How did the drugs get in our water? Simple; when people take the drugs, their bodies absorb some, but not all of them. The unabsorbed amounts leave the body and are flushed down the toilet, where it joins other wastewater. Wastewater is cleansed before it's sent to reservoirs, rivers, and lakes, and the water is often reclaimed by water treatment plants and turned into drinking water. However, the treatments aren't able to completely remove the remaining drug residue.
The amounts of these drugs are extremely tiny and far below the levels of a medical dose, but researchers are concerned because they aren't fully certain what the long term effects from consistent exposure to random combinations of pharmaceuticals are.
People who use home filtration systems or drink bottled water are not protected from exposure to the prescription drugs, either. Bottled water is often from tap water, and water bottling plants don't screen their water for pharmaceuticals.
(Image from CDC)