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(CSWRCB, 1989). The Chico area is situated on recent shady i Sao"° alluvial fan materials derived from volcanic sediments and the 4 Mfg" \\` N mudflows originating in the hills to the east of Chico(DWR, 1984). The alluvial deposits average about 40 to 50 feet in B O L I N A S MESA \`, `; thickness and consist of unconsolidated cobbles,gravel and STUDY AREA south Study sand,and minor amounts of clay.These deposits support a site shallow unconfined aquifer that is recharged directly by infiltration from precipitation, local runoff, and discharge from subsurface sewage disposal. Older alluvium imme- `� diately underlies the recent alluvium and extends to depths of nearly 450 feet. It is characterized mostly by thick clay B O L I N A S v` layers and cemented sand and ravel. In this zone round Y g ,g o°�" == water occurs mainly in thin uncemented sand and gravel �� _ lenses under semiconfined conditions,recharged by vertical -_ -- - leakage from the overlying recent alluvium and from incised streams east of Chico. PACIFIC - _ OCEAN -- _ = Baywood-Los Osos - --- -- -- __ -- -= The Baywood-Los Osos area is an unincorporated coastal community located west of the City of San Luis I ;� t t 3r,4 -- ' �1e Obispo, immediately south of Morro Bay(Figure 6). The majority of the area was subdivided largely for residential • Monitoring Wells Sampled « Ground-Water Flow Direction development in the early 1900s but significant development Fig.4.Map of Bolinas Mesa study area. did not occur until the 1950s.The area impacted by on-site sewage disposal systems comprises about 2,350 acres,with a present density of approximately two to two and a half dwelling units per acre, and typical lot sizes in the range Bolinas Mesa of 5,000 to 10,000 square feet (CRWQCB, 1983). The The Bolinas Mesa area is a residential subdivision of Baywood-Los Osos community is situated in the western approximately 240 acres located about 15 miles north of end of Los Osos Valley, in an area dominated by marine San Francisco.Initially created in the early 1900s,there are sediments and dune deposits(DWR,1973;Zipp, 1979).The presently about 320 single family residences in the subdivi- valley is believed to consist of a single,unconfined aquifer sion,on lots ranging from about 4,000 to 20,000 square feet system with a few isolated confined areas. The primary in area(Figure 4). The subdivision occupies a coastal ter- aquifer consists of alluvium,sand dune deposits,and a thick race, consisting of about 10 to 30 feet of sandy marine underlying siltstone known as the Paso Robles Formation. terrace deposits, overlying a gently sloping, relatively The sand dune deposits are as much as 250 feet in thickness impermeable shale bedrock surface(Questa, 1987).Ground and,historically,this formation has served as the principal water collects in the terrace deposits as a result of local source of supply to pumping wells. The water table in the rainfall percolation, forming an unconfined water-table area occurs at depths ranging typically from 15 to 30 feet aquifer which varies from about five to 20 feet in saturated below ground surface. thickness.The water table fluctuates seasonally,rising typi- cally to within two to four feet of ground surface during the Summary winter months, and receding to depths of five to 10 feet or Table 2 summarizes,for each of the three study areas, more during the summer and fall. The topography of the the development characteristics that are pertinent to the Bolinas Mesa is such that there are no streams or other assessment of nitrate loading impacts. For Chico and significant sources of ground-water recharge that originate Baywood-Los Osos the data and calculated quantities are from outside of the immediate subdivision vicinity,making shown for the respective study areas as a whole. For the the study area relatively isolated from a hydrological Bolinas Mesa area, data are also shown for two smaller perspective. subareas within the overall study area which are labeled, respectively, the North and South study sites. This was Chico Area possible because of the very site-specific data available for The Chico study area consists of approximately 4,550 these two subareas. No similar subarea data were readily acres (7.1 square miles) surrounding the City of Chico, available for the Chico and Baywood-Los Osos study areas. located in the northern part of the Sacramento Valley The overall land area and the number of dwelling units (Figure 5). The city itself is served by a central sewage for each area were obtained from maps and published docu- treatment facility, so it is not considered part of the study ments prepared by the various county and state agencies area.The area around the city consists of a mix of single and that have studied the respective areas.The density(dwelling multifamily residential units and commercial development, units per acre) and average gross acreage per lot(acres per with a density of approximately three dwelling units per acre dwelling unit)were computed directly from the given figures 494