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accurately estimating the quantity of recharge waters is clearly evident, particularly in cases of higher nitrogen load- ing and lower denitrification rates. In Figure 2, the critical minimum gross acreage per lot, A, is plotted against the annual rate of rainfall recharge, R, for a selected range of values for nW and d, with nb = 1.0 mg/1 as before. In this instance the long-term waste -water flow, W, is assumed equal to 150 gal/day per DU, on the basis of an average expected occupancy of three persons per 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 seen to be significant, particularly for lower ranges of R, i.e., drier climates. Thus, in desert areas, very large lots may be necessary. In typical new developments of single family resi- dences, practical lot size limits exist because of minimum space requirements for site development, disposal fields, 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. 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 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 Baywood-Los Osos area in San Luis Obispo County (Figure 3). Description of Study Areas The general physical characteristics of the three study areas are summarized in Table 1. Background on the study sites is discussed below. 0L 0 W=160 pal/day/ dw ailing unit nb=1.0 mp/L 10 16 20 26 NATURAL RAINFALL RECHARGE, R, In/yr 30 Fig. 2. Influence of effluent quality, denitrification, and rainfall recharge on critical lot size. ------------- = BOLINAS= = =_ CHICO AREA T -MESA=_ —== O ` -------------- BAYWOOD-=_ LOS OSOS_= -- ==_=_- Oe ________-'_-_- =_____ ___ 0 100 200 300 400 600 Mlles I I I I I _I Fig. 3. Location of three case study communities in California. Table 1. Physical Characteristics of the Case Study Areas Characteristic 2.6 Chico area < Landform Marine terrace w Coastal dune Topography N 2.0 3 to 5% Soils Sandy loam and Sandy loam 0 sandy clay loam and sand £ O 0 1.6 7 r i O 0: O 0 0 Z J U <e u i 1.0 0.6 4 \\ f a o a Iw G \�0. a� o a ° q �0,� a: Estimated rainfall recharge (in./yr) 6 U 12.0 0L 0 W=160 pal/day/ dw ailing unit nb=1.0 mp/L 10 16 20 26 NATURAL RAINFALL RECHARGE, R, In/yr 30 Fig. 2. Influence of effluent quality, denitrification, and rainfall recharge on critical lot size. ------------- = BOLINAS= = =_ CHICO AREA T -MESA=_ —== O ` -------------- BAYWOOD-=_ LOS OSOS_= -- ==_=_- Oe ________-'_-_- =_____ ___ 0 100 200 300 400 600 Mlles I I I I I _I 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 Baywoodl 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