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i (CSWRCB, 1989). The Chiu area is situated on recent
<br /> .�.. •• alluvW fan materials denved from volcanic sediments and
<br /> mudflows nngmabngmthe h8lstotheeaslol'Chico(DWR,
<br /> 1 J 1984)The alluvial deposits average about 40 to M feet in
<br /> ■OIIMaa YEla thickness and consist of unconsolidated cuhbks,gravel and
<br /> suoiaa'We ` ` ,• �,v w sand,and minor amounts of clay.These depd.its support a
<br /> shallow unconfined aquifer that is recharged directly by
<br /> tration from precipitation,local runoff,and discharge
<br /> �,,,;�..� ,,'♦\ from substuface sewage doposal.Older alluvium imine
<br /> druely underlies the recent alluvium and extends to depths
<br /> h '�� ♦ of nearly 450 feet It is charaeterimcl mostly by thick clay
<br /> 11111116L Alayers and cemented sand and gravel.In this zone,ground
<br /> - - - water oeeuts mainly in thin unmmenled sand and gravel
<br /> lenses under wmiconfined conditions,recharged by vertical
<br /> . leakage from the ovcrlymg recent alluvium and from incise)
<br /> streams cast of Chico.
<br /> v PactsIC
<br /> OCE.. Baywood-Los Osos
<br /> ----_- -- -- The Baywood-Los Osos area u an unincorporated
<br /> coastal community located west of the City of San Luis
<br /> Obispo,immediately south of Morro Bay(Figure 6),hhe
<br /> majority of the area was subdivided largely for residential
<br /> . ......., ,..«.. « a.......,......a....,., development in the early 1900,but significant development
<br /> Fig.4.Map M 8olinas Mets study cern. did not occur until the 1950s.The area impacted by on-site
<br /> sewage disposal systems comprises about 2,150 acre,with a
<br /> prescm density of approximately two to two and a half
<br /> dwelling units per acre,and typical lot sires in the range
<br /> Bofinas Mesa of 5,1x)0 to 10,000 square feet(CRWQCB, IM) The
<br /> The Bolin"Mesa area is a residential subdivision of Baywood-I_os Chat community is situated in the western
<br /> approximately 240 acres located about 15 miles north of end of Lo!Oso!Valley,in an area dominated by marine
<br /> San Francisco.Initially created in the early 1900s,there are sediments and dumcdeptssits(DWR,1971;Ztpp,1979).The
<br /> presently about 120 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).lie subdivision occupies a coastal ter- aquifer consists of alluvium,sand dune deposits,and a thick
<br /> ram,consisting of about 10 to 30 fret of sandy marine underlying ultoonse known As the Pas)Robles Formation.
<br /> rereace deposits, overlying a gently sloping, relatively The sand dune deposits are as much as 250 feet in thickness
<br /> impermeable shak bedrock surfam(Quies a,1987).Ground and,historically,this formation h"served as the principal
<br /> water collects in the terrace deposits As a result of heal source of supply to pumping wells.The water table in the
<br /> rainfall percolation, forming an unconfined water-tabic area occurs at depth. willing typically from 15 to 30 feet
<br /> aquifer which varics 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 Inured surface during the Summary
<br /> wider months,and receding to depths of five to 10 feet or Table 2 summarizes,for each of the three study areas,
<br /> mom during the summer and fall.The topography of the the development characteristics that arc pertinent to the
<br /> Bolin;Mw is such that then are no streams or other assesansem of nitrate loading impacts. For Chico and
<br /> significant sources of ground-water recharge that originate Haywood-I os Osot the data and calculated quantities arc
<br /> from outside of the Immediate subdivision vicinity,making shown for the m,pestive study areas as a whole.For the
<br /> the study arta relatively isolated from a hydrological Bolin.Mesa area,data arc also shown for two smaller
<br /> perspecsiw. subareas within the Overall study arta which an labeled,
<br /> respectively,the North and South study sites.This was
<br /> Chico Area possible because of rise very site-specific data available for
<br /> The Chico study area consists of approximately 4,550 these Iwo 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 Thos study areas
<br /> located in the northern pan of the Sacramento Valley The overall land area and the number ofdwciling units
<br /> (Figure 5).The city itself is served by a mniral sewage foreacharea wereoblainedfrom map and published docu-
<br /> imalmed facility,so is is not considered pan of the study swots prepared by the varum,county and state agencies
<br /> ams.theams amused the city consists of a mix)fsingk and than have studied n he respmirvc arc".I le density(dwelling
<br /> multifamily residential units and commercial development, units per acre)and average gross acreage per lot(acres per
<br /> with adensity ofapprountately three dwelling units per acre dwellmgum.)we,,computed dirmly from the given ftgum,
<br /> 4w
<br /> :�L,INC
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