"The majority of the contaminants found in our drinking water can be traced back to improper or excessive use of ordinary compounds like lawn chemicals, gasoline, cleaning products and even prescription drugs."
Water Contamination And It's Effect On Our Health
"A variety of chemicals, including nitrate, can pass through the soil and potentially contaminate ground water. Nitrate comes from nitrogen, a plant nutrient supplied by inorganic fertilizer and animal manure. Additionally, airborne nitrogen compounds given off by industry and automobiles are deposited on the land in precipitation and dry particles. Other nonagricultural sources of nitrate include lawn fertilizers, septic systems, and domestic animals in residential areas.
Beneath agricultural lands, nitrate is the primary form of nitrogen. It is soluble in water and can easily pass through soil to the ground-water table. Nitrate can persist in ground water for decades and accumulate to high levels as more nitrogen is applied to the land surface every year."
National look at nitrate contamination of Ground Water
List of water contaminants from the EPA
http://www.epa.gov/OGWDW/hfacts.html