State of the Art Technology for Plume Mapping and Water Quality Monitoring in a Dynamic Bay Environment

Demolition of bridges or other marine structures can potentially release pollutants to aquatic environments. Water quality protection effectiveness assessment typically requires complex monitoring. This presentation discusses innovative predictive and monitoring technologies implemented during the controlled implosion of concrete structures located in San Francisco Bay (the Bay).
Following completion of the new East Span of the San Francisco Oakland Bay Bridge for seismic safety in September 2013, the California Department of Transportation (Caltrans) commenced dismantling of the old East Span. Removal of the underwater concrete bridge foundations was required to minimize navigational hazards and avoid an increase of net fill in the Bay. Following the successful Pier E3 pilot removal in 2015, eighteen more bridge piers were removed from 2016 to 2018 using controlled implosion. This talk presents lessons learned adaptively by monitoring water quality effects following each implosion, and noting how the persistence and dispersal of effects changed as the work progressed from deep into shallow water settings.
Plume mapping involves deploying a continuous monitoring device (sonde) to measure pH, turbidity, conductivity, temperature, dissolved oxygen, and depth. The plume mapping boat trolls back and forth across the impact area, raising and lowering the sonde while continuously acquiring data integrated with depth and GPS coordinates. Drift-drogues (underwater sails) approximately follow tidal currents delineating the plume. They are located by high visibility flags and GPS transponders to help monitoring vessel crews follow of the plume after implosion
Pre-project environmental studies predicted that pH would be the most significant water quality impact, and that effects would diminish within a short period of time (hours) and be limited to a small area (hundreds of feet). Numeric models of suspended sediment dispersion predicted worst-case profiles of turbidity that dispersed to background within hours. The studies also predicted small but detectable changes in concentrations of some metals in water and de minimis effects on benthic sediment quality in the area.
The monitoring results of water and sediment quality effects were consistent with project study predictions. Changes in turbidity were not distinguishable from background conditions. Increases in pH were more substantial, exceeding the water quality objective (8.5) within a small area for a brief duration (less than one hour), and returning to background within 3-4 hours as predicted. Transient increases in water column concentrations of some metals (e.g., chromium) were detected at levels below water quality objectives, as predicted. Dynamic monitoring of the plume was augmented by continuous monitoring of water quality using sondes deployed on buoys adjacent to environmentally sensitive eel grass beds. No significant water quality changes were detected at these locations.