For some purposes, it is not necessary for water to be potable (drinkable). For example, the water that is pumped down oil wells to help recover the hard-to-reach oil can have chemicals and microorganisms present. However, drinking water that comes from underground (groundwater) or from sources on the surface (surface water) must meet a higher standard of cleanliness. When harmful chemicals and microorganisms get into the water it is considered contaminated. Contaminants that cause disease must be taken out of the water that humans drink in order for the water to be potable. If not, drinking the water can cause intestinal upsets such as diarrhea if the water contains harmful bacteria, protozoa or viruses. If the contaminant is a compound such as mercury or lead, then the water can be poisonous. For example, mercury-laden water can cause nervous system difficulties...