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The approximation of m??ate concentrations obtained Typical Sfttions \ from equation(1)also ignores dispersion,lateral flow,and Solution of the foregoing equations requires input d mixing with ground-water flow from upgradient areas. for several disposal system and site variables, all of wl; These processes would generally contribute to additional can have a significant effect on the predicted nits reduction of nitrate-nitrogen concentrations in ground nitrogen concentration. Graphical solutions are presen water to the extent that the nitrate-nitrogen concentration here for typical ranges of these variables,as an aid in self. of ground-water flow from upgradient areas is lower. ing appropriate values, and in identifying situations Equation (1) thus provides a conservative(worst case)first potential concern. approximation of ground-water nitrate-nitrogen concentra- The predicted resultant average ground-water nitre tion resulting from the combined effect of on-site sewage nitrogen concentration, nr, computed from equation (1 disposal systems and precipitation.This is for estimation of plotted for convenience in Figure 1 against the fraction long-term effects(i.e.,over years)on ground-water quality, waste-water recharge,I,relative to rainfall recharge,R,fc and is not intended for prediction of seasonal changes. selected range of values for soil denitrification,d,and wa-, A common land use planning dilemma is that of water nitrogen loading, n,r. Background nitrate-nitro] determining acceptable development densities, sometimes loading,n b,typically falls in the range of 0.5 to 1.0 mg/1,a referred to as the carrying capacity of the land. From the is assumed here to be 1.0 mg/I. Exceptions to this would standpoint of ground-water nitrate-nitrogen impacts, the if the area has large numbers of confined livestock or sign critical minimum gross acreage per developed lot,A,may be cant expanses of fertilized crops or turf areas (e.g., park defined as that which would result in a value of nr equal to 10 which would tend to increase background nitrate-nitrog mg/l,the commonly accepted drinking-water limit. By set- loadings above the typical values suggested here.The resu ting I=0.01344 W/A and nr= 10 mg/I,and then rearrang- plotted in Figure 1 show a wide range of potential effec ing equation (1), A is then given by highly sensitive to the initial selection of values for nw and Two curves are plotted for the average value of nw = A ( 0.01344W[nw — dnw— 10] 2) mg/1, with denitrification rates of 0 and 0.25, respective = R(10 — n b) The typical range is represented on the high and low sides the curves for(a)nw=50 mg/1,d=0 and(b)nw=30 mg in which A is expressed in terms of gross acres/dwelling unit d=0.25.The curve for nw=40 mg/1 and d=0.25 would . (DU); W is the average daily waste-water flow,per dwelling considered the most representative of typical on-site sewa unit, in gailons; and 0.01344 is a conversion factor having disposal situations (U.S. EPA, 1980; 1981). In addition units acre inch day DU yr I gal-I. proper selection of values of nw and d, the importance 40 a 36 E nb= 1.0 mg/L = 30 O it d y 0 w 25 OIL' ; v � 60m O O ow m0 201L. d 40 ti a'Iff"G.2b ;hw__AO ntgf�. O Z Drinking v I Z 16 Water x nw= 30 mg/L. d= 0.26 y<t Standard i,�< v f- 7 uj TiY uj 5 0L LL 0 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.60 2.0 .` 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 1/R. 492