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The approximation of nitrate concentrations obtained Typical Solutions from equation(1)also ignores dispersion,lateral flow,and Solution of the foregoing equations requires input data mixing with ground-water flow from upgradient areas. for several disposal system and site variables,all of which These processes would generally contribute to additional can have a significant effect on the predicted nitrate- reduction of nitrate-nitrogen concentrations in ground nitrogen concentration. Graphical solutions are presented water to the extent that the nitrate-nitrogen concentration here for typical ranges of these variables,as an aid in select- of ground-water flow from upgradient areas is lower. ing appropriate values, and in identifying situations of Equation(1)thus provides a conservative(worst case)first potential concern. approximation of ground-water nitrate-nitrogen concentra- The predicted resultant average ground-water nitrate- tion resulting from the combined effect of on-site sewage nitrogen concentration,nr,computed from equation(1)is disposal systems and precipitation.This is for estimation of plotted for convenience in Figure 1 against the fraction of long-term effects(i.e.,over years)on ground-water quality, waste-water recharge,I,relative to rainfall recharge,R,for a and is not intended for prediction of seasonal changes. selected range of values for soil denitrification,d,and waste- A common land use planning dilemma is that of water nitrogen loading, nw. Background nitrate-nitrogen determining acceptable development densities, sometimes loading,nb,typically falls in the range of 0.5 to 1.0 mg/1,and referred to as the carrying capacity of the land. From the is assumed here to be 1.0 mg/l.Exceptions to this would be standpoint of ground-water nitrate-nitrogen impacts, the if the area has large numbers of confined livestock or signifi- critical minimum gross acreage per developed lot,A,may be cant expanses of fertilized crops or turf areas(e.g.,parks), defined as that which would result in a value of nr equal to 10 which would tend to increase background nitrate-nitrogen mg/1,the commonly accepted drinking-water limit.By set- loadings above the typical values suggested here.The results ting I=0.01344 W/A and nr=10 mg/1,and then rearrang- plotted in Figure 1 show a wide range of potential effects, ing equation(1),A is then given by highly sensitive to the initial selection of values for nw and d. Two curves are plotted for the average value of nw=40 A= 0.01344W[nw—dnw— 10] 2 mg/l,with denitrification rates of 0 and 0.25,respectively. R(10—n b) ( ) The typical range is represented on the high and low sides by the curves for(a)nw=50 mg/1,d=0 and(b)nw=30 mg/1, in which A is expressed in terms of gross acres/dwelling unit d=0.25.The curve for nw=40 mg/I and d=0.25 would be (DU);W is the average daily waste-water flow per dwelling considered the most representative of typical on-site sewage unit,in gallons;and 0.01344 is a conversion factor having disposal situations(U.S. EPA, 1980; 1981). In addition to units acre inch day DU yr'gal-'. proper selection of values of nw and d,the importance of 40 j 36 P E nb= 1.0 mg/L tt z 30 O Z 25 d r IV 0 Lu p nw '1i mo 40 m9fl. —0 V 20 nV1 vr.', ktit tt 0 y �5 c. fl►yF Z Drinking `�qN�a i Z 15 Water !s-;;<v'::;: n —30 mg/L.d=0.2 6 < o:Jr3. W Standard ..:..,..e:,nr�...,.::::, Z10 ----- - ---- ------------------------------------ W ¢ 5 0 0 0.20 0.40 0.80 0.80 1.00 1.20 1.40 1.80 1.80 2.00 WASTEWATER RECHARGE RELATIVE TO RAINFALL RECHARGE. I/R 0 Predicted Zone for Most Common Values Fig.1.Resultant ground-water nitrate-nitrogen concentration as a function of effluent quality,denitrification,and I/R. 492