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(CSWRCB, 1989). The Chico area is situated on recent
<br /> "say 5, alluvial fan materials derived from volcanic sediments and
<br /> 9 R 4 Meg" N mudflows originating in the hills to the east of Chico(DWR,
<br /> 1984). The alluvial deposits average about 40 to 50 feet in
<br /> B O L I N A S MESA \`� ; thickness and consist of unconsolidated cobbles,gravel and
<br /> STUDY AREA , ` `, �� Study and,and minor amounts of clay.These deposits support a
<br /> site
<br /> shallow unconfined aquifer that is recharged directly by
<br /> infiltration from precipitation, local runoff, and discharge
<br /> •�X from subsurface sewage disposal. Older alluvium imme-
<br /> �� diately underlies the recent alluvium and extends to depths
<br /> of nearly 450 feet. It is characterized mostly by thick clay
<br /> B O L IN A S layers and cemented sand and gravel. In this zone,ground
<br /> \ - o° == water occurs mainly in thin uncemented sand and gravel
<br /> lenses under semiconfined conditions,recharged by vertical
<br /> leakage from the overlying recent alluvium and from incised
<br /> --- -
<br /> = streams east of Chico.
<br /> -- ---- --..---- - _ __-----.__PACIFIC
<br /> - _ - =OCEAN
<br /> - - aywood-Los Osos
<br /> B
<br /> - The Baywood-Los Osos area is an unincorporated
<br /> coastal community located west of the City of San Luis
<br /> ° 114 1;2 sr. _1 M1e Obispo, immediately south of Morro Bay(Figure 6). The
<br /> majority of the area was subdivided largely for residential
<br /> • Monitoring wells Sampled Ground-water Flow Direction development in the early 1900s but significant development
<br /> Fig.4.Map of Bolinas Mesa study area. did not occur until the 1950s.The area impacted by on-site
<br /> sewage disposal systems comprises about 2,350 acres,with a
<br /> present density of approximately two to two and a half
<br /> dwelling units per acre, and typical lot sizes in the range
<br /> Bolinas Mesa of 5,000 to 10,000 square feet (CRWQCB, 1983). The
<br /> The Bolinas Mesa area is a residential subdivision of Baywood-Los Osos community is situated in the western
<br /> approximately 240 acres located about 15 miles north of end of Los Osos Valley, in an area dominated by marine
<br /> San Francisco.Initially created in the early 1900s,there are sediments and dune deposits(DWR,1973;Zipp, 1979).The
<br /> presently about 320 single family residences in the subdivi- valley is believed to consist of a single,unconfined aquifer
<br /> sion,on lots ranging from about 4,000 to 20,000 square feet system with a few isolated confined areas. The primary
<br /> in area(Figure 4). The subdivision occupies a coastal ter- aquifer consists of alluvium,sand dune deposits,and a thick
<br /> race, consisting of about 10 to 30 feet of sandy marine underlying siltstone known as the Paso Robles Formation.
<br /> terrace deposits, overlying a gently sloping, relatively The sand dune deposits are as much as 250 feet in thickness
<br /> impermeable shale bedrock surface(Questa, 1987).Ground and,historically,this formation has served as the principal
<br /> water collects in the terrace deposits as a result of local source of supply to pumping wells. The water table in the
<br /> rainfall percolation, forming an unconfined water-table area occurs at depths ranging typically from 15 to 30 feet
<br /> aquifer which varies from about five to 20 feet in saturated below ground surface.
<br /> thickness.The water table fluctuates seasonally,rising typi-
<br /> cally to within two to four feet of ground surface during the Summary
<br /> winter months, and receding to depths of five to 10 feet or Table 2 summarizes,for each of the three study areas,
<br /> more during the summer and fall. The topography of the the development characteristics that are pertinent to the
<br /> Bolinas Mesa is such that there are no streams or other assessment of nitrate loading impacts. For Chico and
<br /> significant sources of ground-water recharge that originate Baywood-Los Osos the data and calculated quantities are
<br /> from outside of the immediate subdivision vicinity,making shown for the respective study areas as a whole. For the
<br /> the study area relatively isolated from a hydrological Bolinas Mesa area, data are also shown for two smaller
<br /> perspective. subareas within the overall study area which are labeled,
<br /> respectively, the North and South study sites. This was
<br /> Chico Area possible because of the very site-specific data available for
<br /> The Chico study area consists of approximately 4,550 these two subareas. No similar subarea data were readily
<br /> acres (7.1 square miles) surrounding the City of Chico, available for the Chico and Baywood-Los Osos study areas.
<br /> located in the northern part of the Sacramento Valley The overall land area and the number of dwelling units
<br /> (Figure 5). The city itself is served by a central sewage for each area were obtained from maps and published docu-
<br /> treatment facility, so it is not considered part of the study ments prepared by the various county and state agencies
<br /> area.The area around the city consists of a mix of single and that have studied the respective areas.The density(dwelling
<br /> multifamily residential units and commercial development, units per acre) and average gross acreage per lot(acres per
<br /> with a density of approximately three dwelling units per acre dwelling unit)were computed directly from the given figures
<br /> 494
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