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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/1. 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[n., — dnw— 10] 2 mg/1, 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 n,,=40 mg/1 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 36 0 E nb= 1.0 mg/L c` Z 30 O f- ¢ 0 Z 25 vii d' U f30 to rf 2 0 O n� A0 togfl. Z 20 o Z Drinking d=0.26 Z 15 Water nw— 30 mgfL' w Standard ....;...:: ito ----- — ----- — ————--——————————— — ————————————--———— J rn ul 5 0 0 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 WASTEWATER RECHARGE RELATIVE TO RAINFALL RECHARGE. I/R 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