Bioremediation is the process by which microbes (generally bacteria) or plants transform a harmful water contaminant into a non-harmful substance, much as we turn sugar into carbon dioxide and water. Bioremediation can help clean up ground water contaminated with gasoline, solvents, and other contaminants. Often, the bacteria are already present in the soil or aquifer, and bioremediation takes place naturally.
In some cases, the rate of bioremediation is too slow to effectively clean up a plume of contaminated water before it gets to a spring, well, lake, or stream. In those cases, the rate of bioremediation can sometimes be enhanced by adding a substance that acts like a fertilizer to make the bacteria grow and feed more rapidly. This substance, which depends on the local chemistry and hydrology, might be nitrate, or oxygen, or iron, or something else. Additional information is on a bioremediation fact sheet.