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