The Southern California Bight (SCB) is an ecologically important marine habitat and a valuable resource in terms of commercial fishing and recreation. With a census of over 17 million people, potential impacts of such a large population are cause for concern on coastline water quality. Los Angeles and Orange Counties are home to two of the largest wastewater treatment plants along the U.S. West Coast (Figure 1). The Hyperion Treatment Plant (HTP) and the Orange County Sanitation District (OCSD) discharge secondary-treated wastewater, or effluent, into the SCB from outfall pipes that terminate five miles offshore at a depth of about 60 meters to limit risks to the marine environment and human health (Figure 1, black lines). Treated effluent contains contaminants including oils, various chemicals and metals, and nutrients. The effluent disperses and sufficiently mixes with deeper ambient waters remaining trapped at depth under typical density-stratified conditions; however, on occasion during periods of reduced stratification (September-December timeframe in the SCB) the submerged buoyant plumes can surface. When scheduled or emergency maintenance on the normal outfall system is necessary, effluent is diverted to shorter outfall pipes that terminate one mile offshore at shallower depths (i.e., approximately 20m) (Figure 1, red lines). Such temporary release of wastewater in nearshore environments increases the risk to human health, water quality, and coastal ecosystems, especially in times of reduced water column stratification. Two such SCB diversions include the 2006 HTP (28-30 November 2006) and 2012 OCSD (11 September – 3 October 2012) diversions. During both diversions, extensive in situ and satellite data were collected to detect the wastewater plume and assess its impact on nearshore water quality. Here we examine the results from satellite data.
Figure 1. Overview map of the Hyperion Treatment Plant (HTP) and Orange County Sanitation District (OSCD) outfall pipes. Red and black lines indicate HTP and OCSD short outfall pipes and long outfall pipes. Red box in inset indicates our area of interest within the Southern California Bight.
Cooler sea surface temperatures (SSTs) were observed from satellite in the vicinity of the shorter outfall pipes during the 2006 HTP and 2012 OCSD diversions (Figure 2). The observed SST response is most likely due to the entrainment of cold subsurface water as the buoyant wastewater released at depth rises to the surface.
Figure 2. MODIS-Aqua SST (°C) during the (a) 2006 HTP diversion on 29 November 2006 and (b) 2012 OCSD diversion on 22 September 2012. The long and short outfall pipes are shown. Low SSTs are indicative of the plume signature and are observed in the vicinity of the short outfall pipes.
Enhanced phytoplankton biomass is expected in association with wastewater outfalls, wherein increased nutrient loading into the coastal ocean can stimulate phytoplankton growth and production, but nominally is expected to occur with a lag period of several days. From satellite, chlorophyll-a (chl-a) provides a proxy for phytoplankton abundance and biomass; however, satellite chl-a values are known to be problematic in optically complex coastal waters. The primary reason being that the standard ocean color processing algorithms were developed for open ocean waters and thereby overestimate chl-a in shallow near-shore zones due to the presence of colored dissolved organic matter and suspended matter. As a result, it is likely difficult to distinguish enhanced chl-a responses to wastewater diversions. This was true for the 2006 HTP diversion, wherein the contribution of the wastewater plume to the observed response was unclear given high chl-a in adjacent waters (Figure 3a). In contrast, decreased chl-a concentrations were observed during the 2012 OCSD diversion in the vicinity of the short outfall pipe (Figure 3b). Field and laboratory studies confirm the satellite response, finding low phytoplankton biomass in association with enhanced chlorination of the discharged water that suppressed the phytoplankton response.
Figure 3. MODIS-Aqua chl-a (mg m-3) during the (a) 2006 HTP diversion on 29 November 2006 and (b) 2012 OCSD diversion on 1 October 2012. The long and short outfall pipes are shown. In (a) the wastewater plume is difficult to distinguish from adjacent waters, whereas in (b) it is denoted as decreased chl-a in the vicinity of the short outfall pipe.
Wastewater plumes are rich in oils and grease, creating smooth slicks on the ocean surface that can be identified using sea surface roughness data from synthetic aperture radar (SAR) sensors. SAR imagery is often used to detect and track marine slicks including biogenic surfactants, natural seeps, and spilled oil. The surface expression of the wastewater plume was clearly identified by SAR imagery as a distinct dark feature of decreased backscatter (Figure 4a). During the HTP diversion, wind speed was relatively low allowing the effluent plume to stay intact and be more distinguished in SAR imagery as a conspicuous dark feature. The HTP diversion ceased on 30 November 2006. Remnants of what is believed to be the effluent plume can still be detected with SAR data on 1 December 2006 (Figure 4b)
Figure 4. (a) Envisat ASAR image during the 2006 HTP diversion on 30 November 2006 and (b) Radarsat-1 SAR image after the 2006 HTD diversion on 1 December 2006. The wastewater plume is observed as dark areas of reduced backscatter, indicated by green boxes. The long and short outfall pipes are shown. Drifter tracks are overlaid on (a) with +’s illustrating initial drifter locations. Drifter tracks in (a) support the interpretation that the dark SAR feature is a surface expression of the wastewater effluent plume.
Multi-sensor satellite remote sensing proved useful for detection of wastewater plumes in the SCB and their impacts. It offers a viable and complementary technique to existing in situ sampling methods for monitoring and maintaining water quality and human health standards in the coastal and nearshore environment.
PO.DAAC Science Team in collaboration with Rebecca Trinh, Boyang Pan, and Christine Rains (NASA DEVELOP)