Groundwater Development and Management

Stinson Beach, California

Stinson Beach is a resort community located on the Marin County coast. While having fewer than 800 permanent residents, the community and surrounding parklands are popular recreational destinations that can attract tens of thousands of visitors on summer weekends. Fulfilling this seasonal water demand is a particular challenge for Stinson Beach County Water District, which relies on small local sources of surface water and groundwater for a drinking water supply.

Todd Engineers has assisted the District with development of groundwater supply, beginning with a pumping test on an existing municipal well (Alder Grove No. 2), located in the small alluvial-filled valley of Stinson Gulch.

A review of the construction details, operational history, water quality, and hydrogeologic environment indicated that Well 2 should be replaced with a new well due to chronic and elevated turbidity and decreasing well yields.

Subsequently, the District engaged Todd Engineers to manage and supervise the installation of a replacement well, Alder Grove No. 3. Working closely with the District, we prepared technical specifications, acquired bids, contracted a drilling contractor, and supervised all drilling, construction, development, and pumping test activities.

The well was drilled using cable tool methods and constructed to a depth of 65 feet with a natural filter pack, recognizing that the alluvial aquifer is composed mostly of uniform, coarse-grained sand. A permitting and regulatory variance was requested and granted for a 20 foot surface seal instead of 50 foot seal.

The well is very successful, both in terms of quantity and quality. The new well produces virtually no turbidity and yields over 140 gallons per minute (twice that of Well 2).

Todd Engineers prepared a report documenting the hydrogeologic setting and well drilling, design, installation, and testing. Recommendations also were provided, including conversion of the Alder Grove No. 2 Well to a monitoring well. Todd Engineers assisted the District in working with California Department of Public Health to allow conversion of the well instead of the usual abandonment.