Nitrogen in On-Site Systems

Nitrogen enters domestic on-site systems mainly as organic nitrogen, which means the nitrogen is part of a large biological molecule such as a protein. Bacteria and other microbes oxidize or mineralize the organic nitrogen to ammonium forms. The ammonium can be volatilized to the atmosphere, used by bacteria and plants, or adsorbed by the biomat or soil. Ammonium can also be converted under aerobic conditions to nitrate and nitrite in soils by Nitrosomonas and Nitrobacter bacteria. The nitrate form of nitrogen can be used by bacteria or plants. Under anaerobic conditions, nitrate can be transformed to nitrogen gas, a process known as denitrification. Figure 4.1.2 demonstrates the nitrogen cycle: gains and losses of nitrogen in the atmosphere and soil.

  • Because nitrate is very soluble and is not absorbed by soil, it can move through the soil into the ground water and adjoining surface waters.

  • Hthere are too many on-site systems in one area, nitrate levels in ground water may exceed the U.S. Environmental Protection Agency's Maximum Contaminate Level for nitrogen of 10 milligrams/liter (mg/I). Nitrogen levels above 10 mg/I may cause sickness or death to small babies and at higher levels can be harmful to adults.

  • Denitrification is most likely to occur in anaerobic zones, such as wet soils, or as shallow ground water moves through riparian areas next to streams. However, denitrification is limited in a properly sited system because the aerobic soil conditions will not allow denitrification to occur.

  • Research by Bicki et al. (1985) shows that of the total nitrogen produced from on-site systems, only 20% to 40% is adsorbed or removed during flow through unsaturated soils. Therefore, dilution and denitrification are the mechanisms that must be relied upon to reduce ground water nitrate concentrations.

Figure 4.1.2 The Nitrogen Cycle In Soll. (Sauchelli, Fertilizer Nitrogen -Its Chemistry and Technology, 1964. Reprinted with permission of Van Nostrand Reinhold Publishing Company, New York, NY.)

Figure 4.1.2 The Nitrogen Cycle In Soll. (Sauchelli, Fertilizer Nitrogen -Its Chemistry and Technology, 1964. Reprinted with permission of Van Nostrand Reinhold Publishing Company, New York, NY.)

From the North Carolina Onsite Guidance Manual