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0 114 1;2 314 -- 1 Mlle
<br />• Monitorinp Wells Sampled N Ground -Water Flow Direction
<br />Fig. 4. Map of Bolinas Mesa study area.
<br />Bolinas Mesa
<br />The Bolinas Mesa area is a residential subdivision of
<br />approximately 240 acres located about 15 miles north of
<br />San Francisco. Initially created in the early 1900s, there are
<br />presently about 320 single family residences in the subdivi-
<br />sion, on lots ranging from about 4,000 to 20,000 square feet
<br />in area (Figure 4). The subdivision occupies a coastal ter-
<br />race, consisting of about 10 to 30 feet of sandy marine
<br />terrace deposits, overlying a gently sloping, relatively
<br />impermeable shale bedrock surface (Questa, 1987). Ground
<br />water collects in the terrace deposits as a result of local
<br />rainfall percolation, forming an unconfined water -table
<br />aquifer which varies from about five to 20 feet in saturated
<br />thickness. The water table fluctuates seasonally, rising typi-
<br />cally to within two to four feet of ground surface during the
<br />winter months, and receding to depths of five to 10 feet or
<br />more during the summer and fall. The topography of the
<br />Bolinas Mesa is such that there are no streams or other
<br />significant sources of ground -water recharge that originate
<br />from outside of the immediate subdivision vicinity, making
<br />the study area relatively isolated from a hydrological
<br />perspective.
<br />Chico Area
<br />The Chico study area consists of approximately 4,550
<br />acres (7.1 square miles) surrounding the City of Chico,
<br />located in the northern part of the Sacramento Valley
<br />(Figure 5). The city itself is served by a central sewage
<br />treatment facility, so it is not considered part of the study
<br />area. The area around the city consists of a mix of single and
<br />multifamily residential units and commercial development,
<br />with a density of approximately three dwelling units per acre
<br />494
<br />(CSWRCB, 1989). The Chico area is situated on recent
<br />alluvial fan materials derived from volcanic sediments and
<br />mudflows originating in the hills to the east of Chico (DWR,
<br />1984). The alluvial deposits average about 40 to 50 feet in
<br />thickness and consist of unconsolidated cobbles, gravel and
<br />sand, and minor amounts of clay. These deposits support a
<br />shallow unconfined aquifer that is recharged directly by
<br />infiltration from precipitation, local runoff, and discharge
<br />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 />layers and cemented sand and gravel. In this zone, ground
<br />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 />streams east of Chico.
<br />Baywood-Los Osos
<br />The Baywood-Los Osos area is an unincorporated
<br />coastal community located west of the City of San Luis
<br />Obispo, immediately south of Morro Bay (Figure 6). The
<br />majority of the area was subdivided largely for residential
<br />development in the early 1900s but significant development
<br />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 />of 5,000 to 10,000 square feet (CRWQCB, 1983). The
<br />Baywood-Los Osos community is situated in the western
<br />end of Los Osos Valley, in an area dominated by marine
<br />sediments and dune deposits (DWR, 1973; Zipp, 1979). The
<br />valley is believed to consist of a single, unconfined aquifer
<br />system with a few isolated confined areas. The primary
<br />aquifer consists of alluvium, sand dune deposits, and a thick
<br />underlying siltstone known as the Paso Robles Formation.
<br />The sand dune deposits are as much as 250 feet in thickness
<br />and, historically, this formation has served as the principal
<br />source of supply to pumping wells. The water table in the
<br />area occurs at depths ranging typically from 15 to 30 feet
<br />below ground surface.
<br />Summary
<br />Table 2 summarizes, for each of the three study areas,
<br />the development characteristics that are pertinent to the
<br />assessment of nitrate loading impacts. For Chico and
<br />Baywood-Los Osos the data and calculated quantities are
<br />shown for the respective study areas as a whole. For the
<br />Bolinas Mesa area, data are also shown for two smaller
<br />subareas within the overall study area which are labeled,
<br />respectively, the North and South study sites. This was
<br />possible because of the very site-specific data available for
<br />these two subareas. No similar subarea data were readily
<br />available for the Chico and Baywood-Los Osos study areas.
<br />The overall land area and the number of dwelling units
<br />for each area were obtained from maps and published docu-
<br />ments prepared by the various county and state agencies
<br />that have studied the respective areas. The density (dwelling
<br />units per acre) and average gross acreage per lot (acres per
<br />dwelling unit) were computed directly from the given figures
<br />FO
<br />Sttudy
<br />�5,
<br />Ste
<br />\
<br />BOLINAS MESA
<br />STUDY AREA \\ \ \
<br />south
<br />Study
<br />site
<br />mss,, \\ `\ \ �•� ___- "
<br />''>
<br />SOLINAS
<br />F
<br />OGS
<br />-- -
<br />-- --- - �`---� \ ----- _=-PACIFIC
<br />-- _-- ___-- - _-OCEAN___
<br />---------------------------
<br />0 114 1;2 314 -- 1 Mlle
<br />• Monitorinp Wells Sampled N Ground -Water Flow Direction
<br />Fig. 4. Map of Bolinas Mesa study area.
<br />Bolinas Mesa
<br />The Bolinas Mesa area is a residential subdivision of
<br />approximately 240 acres located about 15 miles north of
<br />San Francisco. Initially created in the early 1900s, there are
<br />presently about 320 single family residences in the subdivi-
<br />sion, on lots ranging from about 4,000 to 20,000 square feet
<br />in area (Figure 4). The subdivision occupies a coastal ter-
<br />race, consisting of about 10 to 30 feet of sandy marine
<br />terrace deposits, overlying a gently sloping, relatively
<br />impermeable shale bedrock surface (Questa, 1987). Ground
<br />water collects in the terrace deposits as a result of local
<br />rainfall percolation, forming an unconfined water -table
<br />aquifer which varies from about five to 20 feet in saturated
<br />thickness. The water table fluctuates seasonally, rising typi-
<br />cally to within two to four feet of ground surface during the
<br />winter months, and receding to depths of five to 10 feet or
<br />more during the summer and fall. The topography of the
<br />Bolinas Mesa is such that there are no streams or other
<br />significant sources of ground -water recharge that originate
<br />from outside of the immediate subdivision vicinity, making
<br />the study area relatively isolated from a hydrological
<br />perspective.
<br />Chico Area
<br />The Chico study area consists of approximately 4,550
<br />acres (7.1 square miles) surrounding the City of Chico,
<br />located in the northern part of the Sacramento Valley
<br />(Figure 5). The city itself is served by a central sewage
<br />treatment facility, so it is not considered part of the study
<br />area. The area around the city consists of a mix of single and
<br />multifamily residential units and commercial development,
<br />with a density of approximately three dwelling units per acre
<br />494
<br />(CSWRCB, 1989). The Chico area is situated on recent
<br />alluvial fan materials derived from volcanic sediments and
<br />mudflows originating in the hills to the east of Chico (DWR,
<br />1984). The alluvial deposits average about 40 to 50 feet in
<br />thickness and consist of unconsolidated cobbles, gravel and
<br />sand, and minor amounts of clay. These deposits support a
<br />shallow unconfined aquifer that is recharged directly by
<br />infiltration from precipitation, local runoff, and discharge
<br />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 />layers and cemented sand and gravel. In this zone, ground
<br />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 />streams east of Chico.
<br />Baywood-Los Osos
<br />The Baywood-Los Osos area is an unincorporated
<br />coastal community located west of the City of San Luis
<br />Obispo, immediately south of Morro Bay (Figure 6). The
<br />majority of the area was subdivided largely for residential
<br />development in the early 1900s but significant development
<br />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 />of 5,000 to 10,000 square feet (CRWQCB, 1983). The
<br />Baywood-Los Osos community is situated in the western
<br />end of Los Osos Valley, in an area dominated by marine
<br />sediments and dune deposits (DWR, 1973; Zipp, 1979). The
<br />valley is believed to consist of a single, unconfined aquifer
<br />system with a few isolated confined areas. The primary
<br />aquifer consists of alluvium, sand dune deposits, and a thick
<br />underlying siltstone known as the Paso Robles Formation.
<br />The sand dune deposits are as much as 250 feet in thickness
<br />and, historically, this formation has served as the principal
<br />source of supply to pumping wells. The water table in the
<br />area occurs at depths ranging typically from 15 to 30 feet
<br />below ground surface.
<br />Summary
<br />Table 2 summarizes, for each of the three study areas,
<br />the development characteristics that are pertinent to the
<br />assessment of nitrate loading impacts. For Chico and
<br />Baywood-Los Osos the data and calculated quantities are
<br />shown for the respective study areas as a whole. For the
<br />Bolinas Mesa area, data are also shown for two smaller
<br />subareas within the overall study area which are labeled,
<br />respectively, the North and South study sites. This was
<br />possible because of the very site-specific data available for
<br />these two subareas. No similar subarea data were readily
<br />available for the Chico and Baywood-Los Osos study areas.
<br />The overall land area and the number of dwelling units
<br />for each area were obtained from maps and published docu-
<br />ments prepared by the various county and state agencies
<br />that have studied the respective areas. The density (dwelling
<br />units per acre) and average gross acreage per lot (acres per
<br />dwelling unit) were computed directly from the given figures
<br />
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