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accurately estimating the quantity of recharge waters is 2.6 clearly evident,particularly in cases of higher nitrogen load- ing and lower denitrification rates. w=,ao o.ve.yraw.nl0o unu N 2.0 nb=1.0 m./L In Figure 2,the critical minimum gross acreage per lot, y A,is plotted against the annual rate of rainfall recharge,R, for a selected range of values for n,,and d, with nb= 1.0 y 4 mg/I as before. In this instance the long-term waste-water o 1 6 4 `� °O flow, W, is assumed equal to 150 gal/day per DU, on the _ ; ° f o a basis of an average expected occupancy of three persons per a 1.0_ residence and 50 gal/person/day.The U.S.EPA(1980)cites m 0 45 gal/day as the typical per capita flow for residential �� a`o? dwellings.The influence of climate and the water balance is 0.6 seen to be significant,particularly for lower ranges of R,i.e., o drier climates.Thus,in desert areas,very large lots may be 0 necessary. 0 6 ,o 16 20 26 30 In typical new developments of single family resi- NATURAL RAINFALL RECHARGE. R. In/yr dences, practical lot size limits exist because of minimum Fig.2.Influence of effluent quality,denitrification,and rainfall space requirements for site development, disposal fields, recharge on critical lot size. roadways,open space,etc.These limits may be on the order of 0.25 to 1.0 gross acres per dwelling unit, depending on local codes and specific development plans. As seen in Figure 2, such practical or statutory limits may often be more stringent than the critical minimum gross acreage per lot, A, determined from equation (2). This is particularly =______= n true as R values increase. --__---_--=- 7 CHICO AREA Case Study Examples To demonstrate and test their validity, the preceding methods for assessing nitrate impacts were compared against the actual ground-water quality data for three BOLINAS California communities.All three of these communities rely __MESA__=___ 0 on individual on-site systems for sewage disposal. In each case ground-water contamination by nitrates has been docu- -________-____--___—__ mented by extensive monitoring programs.The three com- _