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site sewage disposal practices. With each new proposal for disposed of to land, although, according to the EPA, no
<br /> development there is a growing need to quantify and evalu- thorough nitrogen-balance studies have been reported
<br /> ate possible changes in ground-water quality that may which either substantiate or refute this assertion(U.S.EPA,
<br /> result. What are most needed are convenient and reliable 1981). One of the few detailed studies of nitrogen beneath
<br /> analytical tools that can be used by regulatory agencies, septic tank disposal fields is the work of Walker et al.(1973a,
<br /> engineers, and others to make assessments early in the 1973b). This work found denitrification to be an insignifi-
<br /> planning process. cant nitrate removal mechanism in unsaturated sandy soils,
<br /> as deep as 15 to 20 feet, due to the lack of anaerobic
<br /> Nitrogen Contributions and Transformations conditions and organic material which support denitrifying
<br /> Nitrogen is present in high concentrations in septic bacteria. It was thus suggested that the only active mecha-
<br /> tank effluent primarily as ammonium-nitrogen (75-80%), nism of lowering the nitrate content in such situations is
<br /> with organic nitrogen making up the remainder(Otis et al., dilution by higher quality ground water or by recharge
<br /> 1975). Total nitrogen concentrations in such effluent have waters.
<br /> been reported to vary from 25 mg/1 to as much as 100 mg/1,
<br /> the average generally being in the range of 35 to 45 mg/l Simplified Prediction of Ground-Water
<br /> (U.S.EPA, 1980).Walker et al.(1973a)estimated the typical
<br /> annual nitrogen contribution from a family of four to be Nitrate Buildup
<br /> about 33 kg. For a residential lot size of 0.25 acres, this In the long-term,water quality in the upper saturated
<br /> nitrogen contribution would be more than 200 times the zone is closely approximated by the quality of percolating
<br /> amount that would typically be introduced naturally from recharge waters. This is the critical ground-water zone in
<br /> mineralization of soil organic nitrogen and precipitation. which potential nitrate impacts are likely to be most strongly
<br /> Upon introduction into the soil through subsurface expressed. A simplified prediction of the nitrate impacts of
<br /> disposal fields, nitrogen may undergo various transforms- on-site sewage disposal systems over a defined geographical
<br /> tions,the most important being nitrification and denitrifi- area can thus be made by constructing a mass balance,
<br /> cation. considering only inputs from waste water and recharge of
<br /> Nitrification may be broadly defined as the biological rainfall(also meant to include snowmelt)and losses due to
<br /> conversion of nitrogen in organic or inorganic compounds denitrification in the soil column and the upper portion of
<br /> from a reduced to a more oxidized state(Alexander, 1965). the aquifer.
<br /> The predominant end product is nitrate(NO3)because it is The expression for the resultant average concentration,
<br /> a stable anionic species.This also explains its high degree of nr, of nitrate-nitrogen in recharge water is given by
<br /> mobility in the soil. Virtually complete nitrification of Inw(1 — d) + Rnb
<br /> ammonium-nitrogen has been found to occur in the unsatu- nr— (I + R) (1)
<br /> rated zone in well-aerated soil below septic tank disposal
<br /> fields(Walker et al., 1973b).The resulting nitrate may then in which I = volume rate of waste water entering the soil
<br /> pass easily through the soil along with percolating effluent averaged over the gross developed area, in inches per year;
<br /> and other recharge waters. Immobilization of NO3 by nw=total nitrogen concentration of waste water,in milli-
<br /> plants or through microbial uptake into biomass may occur grams per liter;d=fraction of nitrate-nitrogen loss due to
<br /> to a limited extent,but these are generally considered to be denitrification in the soil; R = average recharge rate of
<br /> insignificant NO3 sinks (Alexander, 1965; Lance, 1972), rainfall, in inches per year; and nb = background nitrate-
<br /> and thus largely ineffective in reducing the amount of NO3 nitrogen concentration of rainfall recharge at the water
<br /> available for percolation to ground water. table,exclusive of waste-water influences,in milligrams per
<br /> Denitrification refers to the biological or chemical liter.
<br /> reduction of nitrate and nitrite to volatile gases, usually In this expression,the value of nr is computed simply as
<br /> nitrous oxide and molecular nitrogen or both (Broadbent the weighted average nitrate-nitrogen concentration of per-
<br /> and Clark, 1967). It is the only mechanism in the soil that colating rainfall and waste water, adjusted for expected
<br /> can effect significant reduction of nitrate in percolating losses due to soil denitrification. A critical simplifying
<br /> effluent(Alexander,1965;Lance,1972).The most favorable assumption in equation (1) is that there is uniform and
<br /> soil conditions for denitrification are(a)the abundance of complete mixing of waste water and percolating rainfall
<br /> organic carbon sustrate,(b)high soil moisture content,and over the entire developed area,and that this is completed at
<br /> (c) high soil pH (Broadbent and Clark, 1967; NAS, 1978). the water table. This assumption is made to allow calcula-
<br /> The rate of denitrification appears to be independent of tion of a predicted mean nitrate-nitrogen concentration for
<br /> nitrate concentration over a fairly wide range (Broadbent the area as a whole. In reality, such complete, uniform
<br /> and Clark, 1967). mixing would not be expected to occur because of the
<br /> Most nitrogen balance studies of fertilizer application irregular spatial and temporal distribution of waste-water
<br /> have indicated a large nitrogen deficit attributable to loading and rainfall recharge. Nevertheless, the predicted
<br /> denitrification. Losses range from 1 to 75 percent of the value should correspond with the mean concentration in the
<br /> applied nitrogen, but are typically between 10 and 25 per- ground water determined from representative sampling.
<br /> cent(Broadbent and Clark,1967).These rates of denitrifica- Full conversion of nitrogen to nitrate is also assumed in
<br /> tion are generally considered to also apply to waste waters equation(1).This is a reasonable assumption in most cases.
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