Cigarette butts, plastic bags, animal waste, worn out car tires, automotive oil; take a walk on any city street in the United States and you’ll find pollution. And when it rains, the stormwater washes these contaminants from the urban environment off into near-shore waters. This untreated stormwater contains debris, trash, contaminated bacteria, heavy metals, pesticides, fertilizer, petroleum hydrocarbons, emission pollutants, suspended solids and oils plus other industrial and commercial waste. All this unsavory stuff modifies the physical and biogeochemical conditions of coastal waters and can be a hazard to human health. And because approximately half the world's population lives within 200 km of a coast, monitoring the pollution levels of coastal waters is critical.
"The stormwater problem is increasing because of the growing extent of pavement and buildings throughout the area," Ben Holt, lead author of a recent study published in the Marine Pollution Bulletin titled “Stormwater runoff plumes in the Southern California Bight: A comparison study with SAR and MODIS imagery” said. “That means less water is percolating back into the ground water and more is running off, taking with it whatever waste has accumulated in the streets,” Holt explained.
Determining the full extent and dispersal of pollutants by relying on research vessels alone is difficult because these measurements only provide sparse and coarse spatial sampling of stormwater plumes in coastal waters. Remote sensing could enhance the assessment of coastal pollution and related beach contamination. It could also help guide the collection of in situ water quality data by providing a more comprehensive view and direction of transport.
With this in mind, Holt and a team of oceanographers based out of NASA’s Jet Propulsion Laboratory set out to discover if fine resolution satellite imagery would be useful for examining the environmental impacts of stormwater runoff. In order to determine if a satellite-monitoring concept would be feasible, the science team examined an extensive collection of multi-platform Synthetic Aperture Radar (SAR) images of the surface slicks from stormwater plumes that are composed of the unsavory material listed above from 1992 to 2014 and compared the SAR-derived plumes with the related sediment-laden plumes detected from MODIS-Aqua ocean color imagery in the Southern California Bight (SCB).
The SCB is a coastal ecosystem of particular importance because it’s fed from urban Los Angeles County and Orange County watersheds and serves nearly twenty five percent of the coastal population of the United States. In large urban areas like Southern California, impervious land surfaces (pavement and buildings) have expanded due to population growth. And these impervious land surfaces reduce the amount of rainfall that enters into groundwater and increases the amount of stormwater that runs off.
Additionally, ninety five percent of the annual runoff volume and pollutant load in Southern California comes from episodic storm events that mainly occur during the rainy season from October to March. So during the dry months, pollutants on urban surfaces and in storm drains accumulate until the onset of the storms. During a rain event, the loads of particulate and dissolved compounds that get flushed through man-made waterways and drainage systems are combined with plant debris and trash and these untreated pollutants get discharged into SCB coastal waters. When it rains more often, there is less time to accumulate contaminants and debris which leads to reduced concentration of these pollutants in the stormwater.
Figure 1. Study region illustrating the four major watersheds studied within the Southern California Bight. Also indicated are the locations of the stream gages (circles) that provided discharge data and weather stations (triangles) located at three airports within or near the watersheds. USGS stream gage for the San Gabriel River is the northernmost gage along the river.
To examine and assess the impact of stormwater runoff, the study examined runoff plumes from the four major man-made SCB watersheds: Ballona Creek, the Los Angeles River, the San Gabriel River, and the Santa Ana River using ERS-1, Radarsat-1, Envisat, ALOS-1, and Sentinel-1 SAR imagery, as well as imagery from NASA’s UAVSAR airborne instrument. When available, the remote imagery was combined with in situ measurements of fecal bacteria along beaches adjacent to the river outlets.
SAR instruments are able to detect measurable differences in sea surface roughness between normal sea state and stormwater runoff plume areas. The turbulent nature of stormwater runoff plus the fact that it has high concentrations of buoyant floating material including oils, surfactants, and plant material, means that the runoff plumes smooth out the ocean surface roughness by wave dampening, reducing the radar backscatter within the slick when compared to adjacent waters.
Figure 2. Time series of three Envisat ASAR images of stormwater plumes exiting Ballona Creek over a short time period in late 2008 A) December 15 at 09:58 PST, B) December 17 at 21:47 PST, and C) December 18 at 10:04 PST, with D) showing the discharge rate and precipitation during the overlapping time period, as well as plume surface area of each SAR observation.
Figure 3. Probability or heatmaps showing the distribution of SAR-detected stormwater runoff surface plumes by percentage of plume coverage for the A) SantaMonica Bay and B) San Pedro Shelf.
The team also compared SAR-derived plumes, which give information about the surface slick, with plumes from MODIS-Aqua ocean color imagery, which give information about the sediment load. Stormwater runoff plumes contain sediments, organic matter and nutrients that alter the optical properties of the water, which can be measured using the MODIS instrument on the Aqua satellite. MODIS imagery captured stormwater plumes as significant suspended sediment concentrations in coastal waters adjacent to river mouths within four days of the onset of rain events. The team was able to compare MODIS plume and SAR plume data for 7 out of 16 separate rain events.
Figure 4. Comparison of stormwater plumes observed from two different sensors. A) Envisat ASAR image acquired February 07, 2009, at 10:02 PST, showing surface runoff plumes from the Los Angeles-San Gabriel Rivers and the Santa Ana River, B)MODIS-Aqua remote sensing reflectance (Rrs at 555 nm) on February 11, 2009, at 13:15 PST, nearly 4 days later than the SAR image in A), showing the suspended sediment runoff plume.
The SAR-detected surface plumes are closely tied to concurrent, recent precipitation. The SAR plumes appear to persist only for a short time period until the surficial material composing the radar-dark plume disperses from the source. Eventually, the plumes mix with the surrounding ocean water and the concentration is reduced to a level that is no longer effective in small wave damping, hence a reduction in radar backscatter.
From the series of SAR images of runoff plumes, the study determined the predominant extent of the surface slicks (4-6 kilometers) and if the plumes were likely to be transported offshore or along the coastlines. Using the limited coincident collection of measured beach bacterial contamination, the study found that the highest levels of contamination fell within the extent of the surface slick.
Figure 5. Three Envisat SAR images comparing stormwater plumes and alongshore measured bacterial contamination obtained from beach sampling on the same day. A) San Pedro Shelf on Feb. 07, 2009, B) San Pedro Shelf on Dec. 15, 2008, C) Santa Monica Bay on Dec. 15, 2008, for which no beach sampling took place, D) San Pedro Shelf on Dec. 17, 2008, and E) Santa Monica Bay on Dec. 17, 2008. Exceedance of 104 cfu/100 ml is unsafe for recreational activity.
The study also found that the runoff plumes detected in MODIS are considerably larger than the SAR surface runoff plumes. The MODIS image shows a large area of higher suspended sediment extending along the coast of the three river outlets and further offshore than the SAR surface plumes. This is due to the differences in sensor resolution and highlights the different detection capabilities of radar and optical sensors. Radar sensors detect the surface slick, while optical sensors detect what is in the water itself, such as suspended sediments.
In contrast to SAR instruments, optical sensors cannot penetrate through cloud cover. After the precipitation has ceased and discharge within the watershed has returned to nominal levels, the sediment-laden stormwater plumes are detected by MODIS imagery and persist for several days, well past the end of storm-related precipitation and discharge, until the sediments gradually settle out, become diluted with ambient ocean water, and are no longer detectable. Total suspended sediments make up the greatest load of constituents of stormwater runoff in SCB. So it is not surprising that the sediment plumes, measured by optical sensors, are more extensive and take up a much larger surface area than the SAR surface plumes.
This suggests that combination of radar and optical sensors to detect the extent and direction of stormwater plumes and monitor water quality within the SCB would enhance the assessment of contamination by stormwater runoff from major watersheds. The different detection capabilities of both SAR and ocean color data, including MODIS-type sensors, suggest that an optimal satellite monitoring system for observing urban stormwater runoff in Southern California would incorporate both types of satellite capabilities. SAR can provide all-weather imaging independent of sunlight with fine spatial resolution (<100 m).
While MODIS-type sensors provide the capability of daily observations dependent on cloud cover at coarser resolutions (> 1 km) than SAR. SAR would provide the earliest detection of the stormwater runoff plume, within zero to three days of the storm event, verify the outlet source with fine resolution, indicate the direction of transport either alongshore or offshore and plume extent, and possible contamination of nearby beach water quality by the proximity of the surface plume to adjacent coastlines. After the storm-related cloud cover passes and the storm has ended, the ocean color sensor would map the extent of the suspended sediment component of the plume and its transport over several days, likely over a longer period than the SAR detection of the surface plume.
Utilizing multiple satellite sensors, and combining the use of SAR and ocean color sensors could help offset the limitations of each type of instrument. So using both SAR and ocean color imagery may provide the most complete monitoring approach for any individual storm.
"I do think there’s a way to make the pollution problem better by reducing the pollutants that are entering the ocean," said Holt. "An important lesson from the study is to keep our streets clean, as any contamination will impact the beaches and coastal ecosystems," he added.
Ultimately remote sensing could help predict or forecast the plumes and help municipal agencies do a better job helping the beach communities. Holt and the NASA science team received numerous responses to the study publication, indicating a demand for this type of information.
Most of this study was conducted through NASA’s DEVELOP program, an application oriented program powered largely by undergraduates.
Laura Faye Tenenbaum
PO.DAAC Science Team