Suspended solids analysis using ERTS-A data

REMOTE SENSING OF ENVIRONMENT 3, 69-78 (1974)

69

Suspended Solids Analysis Using ERTS-A Data H. K R I T I K O S and L. Y O R I N K S The Moore School of Electrical Engineering, University of Pennsylvania, Philadelphia, Pennsylvania

and H. S M I T H Environmental Protection Agency, Region 3, Philadelphia, Pennsylvania

The magnetic digital tapes of the imagery obtained by ERTS-A on September23, 1972, have been analyzed for selected areas of the Potomac River. A statistical analysis of all four bands has been carried out. The results

show that band III is useful in determining the water-to-land interface. Data on bands II suggest the existenceof three distinct types of water thosehaving low, medium, and high reflectivity. From previouslypublished results and ground truth measurements the areas of hight reflectivitywere identifiedas containg high concentrations of suspended solids. Areas of low reflectivitywere identifiedas having relativelylower concentrations of suspended solids. A commonly used computer technique with some additional refinementshas been used to generate thematic maps which identify the above areas and show their geographical distribution.

1. I n t r o d u c t i o n

Remote sensing is now rapidly becoming an important tool in the detection and surveillance of water pollution. The basic physical concepts which led to the present developments were developed by a number of investigators, some of which are F. Hulbrut (1946), J. Williams (1970), and S. Duntley (1960). Application of remote sensing techniques to oil slick detection have been made by N. Guinard (1971), J. Aukland (1971), and J. Munday (1971). Water quality measurements have been carried out by M. Querry (1971) and J. Scherz (1971); M. Golberg (1972) has experimented with Raman scattering techniques, and P. White (1971) has carried out a study of the spectral characteristics of sewage outfalls. A great opportunity for the further development of the art of remote sensing appeared in NASA's Earth Resources satellite program. A large number of data for extended geographical areas became available, and it appears that some significant results have been obtained. Some relevant work has been caried out by the following investigators: V. Klemas (1973), B. Bowker (1973), and R. Ruggles (1973) have studied the dispersion of high sedimentation plumes from photographic images. C. Wezernak (1973), H. Yarger (1973), and J. Schuburt

(1973) also have investigated the surveillance of areas with high sedimentation. The present paper is an additional contribution to the effort of assessing water quality from digitized ERTS-A imagery. 2. T h e R e f l e c t a n c e o f W a t e r

The satellite observations were made by using the Multispectral Scanner (MSS), which has four spectral bands. The bands are defined in Table 1. The information is transmitted digitally and then it is converted to a photographic product. Each digital count represents the radiance of a cell of 50 × 80m. It is important to notice that while the information is recorded at the tapes with a 7-bit resolution (127 gray levels), the photographic products are seriously degraded. Only a fraction of the gray level resolution remains at the photographic products (16 gray levels). It is very important, therefore, to recognize that accurate radiometric work can only be carried out with computer compatible tapes (CCT). The total reading which is recorded at the satellite is the product of a number of complex physical phenomena, several of which will be discussed below. © American Elsevier Publishing Company,Inc. 1974

70

H. KRITIKOS, L. YORINKS AND H. SMITH TABLE 1 MSS Spectral Bands

Band 1

II Ill IV

Spectral width in micrometers (X) 0.5 --

0.6

0.6 - - 0.7 0.7 - - 0.8 0 . 8 - t.1

Resolution bits

Gray level

7

127 127 127 64

7 7 6

TABLE 2 Radiance Readings for Barton Pond. Sun, Zenith angle 49°; Date: Sept. 28, 1972 (from H. Rogers)

Band

L mw/cm2/sr

bits

La mw/cm2/sr

bits

T

R%

I II Ill IV

0.476 0.242 0.141 0.234

24.3 15.3 10.3 3.50

0.274 0.118 0.082 0.1062

14.0 7.5 6.0 1.5

0.810 0,865 0.909 0.913

9.3 5.5 2.8 0.9

The radiance L is given by the well known expression L = ~ H r + LA,

where R is the reflectivity of the target, H is the radiance illuminating target, T the transmissivity of atmosphere, and LA is the path luminance. It is important in the course of our investigation to establish the order of magnitude of these quantities. H. Rogers (1973), by measuring separately the ground reflectance, has published some typical values for Barton Pond, which is a small lake. See Table 2. Note in Table 2 that the path luminance is approximately 50% of the total reading. This is potentially a source of large errors because of the uncertainty in its determination. Also, it is significant toobserve that the reflectance of the water in band II and IV is small. This takes place because the incident light is mostly absorbed. A number of other investigators have also studied the optical properties of the water, and it has been established that an optical window exists, which is approximately 0.2g wide in the visible range. The peak of the window lies somewhere between 0.4 and 0.5g depending on the physical composition of the water. For example, for distilled water J. Williams (1970) shows that the peak is at 0.470g and the penetration length is approximately 100 m. The same data show approximately that the average penetration length for band I and II of

H mw/cm2 8,41 8,14 7.38 5,02

MSS is approximately 10 and 3m, respectively. For natural water the peak of the window is shifted towards higher wavelength and the penetration length naturally decreases. A large number of experiments have been made to determine the the spectral response of the water as a function of its constituents. The results are difficult to relate quantitatively with ERTS-A data because of the variable atmospheric conditions, lack of controllable experimental environment, and inherent calibration errors. In general, however, all published results indicate that in band lI high densities of suspended solids correspond to high water reflectance and low densities correspond to low water reflectance. In this study no independent readings were taken at the ground, consequently it was impossible to make absolute radiometric measurements. It was possible, however, from the relative radiance of the scene to derive a substantial amount of information. It was found that the best procedure was to use band III to identify water and then band II to assess the water quality. This is discussed in greater detail in Section IV.

3. Water Quality Conditions In The Upper Potomac River Tidal System The Potomac River is one of the most environmentally stressed areas in the Middle Atlantic states. Aalto (1970) has examined the water

71

SUSPENDED SOLIDS ANALYSIS USING ERTS-A DATA

quality conditions of the estuary. Effluents from a number of major wastewater treatment plants serving a population of about 2,500,000 people are discharged into the system. Figures 1 and 2 show the geography of the region and the major wastewater sources. Some of the important physical phenomena which characterize the Potomac are discussed below. (a) Tides The tidal system extends from the Chesapeake Bay all the way to Little Falls in Washington, D. C. The time at which the satellite data were taken was 9:20.2 A.M.E.S.T., which is 1 h and 40 rain after the high tide. This means that approximately one third of the tidal movement has taken place and that plumes should point downstream. There are two areas which have presented tidally induced problems. These are the Anocostia River and the Piscataway Creek. In the Anocostia River the exchange rate is very small and consequently the high loads of silt which are discharged into it remain entrapped for long periods of time. A similar phenomenon takes place in the Piscataway Bay. In this region, it has been observed that the wastewaters of the Piscata-

way Sewage Treatment Plant (10 MGD) have a very small dispersion rate and occasionally reach Broad Creek, which lies upstream. (b) Water quality measurements Unfortunately, at the time of the satellite overpass no ground data were taken. There are however data taken at an earlier date, September 1972, close to Low Tide Slack (13.32) which provide some insight into the state of water quality of the Potomac. The data is shown in Table 3 and the location of the stations is shown in Figs 1 and 2. From the results the following observations can be drawn.

1. In the Key Bridge region a high chlorophyl reading (54.8 #g//) and low Secchi Disk reading (26.0") probably indicate a high concentration of algae. 2. In the 14th St. Bridge region a low chlorophyl reading (21.0 # g/l) and a high Secchi disk reading indicate a patch of clear water with low concentration of algae and sediments. 3. The most environmentally stressed area is the W. Wilson Bridge station where the dissolved oxygen (2.27 rag/l) has its lowest value, the nutrient loading its highest (1.35PO4,

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S U S P E N D E D SOLIDS ANALYSIS USING ERTS-A D A T A

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1.90NH3 in mg/1) and the chlorophyl count is medium (28.5 /~g/1). This takes place because this region is downstream of the two most important sewage treatment plants in Arlington (20 MGD) and Blue Plains (300 MGD). 4. In the Mathias Point region an interesting phenomenon is observed. Going from Stations 15 to 15A and 16, the salinity nearly goes up by a factor of four, the chlorophyl count decreases by factor of two, and Secchi Disk depths increase by a factor of two. These data indicate that at some point in between lies the salt-tofresh water interface.

4. Identification of Water In order to determine the areas covered by water it was necessary to first examine some

areas where the ground truth was known. Table 4 shows the results of the survey. Notice that the radiance readings for water were very close to those which Rogers observed for Barton Pond This observation indicates that, at the very least, the errors (variability of Path Luminance, MSS calibration error, bottom effects) in both cases are of the same order TABLE 4 Average Radiance for Different Targets in Gray Levels. Readings Include Atmospheric Effects Band

Water

Urban Areas

Vegetation

Max. reading

I II III IV

25.32 16.96 9.46 1.33

28.68 22.37 27.8 14.34

30.61 25.45 38.5 22.15

127 127 127 64

74

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of magnitude. Band III presented the largest contrast between water and its environment, and for this reason it was decided to use it to generate a mask. In order to justify this choice the statistical distribution of the radiance of the water and its environment were determined and are shown in Fig 3. It can be easily seen that there is no overlap and consequently there

was no ambiguity in the determination of the water. The water can also be identified by using an adaptive technique as follows. First choose an ad hoc threshold slightly above the mean level. Find the water and determine the boundary points. Relax the condition at the boundary points by increasing the threshold by a prede-

SUSPENDED SOLIDS ANALYSIS USING ERTS-A DATA

termined amount (e.g. 10%). This has an effect of edge sharpening. This technique was originally tried out and proved to be successful.

5. Data Reduction Once the water has been identified and the appropriate masks have been generated the variations of its brightness are examined. From the statistical distribution of the brightness of all four bands it can be seen that band II shows the maximum spread. The histogram of band II also suggests that three distinct brightness clusters exist (see Figs. 4 and 5.). This observation serves as the basis for identifying three classes of water. Those which have high brightness (shown by 0), medium brightness (shown by .), and low brightness (shown by I ) . The choice of the thresholds is not obvious. The thresholds were arbitrarily chosen to conform with our best judgment. The results show that with very few exceptions regions of high brightness in band II had high or medium brightness in bands I, II, and III. Since band II offered the maximum discrimination, it was decided to work with that band alone rather than consider the spectral signature in all four bands.

6. Interpretation and Significance of Results The CCT (Computer Compatible Tapes) offer the maximum possible gray-level resolution that can possibly be obtained from the ERTSA data. For example, in band II the water brightness covers a region of 9 gray levels (out of a maximum reading of 128) with approximately an error of 1 to 2% (approximately one gray level.) In a photographic product this resolution is greatly reduced because the maximum range has been compressed from 128 to 16 gray levels. In the computer generated thematic maps (Figs. 4 and 5), areas of high suspended solids content are easily determined because of their high reflectivity and consequently brightness. These areas are the Anacostia river and generally most of the western bank of the Potomac. A number of large Sewage Treatment Plants (STP) are known to lie in the western bank. These are the Arlington, Alexandria, and Westgate. In the same vicinity lies the largest one, the Blue Plains STP, which discharges its efflu-

75 ents underwater midstream. The high sediment areas, as predicted from the thematic maps, seem to correlate well with the turbidity of the water, as indicated by low Secchi Disc measurements. Although the ground truth measurements were taken 20 days before the satellite overpass, it is believed that the conditions were similar enough (a few hours hours after high tide with essentially a constant-slope declining river state) to suggest that a substantial correlation exists. This can be seen in Fig. 6 which shows the flow conditions of the Potomac River. From previously published investigations it can be inferred that areas of low reflectivity correspond to clearer water (less sedimentation). In the thematic maps these areas are shown to be in the vicinity of the 14 St. Bridge and generally the lower eastern bank of the Potomac river. Here again there is qualitative agreement between the ground truth measurements and satellite data. The ground measurements have shown large Secchi Disc depths in the same areas. An interesting coincidence can be pointed out here: the water of low reflectivity appears to lie in the vicinity of Thermal Plants (HTP). This is true in both the 14 St. Bridge area and Goose Island one, where it is known that the Pentagon thermal plant (43 MGD) and the PEPCO generating station (315 MGD) discharge their thermal effluents, respectively. The usefulness of the results lies in the formation of qualitative overall pictures of the geographic distribution of pollution in the Potomac river. The most significant observation is that, after high tide, the pollutants seem to accumulate in the west bank of the river. This phenomenon has been reported before and is attributed to the Coriolis forces. There is every reason to believe that from the present data some information on water quality can be obtained for water quality surveillance. A great deal of research, however, has to be carried out to establish the ultimate sensitivity and accuracy of the system and repeatibility of results. We would like to thank Mr. R. Bernstein of I B M for providing us with the CCT tapes, and Mr. M. Tavakoli and Dr. R. Bajcsy for assisting

76

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SUSPENDED

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us with the computer analysis. W e would also fike to acknowledge the valuable assistance of N. Melvin of E P A (Region 3) in gathering the ground truth and in the interpretation of results.

References Aalto, J. (1970), Current water quality conditions and investigations in the upper Potomac River tidal system, Tech. Rep. No. 41, U.S. Dept. of Interior, Federal Water Pollution Control Administration, Middle Atlantic Region. Aukland, J. (1971), Multi-sensor oil spill detection, Proc. 7th Intern. Symp, University of Michigan, 1045-1052. Bowker, D. (1973), Correlation of ERrS multispectral imagery with suspended matter and chlorophyl in lower Chesapeake Bay, Syrup. Significant Results Obtained form ERTS-I, NASA, 1291-1298. Duntley, S. (1960), The Visibility of Submerged Objects, Cambridge. Golberg, M. (1972), Applications of spectroscopy to remote determinations of water quality, 4th Annual Earth Resources Program Review, Vol. III, MSC-05937, Houston, Texas, 81-1, 81-10. Guinard, N. et al. (1971), Remote sensing of oil slicks, Proc. 7th Intern. Syrup., University of Michigan, 1005-1026. Hurlburt, E. (1946), J. Opt. Soc. Amer. 35, No. 11,698-705. Klemas, V. (1973), Applicability of ERTS-I imagery to the study of suspended sediment and aquatic fronts, Sym. Significant Results Obtained from ERTS-1 NASA, 615-624. Lind, A. (1973), Enviromental stuay of ERTS-I Imagery, Syrup. Significant Results Obtainedfrom ERTS-1, NASA, 643-650.

Munday, J. (1971), Oil slick studies using photographic and multispcetral scanner data,Proc 7th Intern. Symp, University of Michigan, 1027-1044. Querry, M. R. (1971), Specular reflectance of aqueous solutions, Proc. 7th Intern. Symp., University of Michigan, 1052-1070. Rogers, R. (1973), A technique for correcting ERTS data for solar and atmospheric effects, Syrup. Significant Results Obtainedfrom ERTS-I, NASA, l 115-1122. Ruggles, R. (1973), Plume development in Long Island sound observed by remote sensing, Symp. Significant Results Obtainedfrom ERTS-I, NASA, 1299-1304. Scherz, J. (1971), Remote sensing considerations for water quality monitoring, Proc. 7th Intern. Symp., University of Michigan, 1071-1088. Schuburt, J. (1973), Digital analysis of Potomac river basin ERTS-1 imagery, Syrup. Significant Results Obtained from ERTS-1, NASA, 659--664. Wezernak, C. (1973), Monitoring ocean dumping with ERTS-I data, Symp. Significant Results Obtained from NASA, 635--642. White, P. (1971), Remote sensing of water pollution, Intern. Workshop on Earth Resources Survey Systems, Vol. II, 303-322, Govt. Publ., Supt. of Doc., Washington, D. C. Williams, J. (1970), Optical Properties of the Sea, United States Naval Institute, Annapolis, p. 39. Yarger, H. (1973), Water turbidity detection using ERTS-1 Imagery, Symp. Significant Results Obtained from ERTS-I, NASA, 651-658.

Received Oct. 18, 1973; revised Dec. 14, 1973