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accurately estimating the quantity of recharge waters is 2.6 clearly evident,particularly in cases of higher nitrogen load W=150 911/dAyid—till,it..It - ing and lower denitrification rates. _ 2'0 In Figure 2,the critical minimum gross acreage per lot, A,is plotted against the annual rate of rainfall recharge,R, g a for a selected range of values for nw and d,with nb= 1.0 a mg/l as before.In this instance the long-term waste-water a o + a flow,W,is assumed equal to 150 gal/day per DU, on the a basis of an average expected occupancy of three persons per o ,.0 residence and 50 gal/person/day.The U.S.EPA(1980)cites 45 gal/day as the typical per capita flow for residential dwellings.The influence of climate and the water balance is 0.a 'OOH 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. o a ,o ,a vo za ao 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 true as R values increase. Y CNICO 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 =__===__======_ - === California communities.All three of these communities rely ==BOLINA3 _MPSA==__-=___= o \ 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- munities reviewed here as case study examples are:(1)the _-=-= _=- =-====- -=== -- Bolinas Mesa area in Marin County;(2)the Chico area in Butte County;and 3 the Ba ood-Los Osos area in San _=_nos Y O Yv ---------------- Luis -____----_-- Luis Obispo County(Figure 3). Description of Study Areas The general physical characteristics of the three study o ,00 zoo aoo 400 aoo Mile. areas are summarized in Table 1.Background on the study I I I I I i sites is discussed below. Fig.3.Location of three case study communities in California. Table 1.Physical Characteristics of the Case Study Areas Characteristic Bolinas Mesa area Chico area Baywood/Los Osos Landform Marine terrace Valley floor Coastal dune Topography 0 to 5% 0 to 2% 3 to 5% Soils Sandy loam and Sandy loam Loamy sands sandy clay loam and sand Depth to ground water(ft) 2 to 6 15 to 20 15 to 30 Average rainfall(in./yr) 30.9 22.5 20.0 Estimated rainfall recharge(in./yr) 14.4 16.8 12.0 Sources:see text. 493