Ocean Imaging. Satellite and Aerial Coastal Water Quality Monitoring in The San Diego / Tijuana Region. Monthly Report for April & May 2003

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1 Ocean Imaging Satellite and Aerial Coastal Water Quality Monitoring in The San Diego / Tijuana Region Monthly Report for April & May 2003 This draft to become final in sixty days. All data and imagery contained in this report are strictly subject to Copyright by Ocean Imaging. No data or imagery contained herein may be copied, digitally reproduced or distributed without written permission by Ocean Imaging. 23 September Lomas Sante Fe Drive Suite 370 Solana Beach, CA Phone: (858) Fax: (858) Internet:

2 INTRODUCTION As part of its remote sensing monitoring project sponsored by the State Water Resources Control Board (SWRCB), the City of San Diego Wastewater Dept. (SDWD) and the International Boundary Water Commission (IBWC), Ocean Imaging Corp. (OI) provides the participating agencies with monthly summaries of remote sensing observations and analyses. The purpose of these reports is to summarize the most relevant remote sensing data available during each period, relate them to field sample results provided to OI by the SDWD and County Dept. of Health, and to analyze the data for any notable trends or anomalies. All image data presented in these reports have been made previously available to the participants through the Internet. This report combines the months of April and May, Important Note on the Deduction of Near-surface Current Direction: The flow-field observations presented in this report are based on careful interpretation of the shapes and directions of turbidity or watercolor features recorded in the multi-spectral imagery. In some cases the actual displacement of such features could be observed and measured by tracking them through a pair of sequential images. The observed and/or derived flows represent net movement of the effluent plumes and other features over the observation interval of hours. Although the net movement is likely the most significant in terms of monitoring the influence of various effluent sources, it must be noted that the current field in the San Diego region tends to be highly variable on shorter time scales. Tidal cycles and rapid changes in wind forcing can alter and even reverse flow in the immediate surface layer. This can be readily seen in hourly flow fields derived from CODAR measurements recently implemented in the coastal area south of Pt. Loma by the Scripps Institution of Oceanography. Mass and momentum characteristics of the water column prevent the effluent and runoff plumes from rapidly changing their travel direction in synch with the surface layer. The satellite and DMSC imagery thus reveals the net or true direction of flow, integrated over a longer time period, likely12-36 hours. GENERAL OBSERVATIONS Both April and May, 2003 experienced relatively mild weather in the San Diego region with no significant rainfall. These conditions were reflected in the near-complete lack of significant effluent signatures in our image data sets, as well as the generally low bacteria counts recorded in shoreline and offshore near-surface samples. Frequent cloud-free MODIS satellite imagery collected during April showed the regional southward nearsurface flow field to strongly dominate during that month, with only one or two periods when a northward net component affected the regional flow pattern for more than hours. Although useful MODIS data from May were more sparse due to persistent cloud cover, they also show a dominant southward flow trend throughout the month. As was noted in the last report, near-surface signatures of the IWTP and Pt. Loma outfall were no longer discernible in aerial and satellite imagery collected during the last few

3 days in March. The IWTP plume was possibly detected again in DMSC imagery on 4/3/03. No further signature from either outfall was detected through the rest of April and May, however, most likely due to the seasonal increase in vertical water column stratification which tends to prevent the outfall plumes from breaking through the thermocline/pycnocline into near-surface waters. The lack of significant rain and hence terrestrial runoff also caused the Tijuana River to cease flowing from upstream into its estuary. Although a river outlet plume was detected in several aerial and satellite image sets, its size and spectral characteristics indicated it is the result of tidal flushing of the estuary, rather than containing input from further upstream. An unusually large effluent plume from the los Buenos creek was detected in satellite data and during aerial over flights on 4/3-9/03 and 4/25-29/03. Since no rainfall occurred during the month, the plume must have represented increased effluent discharge volume from the los Bueños wastewater treatment facility or an additional discharge into the creek from another source. The nearshore current pattern was southward during both occasions. The plume did not, therefore, cross the U.S. border, and spread down coast past Rosarito Beach. INTERNATIONAL WASTEWATER TREATMENT PLANT (IWTP) OUTFALL REGION: April: The first aerial DMSC flight was done on the afternoon of 4/3/03 to coincide with the City s offshore field sampling done on 4/2-4/03 (Figure 1). In order to acquire any image data during the persistent low cloud cover during the 4/2-3 period, the flight was done below the cloud layer at an altitude of approximately Because the DMSC field of view is extremely narrow at this low altitude, the imaging flight lines did not overlap (see Figure 1). Only selected lines were acquired under these compromised conditions. Elevated bacteria counts at 18m depth measured on 4/2 at stations I16, I14 and I22 indicate the plume s presence at that depth and a likely northward trajectory. This corresponds to a slight increase in reflectance characteristics from the longer wavelength (yellow and red) channels of the DMSC data acquired in the vicinity of I14 and I22. The plume was thus likely discernible, however, the generally turbid water conditions over the entire region on 4/3 made its separation difficult. The large-scale turbidity conditions can also be seen on MODIS data collected on 4/5 (Figure 2 and 13). The 04/05 MODIS image does not indicate a persistent northward flow regime. It is thus likely that the northward plume dispersal trajectory detected in field samples collected on 04/02 and the DMSC imagery from 4/3 had ceased within the next 24 hours. A high resolution TM satellite image obtained on 4/9 (Figure 3) also shows somewhat increased overall turbidity patterns nearshore from North County southward. No distinct IWTP outfall plume is evident in that data. The IWTP region was sampled with the aerial DMSC on 4/25/03. Although both flight and offshore water clarity conditions were excellent, no plume signature could be detected (Figure 4).

4 Analysis of flow patterns from MODIS image series during April (Figures 13-16) indicate that the IWTP outfall region was mostly subject to southward flow, with shortlived exceptions around 04/03 and 04/16. May: A DMSC over flight on 5/9 failed to discern any IWTP outfall plume signature (Figure 5). Field sampling done on 5/6 indicates the plume s presence at 18m (but not at 2m) over the outfall at station I12. Although it is possible that the lack of a DMSCdetected plume signature 3 days later over that area is due to a change in the plume s behavior during the 3 days, it is more likely that by early May vertical stratification of the water column has prevented the plume from reaching the upper 10-15m layer imaged by the DMSC. A high resolution TM satellite image from 5/19 was acquired under sun angle geometry that resulted in recording sea surface glint patterns in the visible wavelength channels, thus providing an opportunity to image the occurrence of any surface slicks. No evidence of increased surfactant concentrations were noted around the IWTP outfall (Figure 6A). (No outfall-related surface slicks were also observed during any of the DMSC aerial over flights during April and May.) Additionally, no thermal signature due to the outfall was evident in the TM thermal data (Figure 6 B). These observations indicate that the IWTP outfall plume remained submerged past the 10-15m near-surface layer generally penetrated by the DMSC and TM sensors and the concentrations of surfactants, if any, reaching the surface were insufficient to form any slicks under the existing wind conditions.

5 Figure 1. DMSC image composite of IWTP Outfall region acquired on afternoon 4/3/03. Low cloud ceiling limited flight altitude to 2500, resulting in a very narrow field of view of the sensor and hence data gaps between successive flight lines. Only some lines were scanned due to poor weather conditions. The entire region was subject to high turbidity. A possible outfall plume signature, identified by slightly different spectral (i.e. color) characteristics, is outlined. The nearby dark areas are due to cloud shadows. Bacteria sampling results obtained on 4/2/03 are also shown.

6 Figure 2. MODIS image acquired on 4/5/03 and bacteria sampling data from 4/4/03.

7 Figure 3. Landsat TM-5 image acquired on 4/9/03. Outfall locations are shown. The scattered red nearshore features off Pt. Loma are kelp. The intense turbidity plume south of the U.S./Mexico border is from the los Bueños Creek (see text for discussion).

8 Figure 4. DMSC image composite of the Tijuana River / IWTP outfall region acquired on afternoon of 4/25/03. Bacteria sampling data collected on 4/24 are also shown.

9 Figure 5. DMSC image composite of the Tijuana River / IWTP outfall region acquired on afternoon of 5/9/03. Bacteria sampling data collected on 5/6 and 5/7 are also shown. The bright nearshore area is due to bottom reflectance and wave-stirred sediment.

10 Figure 6. Landsat TM-7 image from 5/19/03. (A) Visible multispectral composite; (B) Thermal infrared with cooler temperatures appearing lighter. The water signal in (A) primarily represents surface glint reflection dark areas are slicks due to surfactant accumulations or wind-sheltering effects (see text for discussion). The small white object and cold line (in B) southwest of Pt. Loma is a ship and its wake.

11 POINT LOMA OUTFALL REGION April: As was already mentioned, April, 2003 experienced calm, rainless conditions throughout the region, with practically no runoff or outfall influence in the Pt. Loma region. A TM-5 satellite image from 4/9 was obtained to correspond with the City s offshore field sampling done on 4/8. A cloud free MODIS image is also available from 4/7. Figure 7 shows the MODIS turbidity patterns in the Pt. Loma area and corresponding bacteria samples collected on the previous day. The field data show significantly increased bacteria concentrations at 60m depth at stations south of the outfall: E10, E8, E5 and E2. The locations of these stations also correspond to a localized southward-directed turbidity plume revealed by the 4/7 MODIS image. A larger-scale view of the MODIS data (see Figure 13) reveals that the turbidity plume extending over the outfall is part of a larger pattern originating along North County and extending southward past La Jolla and Pt. Loma. Although the trajectory direction of this feature corroborates the southward spread of the Pt. Loma outfall plume deduced from the field data, the increased turbidity present around the outfall terminus is likely not related to its effluent discharge. Figure 8 shows the 4/8 field data overlaid on the 4/9 TM imagery. Note that the increased turbidity feature imaged by MODIS over the outfall on 4/7 has receded shoreward along the entire San Diego County shoreline. Along Pt. Loma, it s outer edge location corresponds closely to the location of the relatively persistent inshore/offshore water mass and current gradient noted during our previous historical imagery studies. We believe the existence of this gradient, linked in part to bottom topography, tends to impede any cross-shelf transport. Plentiful cloud free MODIS imagery collected during April show that the Pt. Loma outfall region was persistently subjected to a southward flow regime. The San Diego River/Mission Bay plume tended to be relatively small and did not affect the outfall area or most of the inner waters along Pt. Loma (see Figure 13-16). The outfall area was affected by two large-scale turbidity/plankton bloom events originating mostly north of La Jolla one early in the month described above, the other between 4/28 and 4/30 (see Figure 15 and 16). Figure 9 shows the Pt. Loma inner waters on 4/25 as imaged by the DMSC, with bacteria sampling data collected the previous day. The extremely low bacteria concentrations correlate well with the imagery, which shows clear water conditions with no runoff sources and minimal wind/wave-generated turbidity along the shore. May: The Pt. Loma outfall plume remained relatively deeply submerged in May. A DMSC over flight on 5/9/03 (done in conjunction with the City s offshore field sampling on 5/8) did not reveal any plume signal in the upper water column reached by the imager (Figure 10), in correspondence with the lack of any significant bacteria levels sampled above 40m. No plume signal was noted on any subsequent flights.

12 The TM satellite data from 5/19 (Figure 6) may show an influence of the outfall discharge in the thermal imagery a southwestward directed upwelling plume of cold water off Pt. Loma includes a slightly warmer feature directly over the outfall and spreading southward. A warm thermal anomaly has been the most commonly observed feature related to the Pt. Loma outfall discharge during the winter and spring months.

13 Figure 7. MODIS image from 4/7/03 and bacteria sample data collected on 4/7/03 and 4/8/03.

14 Figure 8. TM multispectral image of the Pt. Loma area from 4/9/03 and bacteria sampling data from 4/7 and 4/8/03. Orange/red areas near Pt. Loma are kelp.

15 Figure 9. DMSC image composite of Pt. Loma shoreline region acquired on afternoon 4/25/03. These data were preceded by extended dry and calm conditions, resulting in no runoff and clear waters. Most of the features seen represent variations in bottom substrate.

16 Figure 10. DMSC image composite of Pt. Loma shoreline region acquired on afternoon 5/9/03 and bacteria sampling data obtained on 5/8/03. Purple areas nearshore are kelp.

17 SHORELINE DISCHARGES AND ANOMALOUS EVENTS Plume from los Bueños Creek: During the 4/3/03 DMSC over flight we noticed that the los Buenos Creek effluent plume is exceptionally large. The southward-directed plume extended at least as far as Rosarito Beach. Because OI has still not been granted permission by Mexican authorities to do any imaging work in Mexican territory (and the pilot s request to do so on 4/3 was denied by the Mexican flight controller), no overhead imaging of the plume was possible. However, MODIS satellite imagery from 4/5, 4/7 and 4/9 (Figure 13) clearly shows the large plume of effluent. A more detailed view was also obtained with TM satellite data on 4/9 (Figure 11). The plume s size and intensity during that period was relatively unusual, especially in view of the fact that no storm runoff could have contributed to the increased effluent volume. The abnormally large plume must thus have been the result of a temporary increase in discharge from the los Buenos wastewater treatment facility or from some other, unknown source. Our historical observations as well as operational monitoring begun in October, 2002 have shown that the los Bueños effluent plume generally does not reach the U.S. border region, even during periods of a northward nearshore current regime. During a southward flow regime the plume rarely reaches as far as Rosarito Beach. During the April, 2003 event, however, the plume consistently reached the Rosarito Beach shoreline and, if the current regime was northward, would have been large enough to cross the U.S. border. A second period of abnormally increased effluent volume from los Bueños likely occurred between 4/25 and 4/30, as evidenced by MODIS imagery (Figures 15 and 16). OI will continue to monitor the los Bueños area for any further plume variability. Tijuana River plume: With no significant rainfall during April and May, 2003 flow of the Tijuana River between its estuary and upstream portions became interrupted. Any flow from the River mouth was thus the result of tidal flushing of the estuary portion. Under such conditions the river plume tends to be much reduced in size, have different spectral characteristics, and pose a significantly lesser health hazard. An example is shown in Figure 12 from the 4/25/03 DMSC over flight. Practically no river outflow was observed at the time and no distinct river plume is visible. Most of the regional turbidity was caused by wind/wave mixing over the shallow areas during relatively windy weather conditions.

18 Figure 11. Detail of effluent plume from the los Bueños Creek in Mexico as imaged by TM-5 on 4/9/03 (see text for discussion).

19 Figure 12. DMSC image composite Tijuana River and Imperial Beach shoreline acquired on 4/25/03. No distinct River effluent plume is visible. The nearshore turbidity represents wave/wind mixing effects in shallow water.

20 Figure 13. Satellite MODIS multispectral imagery from April, 2003.

21 Figure 14. Satellite MODIS multispectral imagery from April, 2003

22 Figure 15. Satellite MODIS multispectral imagery from April, 2003

23 Figure 16. Satellite MODIS multispectral imagery from April, 2003

24 Figure 17. Satellite MODIS multispectral imagery from May, 2003