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The approximation of nitrate concentrations obtained Typical Solutions
<br /> from equation(1)also ignores dispersion,lateral flow,and Solution of the foregoing equations requires input data
<br /> mixing with ground-water flow from upgradient areas. for several disposal system and site variables,all of which
<br /> These processes would generally contribute to additional can have a significant effect on the predicted nitrate-
<br /> reduction of nitrate-nitrogen concentrations in ground nitrogen concentration. Graphical solutions are presented
<br /> water to the extent that the nitrate-nitrogen concentration here for typical ranges of these variables,as an aid in select-
<br /> of ground-water flow from upgradient areas is lower. Ing appropriate values, and in identifying situations of
<br /> Equation(1)thus provides a conservative(worst case)fust potential concern.
<br /> approximation of ground-water nitrate-nitrogen concentra- The predicted resultant average ground-water nitrate-
<br /> tion resulting from the combined effect of on-site sewage nitrogen concentration,n,,computed from equation(1)is
<br /> disposal systems and precipitation.This is for estimation of plotted for convenience in Figure 1 against the fraction of
<br /> long-term effects(i.e.,over years)on ground-water quality, waste-water recharge,I,relative to rainfall recharge,R,for a
<br /> and is not intended for prediction of seasonal changes. selected range of values for soil denitrification,d,and waste-
<br /> A common land use planning dilemma is that of water nitrogen loading, nw. Background nitrate-nitrogen
<br /> determining acceptable development densities,sometimes loading,nb,typically falls in the range of 0.5 to 1.0 mg/1,and
<br /> referred to as the carrying capacity of the land.From the is assumed here to be 1.0 mg/I.Exceptions to this would be
<br /> standpoint of ground-water nitrate-nitrogen impacts,the if the area has large numbers of confined livestock or signifi-
<br /> critical minimum gross acreage per developed lot,A,may be cant expanses of fertilized crops or turf areas(e.g.,parks),
<br /> defined as that which would result in a value of n,equal to 10 which would tend to increase background nitrate-nitrogen
<br /> mg/l,the commonly accepted drinking-water limit.By set- loadings above the typical values suggested here.The results
<br /> ting I=0.01344 W/A and n,=10 mg/l,and then rearrang- plotted in Figure l show a wide range of potential effects,
<br /> ing equation(1),A is then given by highly sensitive to the initial selection of values for ne,and d.
<br /> Two curves are plotted for the average value of nw=40
<br /> A= 0.01344W[nw—dnw—10] (2) mg/1,with denitrification rates of 0 and 0.25,respectively.
<br /> R(10—nb) The typical range is represented on the high and low sides by
<br /> the curves for(a)nu,=50 mg/I,d=0 and(b)ns,=30 mg/1,
<br /> in which A is expressed in terms of gross acres/dwelling unit d=0.25.The curve for ne,=40 mg/I and d=0.25 would be
<br /> (DU);W is the average daily waste-water flow per dwelling considered the most representative of typical on-site sewage
<br /> unit,in gallons;and 0.01344 is a conversion factor having disposal situations(U.S. EPA, 1980; 1981). In addition to
<br /> units acre inch day DU yr-1 gal-i. proper selection of values of nw and d,the importance of
<br /> 40
<br /> J 30
<br /> 0
<br /> E nb=1.0 ma/L
<br /> z 30
<br /> 2
<br /> a
<br /> 0
<br /> Z25- �01L.0 �, S
<br /> C40
<br /> g y
<br /> 0 20
<br /> nets
<br /> War
<br /> Z Drinking 15,��4
<br /> z 10 W•Ler v 6 M' n■c 30 ma/L.0=0.20
<br /> w
<br /> Z 10 Stsndsrtl � v:. .. .'"
<br /> x '{
<br /> ___—__—___________—__——_____———__
<br /> < rN
<br /> F
<br /> J
<br /> 7
<br /> N
<br /> 0
<br /> 0
<br /> 0 0.20 0.40 0.00 0.80 1.00 1.20 1.40 1.00 1.00 T.00
<br /> WASTEWATER RECHARGE RELATIVE TO RAINFALL RECHARGE. I/R
<br /> Predicted Zone for Most Common Values
<br /> Fig.1.Resultant ground-water nitrate-nitrogen concentration as a function of effluent quality,denitrification,and I/R.
<br /> 492
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