Interesting story about well contamination … and Nitrate was at top of the list for man-introduced pollutants. Full story from USGS here
USGS scientists sampled about 2,100 private wells in 48 states and found that the contaminants most frequently measured at concentrations of potential health concern were inorganic contaminants, including radon and arsenic. These contaminants are mostly derived from the natural geologic materials that make up the aquifers from which well water is drawn.
Nitrate was the most common inorganic contaminant derived from man- made sources-such as from fertilizer applications and septic-tanks-that was found at concentrations greater than the Federal drinking-water standard for public-water supplies (10 parts per million). Nitrate was greater than the standard in about 4 percent of sampled wells.