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site sewage dkposal practicer.With each oew propowl for disposed of to lased,although.according to the EPA,no
<br /> development there is a graving seed to quantity and ewlu thorough nitrogen-balance studies have ban reported
<br /> are possible changes in ground-wales quality that may which cit her nrbitantlateorrefute Ihis asartmn(U.S.EPA,
<br /> result What arc most-,,led we eonvenicnl anal reliable 1981).One of the few detailed studies of nitrogen beneath
<br /> analytical took that nes be used by regulatory agencies, septictank dar.—lleidsis the work of Walker et.1.(1973a,
<br /> engincen,and others to make ansessnents early in the 1973b)this work found denitrifcatron to be an insignif-
<br /> planning pries+. cant nitrate removal mechanism on unsaturated sandy soils,
<br /> as deep as 15 In 20 lett,due to the lack of anaerobic
<br /> NIt"Vers Conlrlbutlons and Transformations conduruus and organs,material which supplies denitrifying
<br /> Nitrogen is present in high concentrations in s,plc bacteria.It was thus suggested that the only active maha-
<br /> tank cf mrA primarily as ammonium-nitroll(75-80/H. nhm of lowering the nitrate content in such situations is
<br /> with organic nitrogen making up the remainder((lis ri 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;I to as much as I(X)mgr I,
<br /> the average generally being in the range of 35 to 45 mg;1 Simplified Prediction of Ground-Water
<br /> (U.S.EPA,1980).Walker et al.(1973s)estimated the typical NllraN Buildup
<br /> annual nitrogen contribution from a family of four to br
<br /> about 33 kg.For■—id-tial la sire of 0 25 nares.this In clic Tong-term,water by the
<br /> q the upper saturated
<br /> ng
<br /> nitrogen contnbution world be most than 200 time,the rant n uncal approximated by cal quality of per inne in
<br /> amount that would typically be introduced natutally from recharge waters.tram is the critical groundwater uoe in
<br /> mrneralirarion of vil organic nitrogen and prccipuatwn. which potential sucrate impacts arelikdyto be mosutrongly
<br /> Upon introduction into the soil through subsurface expressed A simplified prediction of the nitrate impacts of
<br /> disposal fields,nitrogen may undergo various ttansforma- .name sewage disposal systems vera defined geognaphural
<br /> lions,the mat important being nitrification and denunf arca can thus be made by eonstructmg a mass balance.
<br /> colon considering only inputs I«sm waste wairr and recharge of
<br /> Nitrification may be broadly defined as the hiologrcal rainfall(also meant In include snuwmck)and losses due to
<br /> eonvmion of nitrogen in organic or inorganic compounds denitrification in the soil column and the upper portion of
<br /> from a reduced to a moreoxilised state(Alexander,1%51the aquifer.
<br /> Iheptedomimnimdpnsductisnittme(NO,)hccauwitis tTe expression fort he resultant average concentration,
<br /> R stableanionic species this also explains its high degme of it...f mrrate-nitrogen in recharge water is green by
<br /> mobility in the soil. Virtually complete niliftcation of In.(I-d)T Roe
<br /> mm
<br /> aonium-nitrogen has been found to occur in the unsulu- 1
<br /> n, 11 4 RI— 1
<br /> rated tone in well-aerated sod below septic tank disposal
<br /> f,i(Wall A,1973b).The resulting abate may then in which I volume rate of wast,water entering the soil
<br /> pus easily through the soil Wong with percolating effluent averaged.ver the it developed arca,in inches per year,
<br /> and usher recharge water. Immobiluatirn of NIL by It.=told nitrogen concentration of waste water,in nulh-
<br /> plants or through microbial uptake into biomass may sxctir grams per liter;d=fraction of nit rate-nitrogen las due to
<br /> to a limited extent,but these arc generally considered to be dcnitrificalon to the sal;R - average recharge rate of
<br /> insignificant NO,-sink.(Alexander, 1965;Lana,1972). rainfall,to Inches es per year;and rte=background nitrate-
<br /> and thus Largely Ineffective in reducing the amount of NO, nitrogen concentiati.n of rainfall recharge at the woes
<br /> available for percolation to ground water. table,exclusive of waste-water inllucnoe>tit mi0yuams per
<br /> Denitrification refer to the biological Or chemical liter
<br /> reduction a(mtram and mint,1n volatile gases,usually In thisexpormion,the I-of n,is computed imply as
<br /> muoau oxide and molecular ntuogen or both(Broadbent the weighmd average nitrate-nitrogen concentration of per-
<br /> and Clark,1967).It is the only mechanbm in the soil that colating rainfall and waste water,adjusted for expected
<br /> can effect signsftcanl reduction of nitrate in percolating losses due to wil denirrifrcation A critical simplifying
<br /> elfluent(Akunder,l%5;lance,1972).The most f—nable assumption in equation(1)is that there Is unifrsrm aid
<br /> sod conditions for denurifiation arc(a)the abwdana of compkic mixing of waste water and percolating rainfall
<br /> organic carbon su.lram,(b)high soil moisture content,and over the entire developed arca.and that this n completed at
<br /> (c)high soil pH 1111nsadhent and Clark,1967;NAS,1978) the water table This aswmption is made to allow caleula-
<br /> The rate of dennrification appcan to he udependent of son of a predicted mean nitrate-nitrogen concentration for
<br /> mtrx,c.neeatnti.n limn a fairly,wide range(Broadbent the arca as a whole In reality,such compkte,uniform
<br /> and Clark,1967) mixing would nes be expected to sxcur because of the
<br /> Mont niroge,balance smdis til fenilira appication regular spatial and temporal distribution of wasm-water
<br /> have indicated a large nitrogen deficit attributable to loading and rainfall recharge.%c,c the'esv,the prdicted
<br /> denonfscatasn.Losses range from I to 75 percent of the valm should correspond with the mean concentration in the
<br /> applied nitrogen,but are typically between 10 and 25 per- gnsurd water determined from representative sampling.
<br /> "In(Broadbent and(lark,1967).Thea rates ofdennnficr Full conversion of nitrogen to rctrate is also assumed in
<br /> tion are g,nerally considered In alar apply to waste waters equation(I i I bis is a may nable assumplsan o most cases
<br /> 191
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