From the U.S. EPA website:
The pH scale measures the logarithmic concentration of hydrogen (H+) and hydroxide (OH-) ions, which make up water (H+ + OH- = H2O). When both types of ions are in equal concentration, the pH is 7.0 or neutral. Below 7.0, the water is acidic (there are more hydrogen ions than hydroxide ions). When the pH is above 7.0, the water is alkaline, or basic (there are more hydroxide ions than hydrogen ions). Since the scale is logarithmic, a drop in the pH by 1.0 unit is equivalent to a 10-fold increase in acidity. So, a water sample with a pH of 5.0 is 10 times as acidic as one with a pH of 6.0, and pH 4.0 is 100 times as acidic as pH 6.0.
While lead is useful in many industrial applications, in water lead is a toxic metal that can cause serious damage to the brain, organs and blood cells over time. Lead occurs naturally in ground waters throughout the world, but most contamination of lead in drinking water comes from lead leaching from service pipes, fixtures, valves, and other materials containing lead. Read More
Copper occurs most commonly in drinking water due to corrosion of copper pipes. Such corrosion can have many causes, but is usually a symptom of overly acidic water or high TDS, chlorine, or oxygen content.