Tidal circulation and sewage pollution in a tropical marine lagoon

TIDAL CIRCULATION A N D SEWAGE POLLUTION IN A TROPICAL MARINE LAGOON J. T. HARDY d~ S. A. HARDY*

US Trust Territory Entomology Laboratory, Koror, Palau, Western Caroline Islands

ABSTRACT Tropical marine lagoons frequently serve as collecting points for domestic sewage, but documentation of pollution in such habitats is scarce. Monitoring of untreated sewage effluent and tidal circulation patterns in lwayama Bay, Palau, US Trust Territory of the Pacific, using relatively simple field techniques, indicates that: (l) during ebb tides, water outflow occurs primarily through two tidal channels at the northwest end of the lagoon and the total volume exchanged with water outside the lagoon during a mean tidal cycle averages 5-97 million m 3 or approximately 15 % of the total lagoon volume; (2) a considerable portion of water in the shallow reef area in the west lagoon is highly polluted and greatly exceeds the water quality guidelines for limits on faecal coliform; (3) the enclosed nature of the lagoon and relatively restricted exchange and flushing with the open ocean make the area an undesirable location for sewage disposal; (4) a public hospital is a major source of this untreated sewage which empties into a popular fishing area; thus, the present condition may represent a health hazard to the local community.

INTRODUCTION

Inhabitants of temperate regions usually envisage tropical south sea islands as surrounding clear deep-blue marine lagoons, rich in multi-coloured corals, teeming with marine life, and unspoiled by human activities. Such habitats are dwindling in number. In the maritime tropics human settlements traditionally developed around * Present address: Department of Biology, American University of Beirut, Beirut, Republic of Lebanon. 195 Environ. Pollut. (3) (1972) pp. 195-203--O Applied Science Publishers Ltd, England--Printed in Great Britain

196

J. T. HARDY~ S. A. HARDY

such lagoons where protected waters offered the best opportunities for boat moorage and fishing. Today, in the face of rapid population growth and economic/ technological shortages, many such communities release untreated domestic sewage into lagoons, often contaminating traditional fishing grounds and degrading formerly pristine environments. The damage thus far, although fairly widespread in the tropics, is poorly documented. As Johannes (1970) states, his paper "is probably the first ever published on the general problems of coral reef pollution". He has recently described Fathoms 0-1

,

Fathoms

Fathoms :. -..'.:....'.~

::::::::::::::::::: , . , . . . . . . . ,.,.. .......... ,.... • • ....:.,.. Be

BRim

20" 25 ""

n

~pgh ( ~

Fig. 1. Iwayama Bay. Clear areas = land. Tidal channels A, B, C and D. Arrows indicate direction of currents on outgoing tides. Broken line encloses area of pollution testing (see Fig. 2). Depth contours in fathoms (see key). Inset represents area of bay at respective depths integrated to give total bay volume. the degradation of once-thriving reef communities in Hawaii where more than 99 of corals in the southern basin of Kaneohe Bay, Oahu, have been killed, primarily by sewage outfall enrichment (Johannes, 1971). Our study was undertaken to determine the tidal circulation patterns and extent of domestic sewage pollution in Iwayama Bay, Palau, US Trust Territory of the Pacific. The relatively simple field techniques employed may prove useful in studying the fate of marine lagoon effluents in other similar remote locations, where sophisticated instrumentation is unavailable.

TIDAL CIRCULATIONAND SEWAGEPOLLUTION

197

Iwayama Bay extends along most of the south and southwest sides of Koror and is completely enclosed by Koror, Auluptagel, and several other islands. Many small limestone islands lie within the bay. Four relatively narrow channels (two at the northwest end and two at the southeast end) serve as the only access of tidal circulation with areas outside the bay (Fig. 1 : A, B, C, and D). The tidal changes are predominantly semidiurnal with two highs and two lows per day. The maximum range in tidal level (2.1 m) usually occurs in May and June and the minimum (9.1 cm) in March and September.

Fig. 2. Pollution sampling stations. The area within the solid line is heavily polluted (faecal coliform > 200/100 ml). The bay has been proposed as a future site of two major sewer outfalls (Austin et al., 1967). At the time of the study, three main sources of pollution emptied into it. One pipe from Koror Hospital, and a second from housing on the hill near the hospital, emptied untreated sewage into the shoreward edge of the mangrove area (Fig. 2 : 1 and 2). A third source was the city dump adjacent to M-Dock. METHODSAND MATERIALS Tidal circulation Using a standard 1.22 m 2 current cross (FAO, 1960, p. 112, Fig. 26), we measured the direction and velocity of tidal currents in the four channel entrances of lwayama

198

J. T. HARDY, S. A. HARDY

Bay (Fig. 1: A, B, C, D). Measurements were taken during periods of spring, mean,* and neap ebb tides during February 1968. No measurements were made of current velocities during flood tides. A small boat was anchored bow and stern in the middle of the channel, the current cross placed in the water, and the time taken for 10 m of line to pay-out timed with a stop-watch to obtain the current velocity in cm/sec. These measurements were generally taken at 1 hour intervals, beginning at high slack tide and continuing until low slack tide. The depths and widths of the four channels were measured with a sounding line. The directions of outgoing tidal currents were measured in several other locations (indicated by arrows in Fig. 1) by the addition of fluorescent dye to the water. The total volume of water in the Bay was estimated from a bathymetric chart. Depth contours were drawn at 1.8, 9.1, 18.2, 27.4, 36.5, 45.6 m (1, 5, 10, 15, 20, 25 fathoms). The respective areas of each of these depth intervals were integrated to give the total volume of water in the bay (Fig. 1, inset). Pollution The concentration of coliform bacteria was used as an index of domestic pollution. Water samples were collected at high and low tides at 20 stations in Iwayama Bay (Fig. 2). A total of 35 water samples was tested between 10 April and 24 June 1968. Two seawater samples from unpolluted areas near Babelthuap Island were tested as controls. Using the multiple-tube fermentation technique (for details see American Public Health Association, 1960), at least three dilutions of each water sample were tested: A total of fifteen lactose-broth fermentation tubes was run for each water sample (five tubes for each dilution). Positive gas-producing tubes were plated on eosin methylene blue agar plates to give colonies which were then re-inoculated into fermentation tubes to complete the test. The EC medium elevated temperature test for faecal coliform was also employed on samples 20L and 22L. In addition to the water samples, we tested two crabs and an oyster for coliform bacteria. We dissected the animals and ground the entire body of each with a mortar and pestle in 100 ml of sterile distilled water. Dilutions were made from these and tested by the multiple-tube fermentation technique.

RESULTS AND DISCUSSIONS

Tidal circulation During ebb tides, the majority of water flowed out of the northwest end of the bay through channels A and B (Fig. I). * ' M e a n tide' refers here to the magnitude of tidal fall on 2 and 5 February.

TIDAL CIRCULATION

199

AND SEWAGE POLLUTION

From zero velocity at high slack tide, the current flow through the four channels increased with time, generally reaching maximum velocities within 2 to 5 h, and then decreasing again to zero within 5.5 to 8 h. As expected, greater current velocities developed during spring tide and lesser current velocities during neap tide. Channel A, shallow and relatively narrow, usually showed the greatest current velocity (58.9 cm/sec on 18 February) followed by channels D, B, and C in order of decreasing maximum current velocity (Table 1). TABLE

1

TOTAL VOLUME OF BAY AND VOLUME OUTFLOW THROUGH CHANNELS FOR NEAP 9 MEAN~ AND SPRING TIDES FEBRUARY 1 9 6 8

Total Total volume of volume of outflow outflow computed computed Tidal Volume of outflow through channels in from from fall (m) million m 3 and (as per cent of total outflow) surface channel (mean for area and current two days) Channel A Channel B Channel C Channel D tidal fall velocities (million (million m 3) m 3)

NEAP Feb. 9 and 10 MEAN Feb. 2 and 5 SPRING Feb. 18 and 19

1-36

1.37

0"21

7'49

4"46

1"17

9.72

8-80

1-52

Approximate crosssectional areas of channel (m 2)

0-132 (9.6) 0"53 (11.9) 0.91 (10.3)

0'422 (30-8) 1"75 (39.2) 5'84 (66"4)

0"361 (26"4) 0.76 (17.1) 0"67 (7.6)

0.463 (33"8) 1"42 (31'8) 1'38 (15'7)

92

980

1300

293

Surface area of bay = 6.4 million m 2. Total volume of bay = 40-2 million m3. The proportion of the total bay outflow was approximately the same through channel A during neap, mean, and spring tides, i.e. 9.6, 11.9 and 10.3 ~o respectively (Table 1). The proportion of bay water flowing out through channels C and D decreased from neap to spring tide, while the proportion flowing out through channel B increased greatly. At spring tide, 5.84 million m 3 of water, or 6 6 . 4 ~ of the total outflow, passes out through channel B. During neap tide only 0.4 million m 3, or 30"8~o of the total outflow, passes out through channel B. The cross-sectioned channel areas used in our calculations do not include the shallow areas adjacent to the main channels, B and C. In addition, the following assumptions and approximations were used: (1) The tidal current velocities in the channels were uniform both vertically and horizontally across the channel.

200

J. T. HARDY, S. A. HARDY

(2) The tidal amplitudes were identical with those listed for adjacent Malakal Harbour in the tide tables of the Trust Territory District Administration. (3) Tidal friction and meteorological pressure effects were assumed to be zero. The total volume of water flowing out of the bay during spring and neap tides was 8-80 million m 3 and 1.37 million m 3, respectively. These values were computed simply by multiplying the surface area of the bay by the tidal fall. When the volumes of water flowing out of each of the four channels (computed from current measurements) were totalled, outflow values of 9-72 million m 3 and 1.36 million m a were obtained. Considering the approximations used, the agreement between these methods was remarkable. During mean tide, however, agreement was not as good. The volume of outflow computed from current velocities was about 60 ~ of that computed from the bay area and tidal fall. Circulation was studied only during February 1968. The volume of water exchanging with that outside the bay should be somewhat greater during the spring tides of May and June and somewhat less during the neap tides of March and September. Pollution Considering the proposed future development of lwayama Bay as a hotel and recreation site, the waters of this bay should be maintained at the highest possible quality. We found the shallow reef area from west of M-Dock to Ngerbeched Dock highly polluted by sewage outfall (Fig. 2). The water within this area did not meet any of the five general standards for water quality outlined by the Water Pollution Control Commission of the Territory of Guam (Territory of Guam, 1967). This commission recommended that in primary contact recreational (swimming) areas the faecal coliform limit should not exceed an arithmetic mean of 200/100 ml. In our study, the concentration of faecal coliform at station 1 near the hospital outfall in Iwayama Bay reached 34 million/100 ml, or more than 100,000 times the recommended limit. Likewise, at all other stations within the approximate boundary shown in Fig. 2 the concentration of faecal coliform was significantly greater than the recommended limit (Table 2). From exceedingly high concentrations at stations 1 and 2, the concentrations of faecal coliform gradually diminished as one moved out towards the channel. During ebb tides, pollution moved from outfalls out across the shallow reef, and at low tide the concentration was often ten times greater than that at high tide. Even at station 19, near the end of M-Dock, faecal coliform reached 16,000/100 ml, or about 80 times the recommended limit. The standard seawater tested from unpolluted areas of Babelthaup Island (samples 8 and 9) gave zero coliform. The crabs and oyster tested in this study apparently concentrated coliform bacteria (and presumably other pollutants) within their bodies by filtration from the

TIDAL CIRCULATION AND SEWAGE POLLUTION TABLE2 CONCENTRATION OF FAECAL COLIFORM BACTERIA AT SAMPLING STATIONS LOCATED IN FIG. 2. H :

MPN

Date

H I G H TIDE,

L

Station No.

16/4

1H

25/4 10/4 16/4 2/5 2/5 7/5 7/5 7/5 14/5

1L 2H 2L 3H 4H 5H 6H 7H 8H stand 9H stand IlH 11L 12H 12L 13 H 13L 14H 14L 15H 15L 16A crab 16B crab 17A oyster 18H 18L 19H 19L 20H 20H 20L 21H 21L 22H 22L 23H 23L 24H 24L 25H 25L

14/5 15/5 28/5 15/5 28/5 20/5 28/5 20/5 9/6 20/5 9/6 24/5 24/5 24/5 31/5 9/6 31/5 9/6 18/6

18/6 14/6 18/6 14/6 18/6 14/6 18/6 24/6 18/6 24/6 18/6 24/6

=

LOW TIDE,

= MOST PROBABLE STATISTICAL NUMBER OF BACTERIA CALCULATED FROM 15 TUBES ( 3 DILUTIONS) OF EACH SAMPLE

MPN Index/ 100 ml 3.3 3"4 1"3 9-1 1"7 2"2

x 106 x 107 x 106 x 106 × 103 x 103 20 7-0 x 102 0 0

3-3 5.4 2.0 2.3 2.0 5.0 2.0 2.0 2.3

x x x x 0 x x x x 0 x

102 103 10t 102 10 t 101 I01 101 106

1"7 × 106 5-4 x 106 4-9 2"0 3"3 1-6 4"0 4-0 1"3 1-3 3'3 2.0 2-7 2-0 5.0

x x x x x x x x x × x x / 0 2.0 x 2.0 x 2.0 ×

102 101 102 104 101 10 t 102 103 102 10I 102 101 101 101 101 101

201

202

J. T. HARDY, S. A. HARDY

water. The crabs (samples 16A and B) contained 0.2 to 1.7 million faecal coliform/ 100 ml of body fluid (see under the heading: Methods and Materials). This represented about 10,000 times the concentration in the surrounding water. The oyster concentrated even more (about 20,000 times the surrounding water). Unfortunately, we were unable to determine coliform concentrations on more than these few invertebrate specimens before termination of the study. However, Stiles (1912) established long ago that oysters can concentrate bacteria and infectious human diseases from sewage-polluted waters.

CONCLUSION

Pollution in Iwayama Bay probably represents a threat to the conservation of coral reefs, fish, shellfish, and other marine life in the area. The enclosed nature of the bay and small volume of mixing with the open ocean make Iwayama Bay an undesirable location for sewage disposal. Sewage effluent emptying into the bay, even after primary treatment, might lead to nutrient enrichment and eutrophication. Effluents could be pumped to other locations on the island, treated, and emptied into deep open-ocean water where circulation would lead to more rapid dispersal. Iwayama Bay is frequently used by fishermen and shellfish collectors. Many times during the period of this study, we observed people collecting clams in polluted areas. And, on one occasion, a fisherman was fishing with a net in the middle of the highly-polluted area of study. Since one outfall pipe of untreated sewage, presumably containing infectious diseases from the Koror Hospital, empties into this area, fishing activities here may represent a health hazard and should be discontinued. ACKNOWLEDGEMENTS

We wish to express our appreciation to the US Peace Corps and Mr Robert Owen, Director of the US Trust Territory Entomology Laboratory for their kind cooperation and support during this study. We also wish to thank Dr Paul Burkholder of the Lamont Geophysical Laboratory for assisting us with needed equipment and advice, and Drs R. del Moral and J. R. Waaland for their critical reading of the manuscript. REFERENCES

AMERICANPUBLICHEALTHASSOCIATIONet al. (1960). Standard methods for the examination o f water and wastewater including bottom sediments and sludges, 11th ed. New York, American Public Health Association,Inc.

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203

AUSTIN, SMITH ~. ASSOCIATES,INC. (1967). Trust Territory of the Pacific Islands engineering report covering a master planned water supply and distribution system as well as a sewerage system for the Koror Area of the Palau Islands, Western Caroline Islands. Honolulu, Hawaii. (Technical Report; Austin, Smith & Associates, Inc.) FAO (1960). Manual of field methods infisberies biology. Rome. Food and Agriculture Organisation of the United Nations. JOHANNES, R. E. (1970). How to kill a coral r e e l Part I. Mar. Pollut. Bull., 1, pp. 186-7 JOHANNES, R. E. (1971). How to kill a coral reef. Part II. Mar. Pollut. Bull., 2, pp. 9-10. STILES,G. W. (1912). Sewage-polluted oysters as a cause of typhoid and other gastro-intestinal disturbances. Bull. Bur. Chem. US Dep. Agric., 156, pp. 1-44. TERRITORY OF GUAM (1967). Water Pollution Control Commission. Standards of water quafity for waters of the Territory of Guam. Government of Guam. Agana, Guam. Mimeographed, 27 pp.