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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 Bolinas Lagoon.

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.

Figure 10 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 inches.

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 this area.

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 Patios area.

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 Figure 3, 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.

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