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Stinson Beach is a community of approximately 650 residences on the California coast that relies on onsite wastewater treatment and disposal systems. The Stinson Beach County Water District (District) provides water supply to the community and manages the onsite wastewater systems through its innovative Onsite Wastewater Management Program (OWMP). Concern over continuing construction of new residences and onsite wastewater systems led the District Board of Directors to impose a temporary building moratorium while a Hydrologic Survey was conducted to provide an improved understanding of local water resource systems, to evaluate impacts of wastewater systems on water quality and beneficial uses, and to provide recommendations to improve District water and wastewater management.

The Hydrologic Survey is summarized in this report. The study focused on areas with rapid soil percolation rates in the Seadrift, Calles, and Patios portion of Stinson Beach. Field work conducted in the late summer of 1997 included installation of nine permanent monitoring wells and three surface water staff gages, five water level measuring events, and three water quality sampling rounds. The new monitoring wells will provide the foundation for future groundwater quality monitoring.

Analysis of groundwater levels and flow revealed that the ocean and Bolinas Lagoon are the receiving surface water bodies for groundwater in Stinson Beach. Specifically, groundwater flow from the oceanside portions of Stinson Beach, Old Town, and Highlands areas generally is toward the ocean, while 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 use in Stinson Beach includes the municipal water supply pumping by the District, which relies on its Aldergrove 2 well, located along Stinson Gulch, for about 30 percent of its supply. The remaining water supply consists of springs and surface water sources. In addition, private wells have been drilled for irrigation purposes. Two wells located in the Highlands also have been identified as possible domestic drinking water sources.

General groundwater quality data showed that groundwater in the Seadrift, Calles, and Patios area consists of fresh water, derived from rainfall, landscape irrigation, and septic systems, that floats on underlying, higher-density seawater. Tidal fluctuations result in a mixing of freshwater and seawater, as does pumping. Groundwater underneath Seadrift and the Calles and Patios is not a likely drinking water supply because of the influence of seawater. In addition, no impacts on drinking water wells of wastewater disposal in the Seadrift, Calles, and Patios areas is expected because these areas are downgradient from the water supply wells.

Nonetheless, wastewater disposal is influencing groundwater quality in the Seadrift, Calles, and Patios areas, as indicated by the detection of ammonia and MBAS in most of the monitoring wells sampled during this study. However, the concentration of such wastewater indicators generally decreases with increasing distance along the groundwater flow path. Detections of fecal coliform were confined to two wells, suggesting that fecal contamination of groundwater is limited and localized. Concentrations of total coliform declined with successive sampling, suggesting that the detections could be the result of insufficient well purging and may not be representative of aquifer conditions.

Beneficial uses of surface water include contact water recreation in the lagoons and shellfish harvesting in Bolinas Lagoon; both of these beneficial uses raise concern over bacterial contamination. Surface water sampling revealed detections of coliform at or below the level of detection in Seadrift Lagoon. Total coliform was detected in Bolinas Lagoon at levels of 79 Most Probable Number (MPN)/100 milliliters (ml); fecal coliform also were detected once. Easkoot Creek showed consistent high levels of total and fecal coliform, amounting to counts of 1600+ and 920 MPN/100 ml, respectively. This suggests that the most significant impacts of wastewater on surface water occur through Easkoot Creek.

Nitrate loading calculations for the Seadrift, Calles, and Patios areas predict that current wastewater disposal practices would result in nitrate concentrations in groundwater that are generally within the drinking water standard of 10 milligrams per liter (mg/L). This estimate is consistent with the concentrations of nitrogen measured in groundwater during this study. In addition, the nitrate loading calculations show that under build out conditions, nitrate loading would increase, but could remain within drinking water standards if the current practice of using sand filter systems is continued. The nitrate loading calculations also demonstrate that the preponderance of nitrate in groundwater is the result of wastewater disposal, and that landscape irrigation is a minor factor. Accordingly, a policy regulating fertilizer use for landscaping is not warranted.

Although the water quality data from this study are limited, wastewater disposal in the Seadrift, Calles, and Patios areas appears to have only a small influence on beneficial uses in Bolinas Lagoon. The impact on Bolinas Lagoon of Easkoot Creek, reflecting drainage from the Highlands and Old Town, appears to be more significant. Accordingly, the focus of future water quality monitoring and protection should be the Highlands, Old Town, and Beach Park areas and Easkoot Creek. Additional permanent monitoring wells will be needed in the area, as well as analysis of the hydrogeology and impacts on water resources of wastewater disposal. Based on the findings of the Hydrologic Survey, the District's routine water monitoring program should be revised to include a network of reliable groundwater monitoring wells that cover not only the Seadrift, Calles, and Patios areas, but also the Highlands and Old Town. Monitoring should be extended to include wet season water level measurements and sampling. A community STEP (Septic Tank Effluent Pump) system may be warranted to serve the Old Town area, as well as portions of the Highlands, Calles, and Patios. The most promising site exists in the vicinity of the Highlands Tank on federal park land. Implementation of a STEP system in Stinson Beach will require cooperative efforts with the federal government, and potentially could be done in a way that would improve the existing waste disposal practices within the park itself.

With regard to the study area including the Seadrift, Calles, and Patios areas, the District has intensively monitored selected sand filter systems, documenting their good to excellent performance. A review of existing design criteria conducted in this study reveals no compelling reason to change the current criteria. Nonetheless, the District should consider broadening the criteria to allow variations in wastewater practices and innovative technologies, including changes in leachfield application rates, recirculating sand filters, aerobic treatment systems, ultraviolet disinfection systems, and subsurface drip irrigation systems.

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