Nitrate contamination occurs when there is more nitrate in the soil than plants can use and when water can move easily through the soil and underlying rock. The excess nitrate is carried through the soil into groundwater supplies by irrigation, rain-water and snowmelt. This occurs particularly where the soil is sandy, gravelly or shallow over porous limestone bedrock. Excess nitrate can accumulate in the soil in several ways
First, applying more nitrate fertilizer to the soil than a crop can use will build up high levels of nitrate.
Second, manure and sewage contain both ammonia and organic forms of nitrogen, organic nitrogen may be converted to ammonia in the soil. This ammonia, along with any ammonia fertilizer that is applied, is converted to nitrate by soil bacteria In a process called nitrification. Nitrification is important because plants can only use nitrogen in the nitrate form. However, when more ammonia is nitrified than plants can use, the unused nitrate will accumulate in the soil.
Finally, some plants, soybeans and alfalfa in particular, can take nitrogen out of the air and put it into the ground through their root nodules. This process is called nitrogen fixation. Small amounts of nitrate also enter the sod with rain during electrical storms. Neither process produces contaminating amounts of nitrate on its own but both should be taken into consideration when determining how much fertilizer is needed.