Caltrans Headquarters - Devil’s Slide Improvement Project Dewatering Dilution Study

Shore-based Sampling to collect data used to model dye dilution in the receiving water
Offshore Sampling & Monitoring to collect data used to model dye dilution in the receiving water
Prime Contractor: 
Period of Performance: 
April, 2007 to October, 2007
Project Description: 

This project consisted of a dilution study to determine whether the San Francisco Bay Regional Water Quality Control Board (RWQCB) could justify granting Caltrans a dilution credit for the discharge of dewatering water from the Devil’s Slide Tunneling Project to the sensitive receiving water (Pacific Ocean).

Scope of Services: 


Key component activities were the following:

  • Development of a Dilution Study Ecological Assessment and project health and safety plan
  • Logistics, mobilization and performance of dye release, and ocean on- and offshore sampling
  • Data analysis, QA/QC, reporting and presentation of results to Caltrans and RWCQB staff

Preparation and Logistics

A Safety Plan was developed under contract with CH2MHILL that focused 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. Under a contract with URS, ADH performed the field study including coordination 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.


Fluorescence and drogue data collected at all sampling stations were analyzed, processed, plotted, and reported in a comprehensive report prepared under contract with URS detailing the dilution characteristics at receiving waters study area. ADH presented the study methods and results in a meeting and discussion with RWQCB and Caltrans staff.