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v
<br /> .r
<br /> Nor1A (CSWRCB, 1989). The Chico area is situated on rec
<br /> Slu Y ! MEg� `001,0 alluvial fan materials derived from volcanic sediments
<br /> `, N mudflows originating in the hills to the east of Chico(DV
<br /> �'. , �� 1984). The alluvial deposits average about 40 to 50 fee,
<br /> BOLINAS MESA ,
<br /> STUDY AREA thickness and consist of unconsolidated cobbles,gravel
<br /> stuth
<br /> udy sand,and minor amounts of clay.These deposits suppo,
<br /> rP a°� .�`;y'v \ s"° shallow unconfined aquifer that is recharged directly
<br /> ( . infiltration from precipitation, local runoff, and discha:
<br /> from subsurface sewage disposal. Older alluvium imr.
<br /> diately underlies the recent alluvium and extends to dep-
<br /> _ --�, a i' , r" ___= of nearly 450 feet. It is characterized mostly by thick c
<br /> oc��a layers and cemented sand and gravel. In this zone,grou
<br /> r--' ---
<br /> ___ water occurs mainly in thin uncemented sand and gra
<br /> ___ lenses under semiconfined conditions,recharged by verti
<br /> =====--======---- leakage from the overlying recent alluvium and from in&
<br /> ---_ _-=_---=_--
<br /> _ ===-=_==========___= streams east of Chico.
<br /> PACIFIC=_
<br /> OCEAN=
<br /> ________ -----------------------— _= =_==_=_________==_ Baywood-Los Osos
<br /> The Baywood-Los Osos area is an unincorporat
<br /> coastal community located west of the City of San L;
<br /> o' �� '12' " ' J Obispo, immediately south of Morro Bay(Figure 6). T
<br /> majority of the area was subdivided largely for resident
<br /> • Monitoring Wells Sampled °round-W.I.r Flo., olr.ollon development in the early 1900s but significant developme
<br /> Fig.4. hlap of Bolinas Mesa study area. r did not occur until the 1950s.The area impacted by on-si
<br /> sewage disposal systems comprises about 2,350 acres,witl
<br /> present density of approximately two to two and a h:
<br /> dwelling units per acre, and typical lot sizes in the ran.
<br /> Bolinas Mesa of 5,000 to 10,000 square feet (CRWQCB, 1983). T}
<br /> The Bolinas Mesa area is a residential subdivision of Baywood-Los Osos community is situated in the weste:
<br /> approximately 240 acres located about 15 miles north of end of Los Osos Valley, in an area dominated by maril
<br /> San Francisco. Initially created in the early 1900s,there are sediments and dune deposits(DWR, 1973;Zipp, 1979).Tl
<br /> w presently about 320 single family residences in the subdivi- valley is believed to consist of a single, unconfined aquif,.
<br /> sion,on lots ranging from about 4,000 to 20,000 square feet system with a few isolated confined areas. The primar
<br /> in area (Figure 4). The subdivision occupies a coastal ter- aquifer consists of alluvium,sand dune deposits,and a chic
<br /> race, consisting of about 10 to 30 feet of sandy marine underlying siltstone known as the Paso Robles Formatio
<br /> terrace deposits, overlying a gently sloping, relatively The sand dune deposits are as much as 250 feet in thickne
<br /> impermeable shale bedrock surface(Questa, 1987). Ground and,historically,this formation has served as the princip.
<br /> water collects in the terrace deposits as a result of local source of supply to pumping wells. The water table in th
<br /> W. rainfall percolation, forming an unconfined water-table area occurs at depths ranging typically from 15 to 30 1`6
<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 area.,
<br /> more during the summer and fall. The topography of the the development characteristics that are pertinent to th
<br /> Bolinas Mesa is such that there are no streams or other assessment of nitrate loading impacts. For Chico an(
<br /> significant sources of ground-water recharge that originate Baywood-Los Osos the data and calculated quantities arc
<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 smalle.
<br /> ' perspective. subareas within the overall study area which are labeled.
<br /> respectively, the North and South study sites. This waL
<br /> r Chico Area possible because of the very site-specific data available fe-
<br /> u The Chico study area consists of approximately 4,550 these two subareas. No similar subarea data were readilN
<br /> = ' acres (7.1 square miles) surrounding the City of Chico, available for the Chico and Baywood-Los Osos study areas.
<br /> _ 5 located in the northern part of the Sacramento Valley The overall land area and the number of dwelling unit!
<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 figure
<br /> 49.3
<br />
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