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Table 2.Development Characteristics of the Case Study Areas Bolinas Mesa area Characteristic North study site South study site Area wide Chico area Baywood/Los Osos Land area(acres) 2.75 1.72 240 4,550 2,350 No.of dwelling units 8 9 320 13,650 5,170 (DU) Density(DU/gross 2.9 5.2 1.3 3.0 2.2 acre) Gross average acreage 0.34 0.19 0.69 0.33 0.45 per lot,A(acres) Average waste-water 5.8 10.5 2.6 6.0 4.4 loading over gross area,I(in./yr) Relative waste-water 0.40 0.73 0.18 0.35 0.37 loading,I/R Sources:see text. Table 3.Ground-Water Nitrate-Nitrogen Data Summary Range of NO3-N concentrations Density of wells sampled No.of wells Total (mg/0 Study area (wells per acre) sampled samples Minimum Mean Maximum Bolinas Mesa •North study site 2.55 7 21 1.5 11.7 64.9 •South study site 4.07 7 21 1.5 13.9 51.0 •Total area 0.125 30 58 0.7 5.3 64.9 Chico area 0.0286 130 289 0.0 9.6 40.6 Baywood-Los Osos •Upgradient wells 0.00511* 6 21 0.0 4.5 13.4 •Downgradient wells 0.00681* 8 32 0.0 10.4 40.0 *Based on estimated apportionment of total area. Sources:See text. (Questa, 1987). The monitoring wells representing condi- over the Baywood-Los Osos area,as shown on Figure 6.In tions in the North and South study sites were sampled three comparison with the other two study areas,in the Baywood- times,and the others were sampled once.Table 3 shows the Los Osos area a significantly larger fraction of the monitor- total nitrogen expressed as NO3-N,assuming that the other ing wells(6 out of 14)is upgradient of the major concentra- forms of nitrogen,which occur in small quantities,will in tion of development;these wells are shown in the southeast time convert to nitrate within the ground water. These quadrant of Figure 6. Therefore, the data are shown values were obtained by summing the nitrate-nitrogen and separately for the upgradient and downgradient group of total Kjeldahl nitrogen data. wells.The downgradient wells would be expected to show Nitrate-nitrogen data for the Chico area were obtained the full effect of nitrate-nitrogen additions from the entire from the sampling of existing water-supply wells located developed area. within the defined study area,and drawing from the shallow ground-water zones.Well locations are shown on Figure 5. These data were obtained and reported by the California Comparison with Predicted Values Department of Water Resources, Department of Health The mean values for nitrate-nitrogen shown in Table 3 Services and Regional Water Quality Control Board during represent, for each of the study areas and subareas, the the period of 1984-1989(DWR, 1984;CSWRCB, 1989). resultant concentration that may be compared with pre- The data shown for Baywood-Los Osos were obtained dicted values obtained from equation(1).A graphical plot of from a special monitoring study conducted by the California the mean nitrate-nitrogen data for the various areas is pro- Regional Water Quality Control Board during 1983-1984 vided in Figure 7;for comparison with predicted values,the water year(CRWQCB, 1983/84). Samples were obtained curves of Figure 1 are included. quarterly from a network of wells completed in the upper As indicated,the observed values for all areas,except aquifer;these included active water-supply wells and moni- the upgradient wells for Baywood-Los Osos,fall within the toring wells.The wells were distributed relatively uniformly envelope defined by the curves of predicted values. This 496