy metals, can cause health effects in dairy herds. This fact sheet describes some of the common pollutants found in water supplies in Pennsylvania and their potential effect on dairy cows.