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evidence of close correspondence between actual and pre- priate. However, even in such cases, the vertical recharge dicted values confirms the validity of this method for esti- from waste water and rainfall will tend to accumulate and mating the area-wide nitrate effects on ground water from remain in a layer at the water table, largely unaffected by on-site sewage disposal systems. lateral inflow. This is due to the slow vertical mixing that With respect to the upgradient group of wells for occurs in horizontal ground-water flow.Use of the methods Baywood-Los Osos, one would expect the nitrate-nitrogen in this paper will protect against nitrate-nitrogen concentra- concentration to be considerably less than that predicted by tions in such upper lavers exceeding safe limits. equation (1), because these wells are not affected by the 2. The nitrate-nitrogen concentrations predicted by the majority of the development in the study area.This is borne methods of this paper are long-term values. First, the out by the results in Table 3 and Figure 7,which show that development of an area to its ultimate density and waste- the mean nitrate-nitrogen concentration in the upgradient water loading rates may take many years. Second,depend- wells is 43 percent of that observed in the downgradient ing upon the thickness and nature of the unsaturated zone, wells. the travel time of effluent to the water table could vary from days to years. Finally, where the vertical recharge of waste DISCUSSIOn water and rainfall adds to ground water in deep aquifers having little lateral flow, deep mixing will be a long-term Factors to be considered when using the simplified process. Such deep mixing could be caused by deep pump- mass balance method presented in this paper include the ing wells,leakage to even deeper aquifers,and ground-water following: outflow. 1. The method incorporates only the vertical compo- 3• The predictive equations are intended to be used to tions nent of ground-water recharge,ignoring any dilution effects evaluate average,area-wide ground-water conditions.They of lateral ground-water inflow from upgradient areas. From do not yield results that can be applied to a single point,such inium a planning and regulatory perspective, this is an appro- as might be required for siting or protecting an individual priate,conservative (worst case) approach. One must con- well.This would entail a more detailed analysis of the areal 1.9 sider that the nitrate-nitrogen concentrations in ground- and vertical distribution of nitrate-nitrogen in the ground 1.0 water inflow from upgradient areas may also increase over water. 1.9 time in response to waste-water loading or other land use 4_ The simplified methods here do not explicitly account 1.6 activities in those areas, thus making unreliable any esti- for other identifiable sources of nitrate-nitrogen, such as mates of the degree of dilution due to lateral ground-water animal wastes and fertilizer applications. Livestock wastes 1.4''`.. inflow. In circumstances where lateral ground-water inflow contain very high levels of nitrogen which may be a signifi- i•0 is determined to be significant and can be assigned a reliable cant contributor to ground-water nitrate-nitrogen concen- constant long-term nitrate-nitrogen concentration,then the trations,depending upon livestock densities,soil conditions, use of a mass balance model which includes such a lateral and waste handling practices. Wastes produced by a single (low component, e.g., Wehrmann (1984), may be appro- 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 6. In where significant livestock populations exist or can be ood- expected within the development area. titor- Lawn fertilizers contribute much less nitrate-nitrogen ntra- 23 ",=1.o mQ/L than do livestock. For typical residential subdivisions and Least rural communities,a reasonable assumption is that about 10 I\,- town , E 20 ,,•° ° ° percent of the gross area is landscaped with turf that is n �, ds ip of °: ,y° mofertilized. The nitrogen fertilizer rate for well-kept lawns is Drinking 0 25 ;how W c „ wala, - .o rAat�. estimated by nurseries to be about 40 to 6 lbs per year per ntire .4 $1nOir° ^'S d 0.25 acre of turf. Typically, 50 to 75 percent of the applied Z ¢ nw X30 mgIL. F 10 _____ _ _ ______ ______ nitrogen can be expected to be consumed by plant uptake W and soil denitrification (WPCF, 1990). The resultant load- s c ing to ground water is then approximately in the range of I ° a � to 3 lbs per year per developed acre. Foran assumed rainfall ble 3recharge rate of 12 inches/year, the resultant nitrate- the °° 0.20 0.40 0.60 o.eD 1 00 nigrogen concentration from the leaching of fertilizer would pre- WASTEWATER RECHARGE RELATIVE TO be about 0.37 to 1.1 mg/1. In the simplified methods of this of of RAINFALL RECHARGE, 1/R paper,this is considered to be substantially accounted for in pr O BoYnu M.ae,Norm Study Site V Chico Ar.a the assumption of a background nitrate-nitrogen concentra- thy' • BOYn..Ma.a,Soul"Study She A 9aywood/Lo.O.o.. tion of 0.5 to 1.0 mg/I.Where substantial portions of the site Bohn...Me.a.Area-Wide Downgradlenl are devoted to turf,special accounting may need to be made Pr.olt l.d Zone for Mo.l 9.ywood/Lo. O.o.. :cept Common Valu.a uoor.dlenl for fertilizer nitrate-nitrogen contributions. Mass balance i the Fig. 7. Comparison of actual and predicted nitrate-nitrogen models by Tinker(199 1)and the Center for Environmental This concentrations in ground water. Research (1985) incorporate a turf fertilizer component. 497