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The approximation of nitrate concentrations obtained from equation (1) also ignores dispersion, lateral flow, and mixing with ground -water flow from upgradient areas. These processes would generally contribute to additional reduction of nitrate -nitrogen concentrations in ground water to the extent that the nitrate -nitrogen concentration of ground -water flow from upgradient areas is lower. Equation (1) thus provides a conservative (worst case) first approximation of ground -water nitrate -nitrogen concentra- tion resulting from the combined effect of on-site sewage disposal systems and precipitation. This is for estimation of long-term effects (i.e., over years) on ground -water quality, and is not intended for prediction of seasonal changes. A common land use planning dilemma is that of determining acceptable development densities, sometimes referred to as the carrying capacity of the land. From the standpoint of ground -water nitrate -nitrogen impacts, the critical minimum gross acreage per developed lot, A, may be defined as that which would result in a value of nr equal to 10 mg/1, the commonly accepted drinking -water limit. By set- ting I = 0.01344 W/ A and nr = 10 mg/ 1, and then rearrang- ing equation (1), A is then given by A0.01344W[n,, — dn,, — 10] (2) = R(10 — nb) in which A is expressed in terms of gross acres/dwelling unit (DU); W is the average daily waste -water flow per dwelling unit, in gallons; and 0.01344 is a conversion factor having units acre inch day DU yr -1 gal-`. 40 36 a E c` Z 30 O F z 25 Lu U Z O U 20 Z I M O Z Z 15 < W f Z 10 J w 5 0 Typical Solutions Solution of the foregoing equations requires input data for several disposal system and site variables, all of which can have a significant effect on the predicted nitrate - nitrogen concentration. Graphical solutions are presented here for typical ranges of these variables, as an aid in select- ing appropriate values, and in identifying situations of potential concern. The predicted resultant average ground -water nitrate - nitrogen concentration, nr, computed from equation (1) is plotted for convenience in Figure 1 against the fraction of waste -water recharge, I, relative to rainfall recharge, R, for a selected range of values for soil denitrification, d, and waste- water nitrogen loading, nw. Background nitrate -nitrogen loading, nb, typically falls in the range of 0.5 to 1.0 mg/ 1, and is assumed here to be 1.0 mg/ 1. Exceptions to this would be if the area has large numbers of confined livestock or signifi- cant expanses of fertilized crops or turf areas (e.g., parks), which would tend to increase background nitrate -nitrogen loadings above the typical values suggested here. The results plotted in Figure 1 show a wide range of potential effects, highly sensitive to the initial selection of values for nw and d. Two curves are plotted for the average value of nw = 40 mg/ 1, with denitrification rates of 0 and 0.25, respectively. 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, d = 0.25. The curve for nw = 40 mg/ 1 and d = 0.25 would be considered the most representative of typical on-site sewage disposal situations (U.S. EPA, 1980; 1981). In addition to proper selection of values of nw and d, the importance of 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