Southern California Coastal Pollution Observed with SAR
by Ben Holt
The rapidly expanding southern California megalopolis, which includes San Diego, Orange, Los Angeles, Ventura and Santa Barbara counties, is home to approximately 20 million people who represent nearly 25 percent of the total U.S. coastal population. Activities of this large human population result in the discharge of a broad range of pollutants—pesticides, fertilizers, trace metals, synthetic organic compounds, petroleum, and pathogens—into the coastal waters of the Southern California Bight (SCB). The area has a complex physical circulation pattern due to varying bathymetry, offshore islands, and numerous prominent headlands, which affects transport of these pollution hazards.
Urban stormwater runoff is currently the most significant source of pollution hazard for coastal waters in the SCB. Stormwater runoff rates and volumes are growing in urban regions due to the expanding population and proliferation of impervious surfaces, i.e., roads and buildings, which limit the area where rainwater can soak into the ground.
Episodic storm events, normally occurring late fall through early spring, contribute more than 95 percent of the annual runoff volume and pollutant load in the SCB. These inputs modify the physical and biogeochemical state of coastal waters while presenting health hazards to swimmers and surfers in the area. Nutrient concentrations become elevated, promoting rapid phytoplankton growth.
Wastewater discharge from publicly owned treatment works (POTWs) and shoreline industries is another source of pollutants entering coastal waters. About half of this effluent receives secondary treatment. Discharge of the effluent offshore at depth (typically ~60 m) and the subsequent formation of submerged wastewater plumes promotes dilution and dispersal of contaminant loadings.
Occasionally, however, the submerged plumes surface, leading to possible onshore transport of contaminants. In addition, natural hydrocarbon seeps in the Santa Barbara Channel, and to a much lesser extent, in Santa Monica Bay, deposit tar and oil over many southern California beaches.
Space-borne Synthetic Aperture Radar (SAR) is a valuable tool for examining these pollution hazards in the SCB. Hazard detection with SAR is possible due to the surfactants deposited on the sea surface, smoothing capillary and small gravity waves to produce areas of reduced backscatter compared with the surrounding ocean.
The smoothed, surfactant-covered areas appear darker on SAR imagery compared with the usually windroughened surrounding ocean, which has higher backscatter and thus appears brighter on SAR imagery. Researchers can also use the imagery to visualize complex, smallscale oceanographic processes, such as coastal eddies, which are thought to be important in controlling the transport, near-shore residence times and fates of pollutants associated with these hazards.
The figures on this page show Radarsat-1 imagery of the Ballona Creek stormwater runoff discharge into Santa Monica Bay (bottom) and a coincident discharge and precipitation time series graph for one storm event (top). The SAR image shows a twolobed stormwater plume, on each side of a breakwater, imaged near the discharge peak.
Cumulative event discharge volume from Ballona Creek into the bay up to the time of the SAR image acquisition was 1.6 x 106 m3. This is associated with a cumulative event precipitation total of ~1.2 cm. Discharge, averaged over 15-minute intervals, peaked at 152 m3s-1 about 1.5 hours prior to image acquisition.
These results suggest that SAR imagery provides valuable information about pollution hazards, supporting improved coastal management in the SCB. Improved access to SAR data is needed, as well as studies linking SAR data with in situ water quality indicators. Together these data may allow synoptic assessments of the consequences of these pollution hazards for human health and ecological impact.