The Fate of Nutrients Added to Soils From Septic Systems
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Collapse ▲Wendell Gilliam
Soil Science Department, NCSU
Raleigh, NC
ABSTRACT
I will focus upon Nitrogen and Phosphorus because these are the nutrients which are of greatest concern in groundwater below septic fields and in surface waters to which this groundwater may flow. The reactions controlling the fate and movement of nitrogen and phosphorus in soils are well documented and as are the factors which influence these reactions. However, nitrogen and phosphorus react very differently when added to soils.
NITROGEN
Nitrogen from septic tanks enters soil as approximately equal parts ammonium nitrogen (NH+4) and organic nitrogen (mostly NH2 groups attached carbon). When the septic field is aerobic (as it must be to perform wee), most of the organic nitrogen is mineralized within a few weeks to form ammonium nitrogen although a portion of the nitrogen will remain as organic nitrogen in the soil microbes which are responsible for the mineralization. Also a small amount of the Nitrogen is present in forms not readily utilized by bacterial and generally stays in the soil as unreactive organic matter. If the septic field is aerobic, the ammonium nitrogen coming form the septic tank directly and the ammonium formed by mineralization is quickly nitrified by soil microbes to nitrate (NO-3). However, if the septic field is anaerobic because of poor drainage conditions, the decomposition is nitrified so the vast majority of the nitrogen remains in either the organic or ammonium forms. Both the organic nitrogen and ammonium nitrogen are generally absorbed to soil particles so their concentration in the waste will decrease dramatically as the water flows through the soil. Thus a septic system which operating properly will produce drainage water in which essentially all of the nitrogen is in the nitrate form. this form of nitrogen moves readily with the drainage water and may contribute to nitrate problems in ground water or eutrophication problems in surface waters. It should be recognized that septic systems which work ideally will retain a small percentage of the nitrogen entering the septic tank. Most nitrogen will leave the system in drainage water as nitrate. If a monitoring system finds significant concentrations of either ammonium or organic nitrogen in the water leaving a septic field, this indicates that system has a problem and is not operating properly.
PHOSPHORUS
Phosphorus also enters the septic field as organic and inorganic forms but a much large fraction is in the organic form. The inorganic phosphorus will react with iron or aluminum associated with the clay particles in the soil to form insoluble mixtures. Much of the organic phosphorus is mineralized during decomposition of the organic matter to form inorganic phosphorus. This phosphorus also reacts readily with soil and generally does not move far form where it enters the soil. The only exceptions to this are in very sandy soils or when the soil is water saturated and water from the septic tanks moves to the surface. When significant amounts of phosphorus added to soils from septic tanks moves more than a few feet from where it enters, it usually indicates drainage problems.