Devil's Slide Improvement Project Dewatering Dilution Study

Client: California Department of Transportation

Prime Contractor: URS Corporation

Location: California State Highway 1 at Devil's Slide, San Mateo County

Goals: Conduct a dilution study to determine whether the San Francisco Regional Water Quality Control Board can justify granting Caltrans a dilution credit for the discharge of dewatering water from the Devil's Slide Tunneling Project to the ocean.

Project Description:

• Preparation, logistics, development of a health and safety plan
• Mobilization, dye release, and ocean onshore and offshore sampling
• Data analysis and QA/QC Reporting

Preparation and Logistics: A study design was developed centering on the calculation of dye concentration and total volume of discharge appropriate for accurate determination of dilution in the receiving waters, and on the identification of onshore and offshore sampling stations/transects necessary for measuring the dilution of the dye plume. ADH coordinated for site placement of four 21,000 gallon potable water holding tanks in which the dye was mixed and used a metered pump in order to convey a consistent flow rate from the holding tanks. Dye water discharge lines were directly connected from the holding tanks to an existing storm water drain pipe. Sampling logistics included three shore-based field crews, and one offshore crew on a sampling vessel equipped with a flow-through fluorometer.

Mobilization and Sampling: Rhodamine WT dye was mixed into holding tanks to achieve 100 ppb. The metered release of this dye discharge took place over two hours, during which coordinated sampling of the receiving waters was being conducted at five sites along the shore and several transects offshore. Drogues to measure current direction and speed were simultaneously deployed. Sampling continued for one hour after the discharge of dye was completed.

Reporting: Fluorescence and drogue data collected at all sampling stations were analyzed, processed, plotted, and reported in a comprehensive report detailing the dilution characteristics at receiving waters study area. ADH presented the study methods and results in a meeting and discussion with San Francisco Bay Regional Water Quality Control Board and Caltrans staff.