Todd Engineers

The Valley of the Moon Water District, located in Sonoma County, serves potable water to a population of about 23,000. Eight-five percent of the District water supply is imported water from the Sonoma County Water Agency (SCWA) while the remainder is local groundwater.

Key Issues
Provision of high-quality groundwater Minimization of impacts on private wells and surface water

Because water deliveries from the SCWA are limited and subject to interruption, the District initiated a project, funded by SCWA, to develop additional well capacity to enhance the overall reliability of its supply, particularly during the peak summer demand season and during drought.

Objectives for the proposed Well No. 5 included a well yield greater than 100 gallons per minute and high water quality that would require no treatment beyond disinfection. Todd Engineers, teaming with Brown and Caldwell, evaluated ten potential well sites according to criteria including anticipated yield, water quality, potential environmental impacts, logistics and engineering feasibility, and cost. The prospect of a new municipal well in the small Sonoma Valley groundwater basin generated considerable concern among agricultural and domestic well owners, and the site selection process was conducted with extensive public outreach, including public presentations, flyers, and press releases.

A site on Verano Avenue was selected (see photograph, below right). Siting of the test well was tightly constrained on this small lot, but important advantages included ownership of the parcel by the District and the presence of a previously-installed nested monitoring well.

Services Provided
Siting and design of a municipal test well Supervision of drilling, construction, logging and testing Assistance with public participation Preparation of a well report

This nested well provided the opportunity for monitoring of shallow, intermediate and deep groundwater levels during aquifer testing of Well No. 5. Groundwater sampling of the 2-inch diameter monitoring well indicated elevated arsenic concentrations.

Verano Site 3

Recognizing that such small diameter monitoring wells are notoriously inefficient and water quality parameters could be in error, Todd Engineers recommended the installation of a small diameter test well.

Todd Engineers provided technical specifications and supervised the drilling and logging of a 1,000 foot exploration boring and construction of a 680-foot test well, later converted to Well No. 5. The test well was designed to production well standards. Testing included both a step- drawdown pumping test and a constant-discharge pumping test with monitoring of both the pumping well and nearby monitoring well to evaluate aquifer properties.

The testing indicated that proposed pumping would not affect shallow aquifer and surface water features. The water quality was also sampled and analyzed; the water parameters met all drinking water standards including arsenic. Todd Engineers also prepared a well report recommending conversion of the test well to a production well with a yield of 200 gallons per minute.