Nitrates in Groundwater

By: John George, P.E., Agricultural Engineering Associates

Q: I keep hearing about feedlots and lagoons contaminating groundwater with Nitrate Nitrogen. What is the real story?

A: Manure contains significant Nitrogen which could be a concern if it reaches concentrations in groundwater higher than 10 mg/l as Nitrate (NO3). Nitrate in drinking water can be a concern for both infants and the elderly. This fact and the presumption that Nitrogen from livestock manure presents widespread potential to convert to Nitrate and migrate into groundwater has been the impetus for a lot of regulatory focus and constraints.

As a student and researcher of the Nitrogen Cycle in lagoons and soils fifty years ago, I learned early on that the chemistry, physics, and biology of Nitrogen transformation and transport constitute an effective “Safety Net” against Nitrate in groundwater below feedlots or in lagoons. How can that be when seemingly the entire environmental establishment is paranoid about this risk? To understand nature’s Nitrate “Safety Net” one needs to understand not only the Nitrogen Cycle but also soil chemistry.

Nitrogen in manure degrades under anaerobic conditions from complex organic Nitrogen to the simple Ammonia form (NH3). Anaerobic denotes the lack of free Oxygen in the biological stabilization processes which act upon organic Nitrogen in the environment. Microbes convert Ammonia to Nitrate only in the presence of free Oxygen by a process called Nitrification. There is no free Oxygen in a feedlot manure pack and/or lagoon because the demand for free Oxygen always exceeds the rate of natural Oxygen replenishment. Therefore Ammonia is normally the terminal form of Nitrogen in the manure pack and/or in a lagoon.

The soil profile under a lagoon or feedlot manure pack has many ion exchange sites which attract and hold onto Ammonia in its cationic form Ammonium (NH4+). Since there are many ion exchange sites compared to Ammonia quantities, Ammonia never moves deep into the soil profile. Free Oxygen cannot exist in the soil under a lagoon or a feedlot manure pack, so the ammonia typically stays locked up indefinitely.

In contrast to this “Safety Net” of Ammonia storage, when Nitrogen is applied to land, the soil profile will undergo aerobic conditions at some point in time. It makes no difference if Nitrate forms from organic or inorganic commercial fertilizer. Once in the Nitrate form, Nitrogen is mobile in the soil solute. If Nitrate moves below the root zone before taken up by plants, it may end up in groundwater. If Nitrate-N encounters anaerobic conditions as it moves downward, however, denitrification reduces it to elemental Nitrogen (N2) and it is released back to the atmosphere.

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