HYDROLOGIC SURVEY
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INTRODUCTION

Background

Stinson Beach is a community of approximately 650 residences located on the California coast 20 miles north of San Francisco. The community is unsewered, having consistently rejected centralized wastewater treatment and disposal plans. Accordingly, local homes and businesses rely on onsite wastewater treatment and disposal systems.

The Stinson Beach County Water District (District) was established in November 1962 to provide water and wastewater management, including conservation of water supply, prevention of water contamination, and improvement of water quality. Protection of the water quality and environmental benefits of Bolinas Lagoon has been a key concern of the local community. The District boundaries encompass the watershed areas tributary and to the east of Bolinas Lagoon, extending beyond Copper Mine Gulch to the north. The developed service area focuses on the community of Stinson Beach itself.

The District provides municipal water supply to Stinson Beach and manages the numerous onsite wastewater systems through its innovative Onsite Wastewater Management Program (OWMP). The OWMP includes regular review of proposed systems with regard to District standards, inspection and monitoring of existing systems, enforcement of District criteria for wastewater system performance, surface and groundwater quality monitoring, and other services such as public education on the proper use of onsite wastewater systems. The District summarizes its onsite wastewater management program in quarterly and annual reports, which are submitted to the San Francisco Bay California Regional Water Quality Control Board (RWQCB). In addition, the District has conducted a number of special investigations of water resources and the effects on water quality of wastewater disposal and treatment.

In recent years, construction of new homes and onsite wastewater disposal systems was allowed in areas with soil permeabilities that exceed percolation rate requirements through the granting of regulatory variances. This practice and the overall, continuing construction of new systems raised concerns with the District and RWQCB that increased use of onsite wastewater disposal systems would result in a threat to public health through contamination of local groundwater and surface water. As a result, the District Board of Directors adopted ordinance 1996-02 on November 16, 1996, which imposed a temporary moratorium on the granting of variances to the percolation rate requirement to allow construction of wastewater systems for new construction. In addition, the District authorized the performance of this Hydrologic Survey.

Purpose

The purpose of the Hydrologic Survey is to provide an understanding of the local groundwater system that is the physical context for onsite wastewater disposal. In addition, the Survey provides an overall evaluation of wastewater systems on groundwater quality, an assessment of the OWMP, and recommendations for improving District activities to ensure future water quality protection that is consistent with the beneficial uses of water defined by the RWQCB. This Hydrologic Survey provides the foundation for future water management by the District. In addition, the Hydrologic Survey establishes the basis for determining if the moratorium should continue or be rescinded.

Scope

The Hydrologic Survey was initiated on May 24, 1997 and completed on December 20, 1997. The survey included field work during the summer and early autumn, including installation of nine permanent groundwater monitoring wells and three surface water staff gages, five water level measuring events, and three water quality sampling rounds. Considerable water quality and level information was compiled, reviewed, and organized to assist in the documentation of local hydrogeologic conditions, wastewater practices, and evaluation of water quality. Existing reports and data compilations are listed in the Bibliography. A major focus was analysis of available groundwater and surface water quality data to evaluate background groundwater quality and to assess impacts of wastewater discharge on groundwater and associated surface water. Analysis also included preparation of a nitrate balance to evaluate nitrate loading from onsite wastewater systems and from landscaping fertilizer applications. In order to provide recommendations for improved water and wastewater monitoring and management, current onsite wastewater disposal practices and the OWMP were evaluated, and the potential for a STEP (Septic Tank Effluent Pump) community system was considered.

Study Area

The study area encompasses the community of Stinson Beach, shown in Figure 1. The community often is described in terms of distinct areas, including the Highlands, Old Town, Stinson Beach Park, Calles, Patios, and Seadrift. Seadrift, in turn is subdivided into the Seadrift Road and Dipsea Road areas. The Hydrologic Survey is focused on the areas with rapid soil percolation rates in the Seadrift, Calles, and Patios areas.

Acknowledgment

This report was prepared by Iris Priestaf and David Abbott of Todd Engineers and Norman Hantzsche of Questa Engineering, with assistance by staff members of both firms, notably Thomas Suggs and Alain Boutefeu of Todd Engineers and Randall Smith and Noahdiah Eckman of Questa.

We appreciate the considerable assistance of District staff, most notably Richard Dinges and Bonnie Jones, and District consultant Troy Pearce. Acknowledgement also is due to staff of the Regional Water Quality Control Board, Rich Condit and Blair Allen, who provided invaluable guidance.




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