DEVIL’S SLIDE TUNNELS
Location: Pacifica, CA
About the Project
Devil’s Slide Tunnels were designed by California Department of Transportation (Caltrans) to re-route existing Highway 1 traffic around an unstable geologic formation called Devil’s Slide. Two 28′ diameter tunnels approximately 1 mile long with 10 cross passages were constructed. During excavation, groundwater and process drilling water was expected to reach a maximum of 650 gpm. Treated groundwater was discharged directly to the Pacific Ocean under an individual NPDES permit. A separate 650 gpm stormwater system was also set up for discharge.
The non-storm water treatment was originally designed for pH, turbidity, and hydrocarbons. Treatment components included pretreatment open top weir tanks, modular panel style treatment tanks, a control module (water quality monitoring-reporting), chemical injections system, bag and cartridge filter system, and hydrocarbon removal system. Limits for pH were 6.5 to 8.5 S.U. with continuous monitoring. Turbidity limit was <50 NTU’s instantaneous maximum. During annual testing, Chromium VI was detected above the Basin Plan, which necessitated a project shut down. Clear Creek solved this problem overnight by reducing it to Chromium III through an on-site treatment, putting the project back on schedule for our client. A similar 650 gpm system was set up for storm water discharges using a lined earthen basin. Clear Creek provided 24 hour monitoring and operations on-site.