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evidence of close correspondence between actual and pre- dicted values confirms the validity of this method for esti- mating the area -wide nitrate effects on ground water from on-site sewage disposal systems. With respect to the upgradient group of wells for Baywood-Los Osos, one would expect the nitrate -nitrogen concentration to be considerably less than that predicted by equation (1), because these wells are not affected by the majority of the development in the study area. This is borne out by the results in Table 3 and Figure 7, which show that the mean nitrate -nitrogen concentration in the upgradient wells is 43 percent of that observed in the downgradient wells. Discussion Factors to be considered when using the simplified mass balance method presented in this paper include the following: 1. The method incorporates only the vertical compo- nent of ground -water recharge, ignoring any dilution effects of lateral ground -water inflow from upgradient areas. From a planning and regulatory perspective, this is an appro- priate, conservative (worst case) approach. One must con- sider that the nitrate -nitrogen concentrations in ground- water inflow from upgradient areas may also increase over time in response to waste -water loading or other land use activities in those areas, thus making unreliable any esti- mates of the degree of dilution due to lateral ground -water inflow. In circumstances where lateral ground -water inflow is determined to be significant and can be assigned a reliable constant long-term nitrate -nitrogen concentration, then the use of a mass balance model which includes such a lateral flow component, e.g., Wehrmann (1984), may be appro- a E 0.20 0.40 0.80 0.80 1.00 WASTEWATER RECHARGE RELATIVE TO RAINFALL RECHARGE, I/R O BoNnas Mesa, North Study Site V Chico Area • Bolinas Mesa, South Study Site A Baywood/los Osos, Salinas Mese, Area -Wide Downpradient QPredicted Zone for Moat Baywood/Los Osos, Common Values Upprsdient Fig. 7. Comparison of actual and predicted nitrate -nitrogen concentrations in ground water. priate. However, even in such cases, the vertical recharge from waste water and rainfall will tend to accumulate and remain in a layer at the water table, largely unaffected by lateral inflow. This is due to the slow vertical mixing that occurs in horizontal ground -water flow. Use of the methods in this paper will protect against nitrate -nitrogen concentra- tions in such upper layers exceeding safe limits. 2. The nitrate -nitrogen concentrations predicted by the methods of this paper are long-term values. First, the development of an area to its ultimate density and waste- water loading rates may take many years. Second, depend- ing upon the thickness and nature of the unsaturated zone, the travel time of effluent to the water table could vary from days to years. Finally, where the vertical recharge of waste water and rainfall adds to ground water in deep aquifers having little lateral flow, deep mixing will be a long-term process. Such deep mixing could be caused by deep pump- ing wells, leakage to even deeper aquifers, and ground -water outflow. 3. The predictive equations are intended to be used to evaluate average, area -wide ground -water conditions. They do not yield results that can be applied to a single point, such as might be required for siting or protecting an individual well. This would entail a more detailed analysis of the area] and vertical distribution of nitrate -nitrogen in the ground water. 4. The simplified methods here do not explicitly account for other identifiable sources of nitrate -nitrogen, such as animal wastes and fertilizer applications. Livestock wastes contain very high levels of nitrogen which may be a signifi- cant contributor to ground -water nitrate -nitrogen concen- trations, depending upon livestock densities, soil conditions, and waste handling practices. Wastes produced by a single horse, for example, contain twice as much nitrogen as that from a typical household. This potential source should be added to the mass balance analysis when considering areas where significant livestock populations exist or can be expected within the development area. Lawn fertilizers contribute much less nitrate -nitrogen than do livestock. For typical residential subdivisions and rural communities, a reasonable assumption is that about 10 percent of the gross area is landscaped with turf that is fertilized. The nitrogen fertilizer rate for well -kept lawns is estimated by nurseries to be about 40 to 65 lbs per year per acre of turf. Typically, 50 to 75 percent of the applied nitrogen can be expected to be consumed by plant uptake and soil denitrification (WPCF, 1990). The resultant load- ing to ground water is then approximately in the range of 1 to 31bs per year per developed acre. For an assumed rainfall recharge rate of 12 inches/year, the resultant nitrate- nigrogen concentration from the leaching of fertilizer would be about 0.37 to 1.1 mg/ 1. In the simplified methods of this paper, this is considered to be substantially accounted for in the assumption of a background nitrate -nitrogen concentra- tion of 0.5 to 1.0 mg/ 1. Where substantial portions of the site are devoted to turf, special accounting may need to be made for fertilizer nitrate -nitrogen contributions. Mass balance models by Tinker (199 1) and the Center for Environmental Research (1985) incorporate a turf fertilizer component. 497