Occurrence of Groundwater
Groundwater in Stinson Beach exists in the bedrock, alluvium, and sand dune geologic
units. Local groundwater ultimately is derived from rainfall across the study area and the tributary
watersheds along Bolinas Ridge. Locally, the groundwater also is recharged by streamflow and by
onsite wastewater disposal systems.
Groundwater generally occurs under unconfined or water table conditions and is
hydraulically continuous with, and discharges to, surface water in the ocean, Bolinas Lagoon, and
Seadrift Lagoon. Groundwater also is hydraulically continuous with Easkoot Creek along portions
of its channel. In general, Easkoot Creek is a drain for groundwater in its upper reaches (above
Highway 1), where groundwater seeps and springs yield a year-round flow of groundwater to the
creek. Along the middle reach of Easkoot Creek, roughly between Highway 1 and the Calles,
flow dwindles seasonally and may cease near the end of the dry season and during droughts. This
suggests that the water table falls to a level below the creek channel bottom, and that the creek
provides recharge to the groundwater. In the lower reach below Calle del Arroyo, Easkoot Creek
is tidal, and again hydraulically continuous with the wetlands groundwater and Bolinas Lagoon.
Confined conditions (in which groundwater occurs under pressure greater than atmospheric
pressure) can occur in the alluvium in lower Stinson Gulch following the rainy season, as
indicated by artesian flow from Aldergrove 2 in spring 1990. The artesian conditions indicate that
the clay layer beneath Stinson Gulch is sufficiently extensive and impermeable to confine
groundwater in underlying sand and gravel zones, and that vertical groundwater gradients are
upward. Such conditions also likely extend beneath Bolinas Lagoon and the Dipsea side of
Seadrift; a noticeable inflow of water into MW-2 during drilling into the sand and gravel zone
suggests a relatively high hydraulic head in the lower zone.
Groundwater Levels and Flow
In Stinson Beach, the water table typically mimics the ground surface in a subdued way.
Accordingly, highest groundwater levels (relative to mean sea level) occur in the Highlands, with
levels declining toward mean sea level at the shore. The water table generally is less than 20 feet
below ground surface in the Highlands area and less than 10 feet deep in the Old Town, Calles,
Patios, and Seadrift areas.
Figure 8 is a water table contour map for Seadrift and part of the
Patios, while Figure 9
shows water table contours for part of the Patios, Calles, Highlands, and Old Town. The maps
reflect generalized conditions and are based on water level measurements not only from this study,
but also previous investigations in 1975-1976 and 1987. Use of information spanning 20 years
was deemed appropriate because local groundwater production is small and no evidence exists for
progressive groundwater declines. In addition, groundwater level fluctuations in the lowland areas
are restricted by topography and the proximity of the ocean and lagoons. Data used in the maps
are presented in Appendix B.
As shown in Figure 8, groundwater levels
along the Seadrift sandspit range from mean sea
level (MSL) to nearly four feet MSL. Overall, groundwater occurs as a long, linear mound
furrowed along the center axis by Seadrift Lagoon. The water level in Seadrift Lagoon is
relatively steady at about 2.5 feet MSL. Two secondary mounds occur respectively under the
Seadrift Road and Dipsea Road sides of the sandspit, reaching elevations exceeding three feet
MSL. The mound under the Seadrift side extends from the sandspit into the patios area.
The water table contour map indicates that groundwater flow along the entire Seadrift
sandspit is generally divergent outward to the ocean and Bolinas Lagoon. Under the Seadrift Road
side, groundwater flow is towards the ocean and Seadrift Lagoon. Groundwater flow on the
Dipsea side probably includes some flow toward Seadrift Lagoon, but appears to be largely toward
Figure 9 illustrates groundwater levels in
eastern Stinson Beach, including the Highlands,
Old Town, Calles, and Patios. Note that the water table contour interval in the Calles and Patios
is one foot, while the contour interval for the Highlands area is forty feet. The very steep
groundwater gradients suggested for the Highlands area indicates that the permeability of the
bedrock is low. Under the Highlands and Old Town, groundwater flow generally is toward the
ocean. Groundwater also flows locally toward springs and Easkoot Creek.
The Patios area is characterized by an elongated groundwater mound mirroring the line of
dune sands. This mound is generally continuous with the groundwater mound underlying the
Seadrift Road portion of the sandspit. Groundwater flow is toward the ocean and toward the
Easkoot Creek wetlands.
The water table contour maps shown in Figure 8 and
Figure 9 represent average and regional
groundwater levels and flow conditions. However, groundwater levels and flow at a specific time
and place in Stinson Beach are the result of the dynamic interaction of rainfall, stream, and
wastewater system recharge with discharge to tidal surface water bodies. These factors vary daily,
seasonally, and over a period of years.
presents three schematic cross sections that illustrate the variability of
groundwater levels and flow patterns across Seadrift, as documented in the new monitoring wells
during this Hydrologic Survey. The general position of residences and septic tanks are illustrated.
Groundwater level patterns are shown for the monitoring wells and staff gages on each cross
section for September 9, September 17, and October 28. The actual measurements for these dates
(and the other measurement events in September and October) are also recorded in
Table 2 and
Appendix B. It should be noted that groundwater levels are not
shown to scale, and that levels are
vertically exaggerated; review of measurement values reveals that they vary by tenths of a foot.
Review of the Seadrift side of the sandspit for all three dates shows a relatively constant
pattern of a groundwater mound with groundwater flow diverging toward the ocean and Seadrift
Lagoon. Groundwater levels probably fluctuate along the ocean beach as a result of tides, but no
data are available. Documented groundwater levels and flow patterns are relatively stable on the
Seadrift Lagoon side, reflecting the stability of the lagoon and the presence of numerous
residences providing recharge through landscaping and septic return flows.
The Dipsea side of Figure 10 shows
greater variability in groundwater levels and flow
patterns. As shown for September 9, Seadrift Lagoon remained steady, but Bolinas Lagoon
experienced a low tide prior to the groundwater level measurements. Groundwater levels decline
from Seadrift Lagoon to Bolinas Lagoon, suggesting groundwater through-flow under Dipsea.
On September 17, the Seadrift Lagoon level was relatively high and Bolinas Lagoon was
rising toward a high tide. Under these conditions, groundwater levels are relatively high near the
lagoons and relatively low in the interior, suggesting a brief period of groundwater flow
converging toward the center of Dipsea. However, this pattern of converging flow is only a
temporary phenomena due to high tides. A pattern of throughflow or diverging flow must be
more typical, allowing groundwater discharge or outflow to occur, because recharge from rainfall,
irrigation, or wastewater disposal is known to occur.
A third pattern is apparent on October 28. During this well monitoring event, Seadrift
Lagoon showed a typical water level, while Bolinas Lagoon experienced a very low tide.
Groundwater levels under Dipsea are higher than those in the lagoons, and groundwater flow
diverges outward toward both lagoons, albeit with a steeper gradient and greater flow to Bolinas
Lagoon. This pattern of divergent groundwater flow also was recorded on October 14, despite a
rising tide in Bolinas Lagoon.
The greater variability shown on the Dipsea side relative to the Seadrift side of the sandspit
probably reflects the relatively narrow area between Seadrift and Bolinas Lagoons and the lower
number of residences. Because only a single row of homes exists (as compared to the double row
along Seadrift Road), recharge from landscape and septic return flow is less likely to be sufficient
to maintain a stable groundwater mound as under the Seadrift side.
It should be cautioned that Figure 10
represents only a single section across Seadrift with
measurements taken during a relatively brief, late summer period. Nonetheless, a range of flow
patterns is indicated. Different portions of the sandspit may be best characterized by a single flow
pattern. For example, the eastern portion of Dipsea is characterized by a relatively high
groundwater mound (see Figure 8);
this mound may be relatively stable, leading to predominance
of the October 28 divergent flow pattern. Similarly, the westernmost end of Seadrift, or areas
with low housing density may be better characterized by the pattern recorded on September 9.
The groundwater flow patterns recorded in September and October also change with the seasons
and through drought and high rainfall periods. For example, it stands to reason that the rainy
winter season and high rainfall years would be marked by increased rainfall recharge, greater
development of groundwater mounds, and increased divergent groundwater flow and discharge to
the ocean and lagoons.
Figure 11 presents two schematic cross sections
that illustrate the variability of
groundwater levels and flow patterns along Calle del Resaca. Groundwater level measurements
are shown for the monitoring wells and staff gages for September 9 and October 28. It should be
reiterated that the groundwater levels are not shown to scale, and actually vary only by feet and
As shown by the measurements on September 9, a continuous groundwater gradient exists
from the edge of the Highlands to the ocean, indicating a southward groundwater flow. Seven
weeks later on October 28, groundwater levels under the sand dunes were higher and water levels
in the vicinity of Easkoot Creek were relatively low, resulting in groundwater flow converging in
Consideration of these two distinct patterns and review of the water table contour map
(Figure 9) suggests that the Calles are transitional between Old Town, where groundwater flow is
toward the ocean, and the Patios, where groundwater flow diverges toward the ocean and Easkoot
Creek wetlands. This transition probably occurs seasonally. Under wet conditions, Easkoot
Creek would be flowing and providing recharge to the underlying groundwater. This would help
maintain a continuous groundwater gradient to the ocean. Under dry, low streamflow conditions
(e.g., late dry season or drought) Easkoot Creek flows diminish or cease and local groundwater
levels would decline. However, recharge from landscaping and septic return flows would be
maintained along the dune line, resulting in a divergent groundwater flow pattern like that in the
Overall, the ocean and Bolinas Lagoon are the discharge points or receiving surface water
bodies for groundwater in Stinson Beach. Groundwater flow from the oceanside portions of
Stinson Beach, Old Town, and Highlands areas generally is toward the ocean, while groundwater
flow from the remaining portions of Seadrift, Calles, and Patios is toward Bolinas Lagoon.
Easkoot Creek intercepts groundwater and conveys it to Bolinas Lagoon.
Groundwater Level Changes
Only limited information is available on groundwater levels through the seasons and over
the long term. As noted previously, sustained groundwater trends are unlikely, given the limited
use of groundwater resources locally. Seasonal fluctuations were not documented during this
study; however, some data are available from previous investigations.
Groundwater levels in test wells and piezometers were measured between October 1970
and January 1971 for a soils investigation (Lee and Praszker, February 1971). Prior to and during
the investigation, rainfall amounted to 22 inches and groundwater levels rose, with increases
ranging between 1 and 3.5 feet in the Seadrift, Patios, and Calles areas. Highest increases
occurred in piezometers adjacent to Bolinas Lagoon, while groundwater level increases near
Seadrift Lagoon and near Easkoot Creek at Calle del Ribera were about one foot.
Groundwater level change data also are available from the routine groundwater monitoring
program of the District, which has included regular measurement of groundwater levels as part of
sampling procedures. These water level data were reviewed for seven wells to assess the range of
seasonal water level changes; of these, the water level data for Well G4, located near the beach at
Joaquin Patio (see Figure 2) are most complete and
representative. Figure 12 shows the water
level fluctuations from October 1991 to April 1997 in Well G4 along with monthly rainfall for
comparison. The water level data reveal a regular seasonal pattern of winter rainy season peaks
ranging between 2.3 and 4.5 feet MSL and summer dry season lows of 0.8 to 1.0 feet MSL. The
typical seasonal fluctuation is 2.5 to 3.5 feet.
Groundwater Use and Wells
Table 3 summarizes information on production wells that is available from water well
drillers reports filed with the California Department of Water Resources. As indicated,
information is available on fourteen production wells. The locations of these wells are shown on
Figure 2 and
with the exception of the Ranch Tank Wells 1 and 2, Highlands Tank Well, and
6500 Panoramic Highway well, located off the maps and generally uphill of the study area.
The known production wells are grouped in two areas, Stinson Gulch and in the eastern
portion of Stinson Beach including the Highlands, Panoramic, and Beach Park areas. The wells
drilled in Stinson Gulch include the Stinson Beach school irrigation well, and the District's
Aldergrove, Ranch, and Ranch Tank wells. The school well is shallow with a ten-foot sanitary
seal, but is used only for irrigation purposes. Of the District municipal wells, only Aldergrove 2
is currently in production, yielding approximately 30 percent of District municipal supply, with
the remaining supply provided by springs and surface water. Aldergrove 2 is 80 feet deep with an
18-foot sanitary seal. It penetrates nearly 70 feet of alluvium, including shallow clayey materials
overlying sand and gravel. The well screens are opposite the sand and gravel in the lower portion
and also include a section in underlying bedrock.
The wells include three private wells in the Highlands (15 Avenida Las Baulinas, 200
Belvedere, Leonard), one well on Panoramic Highway, and one well at the Beach Park. Three of
these wells are intended for irrigation purposes only, but two of the Highlands wells also were
intended for domestic purposes.