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Field measurements of SOD and sediment nutrient fluxes in a land-locked embayment in Hong Kong
Advances in Environmental Research 6 Ž2002. 135᎐142
Field measurements of SOD and sediment nutrient fluxes in a land-locked embayment in Hong Kong K.W. ChauU Department of Ci¨ il and Structural Engineering, The Hong Kong Polytechnic Uni¨ ersity, Hunghom, Kowloon, Hong Kong
Abstract The pollution problem in Hong Kong has increased over the past two decades, and the situation in the nearly land-locked Tolo Harbour, located at the north-eastern part of the territories, has been particularly problematic. Although the exogenous organic materials dissolved in the overlying water have been substantially reduced, the soft sediments at the bottom of the harbour continue to act as sources of nutrients. This sediment oxygen demand ŽSOD. amounts to a significant value, hence increasing the total oxygen demand load of the harbour. In this paper, the current state of the water environment is reviewed. The SOD and the rate of nutrient release in different forms of nitrogen and phosphorus were measured in laboratory experiments. It was found from the laboratory tests that the average SOD value is 38 mgO 2rm2-h and that the mean release rates of phosphate phosphorus and ammonia nitrogen are, respectively, 3 and 62 mgrm2-d. These measured field data will be useful in understanding the water quality of the water body and also provide parameters that may be used when developing numerical water quality models for Tolo Harbour. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Tolo Harbour; Water quality; Sediment oxygen demand; Nutrient release
1. Introduction Tolo Harbour, an almost land-locked estuary located in the north-eastern territories of Hong Kong, is most vulnerable to environmental pollution. The main water body is approximately 16 km long and 3 km wide. It is connected to the Mirs Bay through Tolo Channel, which is a very narrow channel with a maximum width of only 1.3 km. The mean depth of Tolo Harbour and Tolo Channel are 12 and 20 m, respectively. A few decades ago, the harbour was enclosed by rural areas, and hence the water quality problem was not acute.
U
Corresponding author. Tel.: q852-2766-6014; fax: q8522334-6389. E-mail address: [email protected] ŽK.W. Chau..
However, in the past 30 years, enormous population expansion in Hong Kong caused the government to shift infrastructure developments to the New Territories, previously a rural area. Land use changes in the Tolo Harbour area included: a large water reservoir scheme, Plover Cove Reservoir; two new satellite towns, Shatin New Town and Tai Po New Town; and a large industrial estate, Tai Po Industrial Estate. In total, the water area was decreased by 30% or so, mainly due to extensive reclamation to acquire land for the new towns and the industrial estate. The length of the coastline was shortened from 139 to 109 km, representing a 22% decrease. Due mainly to the large scale fresh water Plover Cove Reservoir Scheme, the catchment area of Tolo Harbour was reduced to 5000 ha representing a 68% decrease, and freshwater run-off was also reduced ac-
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K.W. Chau r Ad¨ ances in En¨ ironmental Research 6 (2002) 135᎐142
cordingly ŽHolmes, 1988.. Waste from the livestock industry continued whilst sewage from the increasingly populated municipality and effluent discharge from the manufacturing and process industries built up drastically. All the above were discharged into Tolo Harbour via rivers and watercourses. The cumulative effects of these human activities resulted in drastic deterioration of the water quality of Tolo Harbour. Due to its topography, the tidal flushing rate in the harbour is very weak. The hydraulic residence time can be up to 36 days ŽOakley and Cripps, 1972. and hence most of the exogenous pollutants discharged into it were decomposed and deposited within the harbour itself. These uncontrolled discharges caused upset to the natural ecosystem. Hence, frequent occurrences of red tides were reported during the last decade. It also resulted in a loss of coastal amenity value. The problem came to the attention of the Hong Kong Government, which started to take precautionary measures to prevent further deterioration of the environmental quality. For example, the Water Pollution Control Ordinance was enacted in the early 1980s. The Waste Disposal Ordinance put into operation in 1988 prohibited the rearing of livestock in Sha Tin New Town and part of Tai Po New Town. The domestic sewage treatment plants and specified industries in the Tolo Harbour catchment had to apply for exemption or licences. The Nutrient Export Scheme, which diverted most of the domestic wastewater into Kowloon Bay, was implemented in the early 1990s. Now the pollutants flowing into the harbour have been substantially reduced. Both the depth-averaged E. coli and 5-day biochemical oxygen demand ŽBOD5 . levels showed decreasing trends from 1988 to 1995. E. coli was reduced from 100 to 50 cfur100 ml whilst BOD5 decreased from 2.1 to 1.1 mgrl ŽHong Kong Environmental Protection Department, 1995.. Nevertheless, the effect of the sediment on the overlying water quality remains significant. It is well established that sediment oxygen demand ŽSOD. is an important parameter in water quality models and can be a significant component of the dissolved oxygen budget in the overlying waters ŽThomann and Mueller, 1987.. Hanes and Irvine Ž1968. reported that SOD in certain water bodies accounted for as much as 50% of the total oxygen demand. Promeroy et al. Ž1965. demonstrated that sediment release could maintain nutrient concentrations in overlying waters enough to support significant algal growth. Hater Ž1968. found that eutrophic lake sediments have a large capacity to temporarily absorb nutrients and later release them, which might prevent or delay anticipated improvement in the lake following nutrient diversion. Of course, water in the Tolo Harbour zone is saline rather than freshwater and hence direct comparison with this case may not be totally applicable. Nevertheless, marine
sediments can impose a severe impact on both the oxygen and nutrient concentrations ŽWhittemore, 1986.. Sediments can exert a measurable oxygen demand upon overlying waters, and can act as a source of nutrients that can contribute significantly to eutrophication of the water bodies ŽLee et al., 1991; Wu, 1990; Yung and Lee, 1995.. In this paper, the current situation of the water environment in the Tolo Harbour is outlined and field measurements of SOD and nutrient release from the sediment are detailed.
2. Changes of water quality in Tolo Harbour zone 2.1. Ri¨ er water quality in Tolo Harbour Water Control Zone Environmental Protection Department of Hong Kong Government delineated the territorial waters of Hong Kong into 10 Water Control Zones. Specific regulations, pinpointing the existing situation and its significance, were applied in each Water Control Zone to control effluent discharges. In 1982, Tolo Harbour and Channel became the first declared Water Control Zone, due to the serious nature of the pollution. There are three major river systems, namely Shing Mun, Lam Tsuen and Tai Po, within the catchment of the Tolo Harbour Water Control Zone ŽHong Kong Environmental Protection Department, 1986.. The three main tributaries of Shing Mun River were previously seriously polluted by alum sludge from water treatment works, domestic sewage and livestock waste and sporadic discharges of industrial effluents particularly from the Fo Tan area. In the early 1980s, the water quality in the Shing Mun River was generally poor. After the 1980s, a progressive improvement in water quality was noted in various sections of the river. Such improvements were mainly attributed to the enactment of the Water Pollution Control Ordinance aiming to penalise unauthorised effluent discharges. The Lam Tsuen and Tai Po Rivers have also been showing progressive improvement in water quality since 1983, and the water quality was well maintained ŽHong Kong Environmental Protection Department, 1998..
2.2. Water quality in Tolo Harbour As early as 1972, the Drainage Works Division, of the then Public Works Department in the Hong Kong Government, commenced a bimonthly sampling programme in the territorial waters in Hong Kong including Tolo Harbour. It was found that, at Tide Cove and Centre Island of Tolo Harbour, oxygen was in severe depletion in certain months of 1972, 1975 and 1977. At the middle of Tolo Channel, where the current speed is
K.W. Chau r Ad¨ ances in En¨ ironmental Research 6 (2002) 135᎐142
faster and nearer to the outer Mirs Bay and hence better result is expected, a bottom level of only 15% saturation was recorded in 1977. Phosphate and nitrate concentrations were less satisfactory and mean values were 40 and 200 mgrm3, respectively, in the inner harbour. The mean concentration of chlorophyll-a in surface water in the inner harbour was 10 mgrm3 or so, which is much higher than in unpolluted waters. ŽMackay, 1980. In early 1980s, the Environmental Protection Agency ŽEPA., which was the former establishment of the Environmental Protection Department, commenced the Tolo Harbour monitoring programme and undertook a comprehensive review of water quality in Tolo Harbour and Channel. The quantities of pollutant loading generated in the Tolo catchment area, due to different sources such as sewage discharges, poultry wastes, and industrial effluents etc., were measured. It was concluded that the increase in pollutant loading during the period from 1976 to the end of 1983 had exceeded the receptive capacity of the Tolo waters ŽHong Kong Environmental Protection Department, 1986.. The Hong Kong Government is obliged to ensure that the waters of Hong Kong measure up to specific water quality objectives and hence has implemented a series of pollution abatement measures to rectify the situation. In order to restore the beneficial uses as well as to provide pleasant and hygienic water quality surroundings in the Tolo area, the Hong Kong Government began to enforce the Tolo Harbour Action Plan in 1986. Through the Waste Disposal ŽLivestock Waste. Regulations, controls on discharges of livestock waste and industrial effluent were in operation in the early 1990s. Industrial effluents were kept under strict control through the licensing procedures of the Water Pollution Control Ordinance ŽHong Kong Environmental Protection Department, 1991.. High nutrient loading has caused increased frequency of algal blooms and red tides occurrences. To alleviate this problem, the treated sewage effluent from both Sha Tin and Tai Po sewage treatment works were piped to other less sensitive waters. Construction of the Sha Tin effluent export system was completed in 1994. Flows from the Tai Po sewage treatment works were connected into the scheme in 1995 ŽHong Kong Government Information Services, 1994.. The discharge of alum sludge from the Sha Tin Water treatment works was ceased with the completion of the sludge transfer and handling system in 1991 ŽHong Kong Environmental Protection Department, 1992.. Implementation of the Tolo Harbour Action Plan has already produced results to a certain extent. An increase in the compliance rates with the water quality objectives, most of them from 90 to 100%, has been noted for water column average dissolved oxygen ŽG 4 mgrl ., surface chlorophyll-a ŽF 10 mgrm3 . and E. coli ŽF 50 cfur100 ml. levels in the
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last few years. The substantial decrease in the frequency of red tides in the last few years is also indicative of the improvement in water quality ŽHong Kong Environmental Protection Department, 1999.. However, the improvement in the levels of bottom dissolved oxygen is less obvious. There are many factors contributing to the low bottom DO, including substantial nutrient loads from non-point sources, freshwater runoff and fish-farm wastes, and also the effect of water temperature in the unusually warm recent El Nino year. The SOD and sediment nutrient fluxes are also contributing factors. It is necessary to study the SOD and nutrient release from the sediment.
3. Measurement technique 3.1. Choice of method There are generally two categories of techniques for measuring SOD and sediment nutrient fluxes: laboratory measurement of sediment core samples, or in situ field measurement. Both methods have their own pros and cons. Laboratory measurements are generally more accurate than field measurements as they are carried out in a more controlled environment. However, transportation of sediment from the site to the laboratory inevitably involves a certain degree of disturbance to it, no matter how careful the process is. The extent of reproducibility of ambient field conditions in the laboratory may also affect the accuracy of the measurements. The delayed measurement of sediment samples may also have an effect. In situ field measurements can minimise manipulation of sediment and can reflect ambient field conditions well. But the measurement is carried out on site, usually under a more dynamic ambient environment, which affects the degree of accuracy to a certain extent. Moreover, depending on the actual configuration details and design of the equipment, the process of in situ measurement may also disturb the ambient conditions. Murphy and Hicks Ž1986. concluded that current techniques of in situ SOD measurement are still far from satisfactory and that a universally accepted or standard method has not yet been developed. The present work reports laboratory SOD and nutrient flux measurements made on grab sediment core samples. These measurements complement previous in-situ respirometric SOD measurements ŽLee et al., 1991; Wu, 1990; Yung, 1994; Yung and Lee, 1995..
3.2. Core and water sampling By employing a Phlege Core Sediment Sampler ŽKahl Scientific Instrument Corporation, U.S.A., Model No.
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K.W. Chau r Ad¨ ances in En¨ ironmental Research 6 (2002) 135᎐142
217WA200., sediment samples were acquired from Tolo Harbour and Tolo Channel on a weekly and continuous basis from June to November in 1995. At each sampling station ŽSS1 to SS5 with locations as shown in Fig. 1., three undisturbed sediment cores were gathered Žone for SOD measurement and two for nutrient release measurements.. In order to ensure the minimum disturbance, the core samples were well protected in an ice chest for delivery to the laboratory. To avoid the formation of an oxidised layer, tests for SOD and release of nutrients were immediately performed once they were transported back to the laboratory, with travel time usually less than 1 h. In addition, by employing an Acrylic Horizontal Water Sampler ŽWildlife Supply Company, U.S.A.., raw water samples were collected at 2 m below the water surface at each of the stations.
3.3. SOD measurements Prior to the measurements, the raw water samples were pre-treated by filtering with glass microfibre filter paper ŽWhatman Company, England, Model: GFrC. and aerated for more than 8 h in a dark room. The core sediment sample was then placed into several BOD bottles. The bottles were then filled to the rim with the pre-treated water samples. Extreme care had to be exercised in these steps, so as to ensure the absence of formation of air bubbles and also to avoid disturbance to the sediment sample. The set-up was initially allowed to stay for 20 min. Continuous measurement of the amount of dissolved oxygen in the overlying water was commenced every 30 min for a
total duration of 4᎐5 h with a Dissolved Oxygen Meter ŽYellow Spring Instrument Company, USA., Model 58..
3.4. Nutrient release measurements For measuring the nutrient release fluxes, settling columns with a height of 50 cm and diameter of 9.6 cm were used. Two core sediments acquired from each sampling station were first placed in the columns. A pre-filtered water sample Ž1800 ml. was slowly added with great care to each settling column. The overlying water was continuously and mechanically stirred at a rotational speed of 4 rev.rmin. This set-up was placed in a dark, air-conditioned room and the temperature was maintained at 20⬚C. The initial nutrient concentrations of the overlying water in the settling column were first determined. The overlying water was then periodically analysed for nutrient concentrations for a total duration of 4᎐5 days. Prior to analysing for nutrient concentration, the water samples were filtered. The procedures for all these nutrient analyses were determined in accordance with standard methods ŽGreenberg et al., 1992.. Phosphorus concentration was determined using the ascorbic acid reduction technique catalysed by antimony for ortho-phosphate phosphorus and total phosphorus. Nitrate and nitrite nitrogen was determined by using the cadmium reduction method with a Spectronic 601 spectrophotometer ŽMilon Roy Company, USA... Ammonia nitrogen and kjeldahl nitrogen were determined by the ammonia selective electrode method. An Ion Analyser EA940 ŽOrion Research Incorporation, U.S.A.. and automatic TKN analyser ŽTecator Company, Sweden, System 6 1007
Fig. 1. Sampling stations in Tolo Harbour.
K.W. Chau r Ad¨ ances in En¨ ironmental Research 6 (2002) 135᎐142
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Table 1 Statistical analysis of SOD measurements at station SS1᎐SS5 Sampling station SOD mgO2rm2-h
No. of samples Mean S.D. Range
SS1
SS2
SS3
SS4
SS5
96 37.9 3.4 32.5᎐44.6
96 33.3 2.8 25.8᎐39.6
96 40.8 3.6 32.1᎐51.7
96 40.3 3.5 19.6᎐53.2
96 35.4 2.9 25.8᎐41.2
Digester and Kjeltec Auto 1030 Analyser. were employed, respectively.
4. Results and discussions 4.1. Sediment oxygen demand The sediment oxygen demand ŽSOD. is the rate of removal of dissolved oxygen by the bottom sediment from the overlying water due to the decomposition of settled organic matter. It includes both the respiration rates of benthic communities or benthic animals, i.e. living organisms in the sediment, and chemical oxidation of reduced substances in the sediment such as divalent iron and manganese, sulfide, etc. Hence, the mechanisms of SOD incorporate numerous complicated physical, chemical as well as biological processes. Fig. 2 shows the oxygen consumption by the sediment in Tolo Harbour at different stations. By using the slope of the curve in Fig. 2, the volume of the overlying water and the surface area of sediment, SOD could be computed with the amount of oxygen consumption per unit interfacial area per unit time ŽmgO 2rm2-h.. Table 1 shows the summary of statistical analysis results of SOD measurements taken at all the five sampling stations SS1 to SS5. The number of samples, mean values, S.D. values as well as the ranges of data are all listed. SOD values at each station did not differ too much. From the locations of the five stations, it was expected that water pollution at stations SS1 and SS2, which are located in the inner harbour where the
Fig. 2. Consumption rate of dissolved oxygen by sediment in SOD measurement.
flushing effect is extremely poor, would be more serious than at the other three stations. However, the measured SOD values at these two stations were not particularly higher than their counterparts at the other three stations. This can be explained by the different nature of sediments at these stations. Sediments at stations SS1 and SS2 were mainly sandy while those at stations SS3 to SS5 were fine silty-clay, which are more vulnerable to absorption and accumulation of pollutants. It is also noted that the measured SOD values at stations SS3 and SS4 were quite high, which may be related to the fish-farm activities in the vicinity of Yim Tin Tsai, and inside Three Fathoms Cove, respectively. The SOD values agreed closely with those of Spies and Merle Ž1984., who reported a few mgO 2rm2-h for sand and larger than 100 mgO 2rm2-h for organically enriched mud. They were also comparable with those reported in Seiki et al. Ž1989.. Wu Ž1990. measured the SOD of different bottoms Žsandy, muddy and organically enriched muddy. at a fish culture site and obtained values from 21 to 516 mgO 2rm2-h by employing a respirometer. Wu et al. Ž1994. reported that, by using the same respirometer, the average SOD value measured in 1990 at the fish culture zone in Three Fathoms Cove was 400 mgO 2rm2-h. Lee et al. Ž1991. listed the measured SOD values during 1987᎐1989 at the same fish culture zone and found that the values could vary from 33 to 800 mgO 2rm2-h. Yung and Lee Ž1995., by employing a twin-jet cylindrical SOD chamber, obtained a SOD value of 105 mgO 2rm2-h for an organic sediment surface prepared by mixing dewatered sludge with fine sand at a ratio of approximately 3 to 1 and a very high SOD value of approximately 500 mgO 2rm2-h at the tail of an algal bloom that occurred in 1991. After comparison with the above values, it can be stated that the SOD values obtained in this study are within the usual range. Since an increase in temperature will result in increased rates of bacterial respiration and biochemical reaction, higher SOD values are expected at higher temperature. McDonnell and Hall Ž1969. reported that biological processes increased two-fold for each 10⬚C rise in temperature. Hence tests were also performed in this study to determine the relationship of SOD values with different temperatures. Fig. 3 shows the measured SOD values at 12, 22 and 32⬚C. It was
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K.W. Chau r Ad¨ ances in En¨ ironmental Research 6 (2002) 135᎐142
quite a significant amount of phosphorus and nitrogen existing in different forms was released from sediments to the overlying waters. During the whole measurement period, it was ensured that aerobic conditions were maintained. It is logical that sediments in eutrophic water may contain enormous amounts of phosphorus existing in both organic and inorganic forms. Under aerobic conditions, a thin aerobic layer with a thickness of a few millimetres covering the sediments exists, which has been determined to be one of the factors contributing to the assimilation capacity of phosphorus. ŽPromeroy et al., 1965. When the condition changes to anaerobic, the ferric compounds are reduced and the sorption capacity substantially decreases. A free exchange of dissolved substances between the sediments and the overlying water takes place. Under such conditions, phosphorus will be gradually released into the overlying water. Compared with phosphorus, the process of nitrogen release from sediments is more complicated since it involves the inter-conversion of a larger number of nitrogen species. It was noticed that ammonia nitrogen was, among others, the key form of nitrogen released from the sediment, which agreed well with results reported by Boynton et al. Ž1980.. The release of a high concentration of ammonia nitrogen from the sediment is the result of the decomposition of organic nitrogen, which previously accumulated continuously in the sediment. The concentration of nitrate-nitrite nitrogen was found to be low since it can be released from or
Fig. 3. Graph showing relationship between SOD values and temperature.
verified that as temperature rises, SOD value increases. It was also noted that the effect of temperature is larger at lower temperature than at higher temperature, i.e. with the same temperature increment, the change in SOD value was larger at low temperature than at higher temperature. It was also found that SOD value was independent of the degree of salinity of the overlying water. This was confirmed by experimenting with artificially mixing one-third seawater and two-thirds freshwater to form the overlying water. When comparison was made with the results obtained using raw seawater, no differences were noticed.
4.2. Sediment nutrient release Table 2 lists the measurement results on release of nutrients from the sediment, which demonstrate that Table 2 Mean values and S.D. of release of nutrients from sediment Item
SS1 Žmgrm2-d.
SS2
SS3
SS4
SS5
Ortho-phosphate phosphorus
No. of samples mean S.D.
24 3.13 0.29
24 2.39 0.21
24 3.08 0.28
24 2.40 0.21
24 2.25 0.20
Total phosphorus
No. of samples mean S.D.
24 3.50 0.31
24 2.90 0.27
24 3.35 0.30
24 3.50 0.30
24 2.60 0.22
Nitrate-nitrite nitrogen
No. of samples mean S.D.
24 0.23 0.02
24 0.35 0.02
24 0.20 0.01
24 0.10 0.01
24 0.18 0.02
Ammonia nitrogen
No. of samples Mean S.D.
24 82 7
24 46 3
24 65 5
24 57 6
24 62 5
Kjeldahl nitrogen
No. of samples Mean S.D.
24 103 9
24 55 4
24 81 7
24 86 7
24 71 6
K.W. Chau r Ad¨ ances in En¨ ironmental Research 6 (2002) 135᎐142
absorbed into the sediment, depending on the concentration gradient across the interface between sediment and water. When the external nutrient loadings or sources were gradually decreased and removed from Tolo Harbour, sediment previously enriched with nitrogen could still release sufficient nitrogen quantities to support the growth of plankton and hence improvement of water quality could not be achieved immediately. It is also noted that the sediment release rate measurements are of the same order as those computed independently from a diagenesis model ŽLee and Feleke, 1999..
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ery time will be required for the previously contaminated sediment to have no adverse effect on the water quality in Tolo Harbour.
Acknowledgements This study was supported by a research grant from the Hong Kong Research Grants Council. The assistance of F.L. Hua and H. Chua in the field work and laboratory measurements is gratefully acknowledged. References
5. Conclusion In the present study, laboratory measurements of sediments showed that the average SOD was 38 mgO 2rm2-h in the Tolo Harbour. The SOD value appeared to be reasonable and agreed closely with those reported in other literature, bearing in mind the different conditions reported in the previous literature such as nature of sediment, location, year, technique of measurement, etc. From the laboratory measurement results, it was also noticed that the release of nutrients from the sediment was quite serious, especially for ammonia nitrogen and ortho-phosphate phosphorus with average release rates of 62 and 3 mgrm2-d, respectively. The sediment release rate measurements are also of the same order as those computed independently in the literature. These measured field data will be useful in understanding the water quality of the water body and also provide parameters that may be used when developing numerical water quality models for Tolo Harbour. Over the last decade, the Hong Kong Government has exerted continuous effort in the Tolo Harbour Water Control Zone and has imposed strict controls on discharging domestic, poultry and industrial sewage into the practically land-locked embayment. Implementation of the Tolo Harbour Action Plan has already produced results in improving the water quality. An increase in the compliance rates with the water quality objectives, most of them from 90 to 100%, has been noted for water column average dissolved oxygen, surface chlorophyll-a and E. coli levels in the last few years. Though the external nutrient sources were reduced and removed as far as possible from Tolo Harbour by the Government, SOD and nutrients released from previously contaminated sediments are among the factors that delay the improvement of water quality in the overlying water. At the present moment, it still does not fully comply with all the water quality objectives in the Water Control Zone. A much longer recov-
Boynton, W., Kemp, W.M., Osborne, C., 1980. Nutrient fluxes across the sediment-water interface in the turbid zone of a coastal plain estuary. Estuarine Perspectives, Kennedy. Academic Press Inc., New York. Greenberg, A.E., Clescerl, L.S., Eaton, A.D., 1992. Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, USA.. Hanes, N.B., Irvine, R.L., 1968. New techniques for measuring oxygen uptake rates of benthal systems. J. Water Pollut. Control Fed. 40, 223᎐232. Hater, R.D., 1968. Adsorption of phosphorus by lake sediment. Soil Sci. Soc. Am. Proc. 32. Holmes, P.R., 1988. Tolo Harbourᎏthe case for integrated water quality management in a coastal environment. Institution Water Environ. Manage. 2 Ž2., 171᎐179. Hong Kong Government Information Services, 1994. Hong Kong Government Printer, Hong Kong. Hong Kong Environmental Protection Department, 1986. Marine water quality in Hong Kong: results from the EPD Marine Water Quality Programme for 1986, Hong Kong Government Printer. Hong Kong Environmental Protection Department, 1991. Environment Hong Kong 1991, Hong Kong Government Printer. Hong Kong Environmental Protection Department, 1992. Environment Hong Kong 1992, Hong Kong Government Printer. Hong Kong Environmental Protection Department, 1995. Marine water quality in Hong Kong: results from the EPD Marine Water Quality Programme for 1995, Hong Kong Government Printer. Hong Kong Environmental Protection Department, 1998. Environment Hong Kong 1998, Hong Kong Government Printer. Hong Kong Environmental Protection Department, 1999. Marine water quality in Hong Kong: results from the EPD Marine Water Quality Programme for 1999, Hong Kong Government Printer. Lee, J.H.W., Wu, R.S.S., Cheung, Y.K., Wong, P.S.S., 1991. Dissolved oxygen variations in marine fish culture zone. J. Environ. Eng., ASCE 117 Ž6., 799᎐815. Lee, J.H.W., Feleke, A., 1999. Eutrophication dynamics of Tolo Harbour, Hong Kong. Mar. Pollut. Bull. 39, 187᎐192. Mackay, D.W., 1980. Investigations of Water Pollution in Tolo
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Harbour, Hong KongᎏA Review with Conclusions and Recommendations. McDonnell, A.J., Hall, S.D., 1969. Effects of mixing on environmental factors on benthal oxygen uptake. J. Water Pollut. Control Fed. 41, 353᎐363. Murphy, P.J., Hicks, D.B., 1986. In situ method for measuring sediment oxygen demand. In: Hatcher, K.J. ŽEd.., Sediment Oxygen Demand: Processes, Modelling and Measurement. Institute of Natural Resources, University of Georgia, Athens, pp. 307᎐330. Oakley, H.R., Cripps, T., 1972. Marine pollution studies at Hong Kong and Singapore. In: Ruivo, M. ŽEd.., Mar. Pollut. Sea Life. Promeroy, L.R., Smith, E.E., Grant, C.M., 1965. The exchange of phosphate between estuarine water and sediments. Limnol. Oceanogr. 10 Ž2., 67᎐172. Seiki, T., Izawa, H., Date, E., 1989. Benthic nutrient demineralisation and oxygen consumption in the coastal area of Hiroshima Bay. Water Res. 23 Ž2., 219᎐228. Spies, P.A., Merle, G., 1984. Mesure de la demande en oxygene des sediment dans les cours d’eau et les lacs. Cah. Lab. Hydrobiol. Montereau 15, 31᎐38.
Thomann, R.V., Mueller, J.A., 1987. Principles of Surface Water Quality Modelling and Control. Harper and Row. Whittemore, R.C., 1986. Problems with in situ and laboratory SOD measurements and their implementation in water quality modelling studies. In: Hatcher, K.J. ŽEd.., Sediment Oxygen Demand: Processes, Modelling and Measurement. Institute of Natural Resources, University of Georgia, Athens, pp. 331᎐342. Wu, R.S.S., 1990. A respirometer for continuous, in-situ, measurements of sediment oxygen demand. Water Res. 24 Ž3., 391᎐394. Wu, R.S.S., Lam, K.S., MacKay, D.W., Lau, T.C., Yam, V., 1994. Impact of marine fish farming on water quality and bottom sediment: a case study in the sub-tropical environment. Mar. Environ. Res. 38, 115᎐145. Yung, K.S., 1994. Sediment oxygen demand in coastal waters. PhD Thesis, The University of Hong Kong. Yung, K.S., Lee, J.H.W., 1995. Hydraulics of sediment oxygen demand chambers. Proceedings of Sixth International Symposium on River Sedimentation. Central Board of Irrigation and Power Malcha Marg., New Delhi, pp. 1091᎐1100.
Field measurements of SOD and sediment nutrient fluxes in a land-locked embayment in Hong Kong K.W. ChauU Department of Ci¨ il and Structural Engineering, The Hong Kong Polytechnic Uni¨ ersity, Hunghom, Kowloon, Hong Kong
Abstract The pollution problem in Hong Kong has increased over the past two decades, and the situation in the nearly land-locked Tolo Harbour, located at the north-eastern part of the territories, has been particularly problematic. Although the exogenous organic materials dissolved in the overlying water have been substantially reduced, the soft sediments at the bottom of the harbour continue to act as sources of nutrients. This sediment oxygen demand ŽSOD. amounts to a significant value, hence increasing the total oxygen demand load of the harbour. In this paper, the current state of the water environment is reviewed. The SOD and the rate of nutrient release in different forms of nitrogen and phosphorus were measured in laboratory experiments. It was found from the laboratory tests that the average SOD value is 38 mgO 2rm2-h and that the mean release rates of phosphate phosphorus and ammonia nitrogen are, respectively, 3 and 62 mgrm2-d. These measured field data will be useful in understanding the water quality of the water body and also provide parameters that may be used when developing numerical water quality models for Tolo Harbour. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Tolo Harbour; Water quality; Sediment oxygen demand; Nutrient release
1. Introduction Tolo Harbour, an almost land-locked estuary located in the north-eastern territories of Hong Kong, is most vulnerable to environmental pollution. The main water body is approximately 16 km long and 3 km wide. It is connected to the Mirs Bay through Tolo Channel, which is a very narrow channel with a maximum width of only 1.3 km. The mean depth of Tolo Harbour and Tolo Channel are 12 and 20 m, respectively. A few decades ago, the harbour was enclosed by rural areas, and hence the water quality problem was not acute.
U
Corresponding author. Tel.: q852-2766-6014; fax: q8522334-6389. E-mail address: [email protected] ŽK.W. Chau..
However, in the past 30 years, enormous population expansion in Hong Kong caused the government to shift infrastructure developments to the New Territories, previously a rural area. Land use changes in the Tolo Harbour area included: a large water reservoir scheme, Plover Cove Reservoir; two new satellite towns, Shatin New Town and Tai Po New Town; and a large industrial estate, Tai Po Industrial Estate. In total, the water area was decreased by 30% or so, mainly due to extensive reclamation to acquire land for the new towns and the industrial estate. The length of the coastline was shortened from 139 to 109 km, representing a 22% decrease. Due mainly to the large scale fresh water Plover Cove Reservoir Scheme, the catchment area of Tolo Harbour was reduced to 5000 ha representing a 68% decrease, and freshwater run-off was also reduced ac-
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cordingly ŽHolmes, 1988.. Waste from the livestock industry continued whilst sewage from the increasingly populated municipality and effluent discharge from the manufacturing and process industries built up drastically. All the above were discharged into Tolo Harbour via rivers and watercourses. The cumulative effects of these human activities resulted in drastic deterioration of the water quality of Tolo Harbour. Due to its topography, the tidal flushing rate in the harbour is very weak. The hydraulic residence time can be up to 36 days ŽOakley and Cripps, 1972. and hence most of the exogenous pollutants discharged into it were decomposed and deposited within the harbour itself. These uncontrolled discharges caused upset to the natural ecosystem. Hence, frequent occurrences of red tides were reported during the last decade. It also resulted in a loss of coastal amenity value. The problem came to the attention of the Hong Kong Government, which started to take precautionary measures to prevent further deterioration of the environmental quality. For example, the Water Pollution Control Ordinance was enacted in the early 1980s. The Waste Disposal Ordinance put into operation in 1988 prohibited the rearing of livestock in Sha Tin New Town and part of Tai Po New Town. The domestic sewage treatment plants and specified industries in the Tolo Harbour catchment had to apply for exemption or licences. The Nutrient Export Scheme, which diverted most of the domestic wastewater into Kowloon Bay, was implemented in the early 1990s. Now the pollutants flowing into the harbour have been substantially reduced. Both the depth-averaged E. coli and 5-day biochemical oxygen demand ŽBOD5 . levels showed decreasing trends from 1988 to 1995. E. coli was reduced from 100 to 50 cfur100 ml whilst BOD5 decreased from 2.1 to 1.1 mgrl ŽHong Kong Environmental Protection Department, 1995.. Nevertheless, the effect of the sediment on the overlying water quality remains significant. It is well established that sediment oxygen demand ŽSOD. is an important parameter in water quality models and can be a significant component of the dissolved oxygen budget in the overlying waters ŽThomann and Mueller, 1987.. Hanes and Irvine Ž1968. reported that SOD in certain water bodies accounted for as much as 50% of the total oxygen demand. Promeroy et al. Ž1965. demonstrated that sediment release could maintain nutrient concentrations in overlying waters enough to support significant algal growth. Hater Ž1968. found that eutrophic lake sediments have a large capacity to temporarily absorb nutrients and later release them, which might prevent or delay anticipated improvement in the lake following nutrient diversion. Of course, water in the Tolo Harbour zone is saline rather than freshwater and hence direct comparison with this case may not be totally applicable. Nevertheless, marine
sediments can impose a severe impact on both the oxygen and nutrient concentrations ŽWhittemore, 1986.. Sediments can exert a measurable oxygen demand upon overlying waters, and can act as a source of nutrients that can contribute significantly to eutrophication of the water bodies ŽLee et al., 1991; Wu, 1990; Yung and Lee, 1995.. In this paper, the current situation of the water environment in the Tolo Harbour is outlined and field measurements of SOD and nutrient release from the sediment are detailed.
2. Changes of water quality in Tolo Harbour zone 2.1. Ri¨ er water quality in Tolo Harbour Water Control Zone Environmental Protection Department of Hong Kong Government delineated the territorial waters of Hong Kong into 10 Water Control Zones. Specific regulations, pinpointing the existing situation and its significance, were applied in each Water Control Zone to control effluent discharges. In 1982, Tolo Harbour and Channel became the first declared Water Control Zone, due to the serious nature of the pollution. There are three major river systems, namely Shing Mun, Lam Tsuen and Tai Po, within the catchment of the Tolo Harbour Water Control Zone ŽHong Kong Environmental Protection Department, 1986.. The three main tributaries of Shing Mun River were previously seriously polluted by alum sludge from water treatment works, domestic sewage and livestock waste and sporadic discharges of industrial effluents particularly from the Fo Tan area. In the early 1980s, the water quality in the Shing Mun River was generally poor. After the 1980s, a progressive improvement in water quality was noted in various sections of the river. Such improvements were mainly attributed to the enactment of the Water Pollution Control Ordinance aiming to penalise unauthorised effluent discharges. The Lam Tsuen and Tai Po Rivers have also been showing progressive improvement in water quality since 1983, and the water quality was well maintained ŽHong Kong Environmental Protection Department, 1998..
2.2. Water quality in Tolo Harbour As early as 1972, the Drainage Works Division, of the then Public Works Department in the Hong Kong Government, commenced a bimonthly sampling programme in the territorial waters in Hong Kong including Tolo Harbour. It was found that, at Tide Cove and Centre Island of Tolo Harbour, oxygen was in severe depletion in certain months of 1972, 1975 and 1977. At the middle of Tolo Channel, where the current speed is
K.W. Chau r Ad¨ ances in En¨ ironmental Research 6 (2002) 135᎐142
faster and nearer to the outer Mirs Bay and hence better result is expected, a bottom level of only 15% saturation was recorded in 1977. Phosphate and nitrate concentrations were less satisfactory and mean values were 40 and 200 mgrm3, respectively, in the inner harbour. The mean concentration of chlorophyll-a in surface water in the inner harbour was 10 mgrm3 or so, which is much higher than in unpolluted waters. ŽMackay, 1980. In early 1980s, the Environmental Protection Agency ŽEPA., which was the former establishment of the Environmental Protection Department, commenced the Tolo Harbour monitoring programme and undertook a comprehensive review of water quality in Tolo Harbour and Channel. The quantities of pollutant loading generated in the Tolo catchment area, due to different sources such as sewage discharges, poultry wastes, and industrial effluents etc., were measured. It was concluded that the increase in pollutant loading during the period from 1976 to the end of 1983 had exceeded the receptive capacity of the Tolo waters ŽHong Kong Environmental Protection Department, 1986.. The Hong Kong Government is obliged to ensure that the waters of Hong Kong measure up to specific water quality objectives and hence has implemented a series of pollution abatement measures to rectify the situation. In order to restore the beneficial uses as well as to provide pleasant and hygienic water quality surroundings in the Tolo area, the Hong Kong Government began to enforce the Tolo Harbour Action Plan in 1986. Through the Waste Disposal ŽLivestock Waste. Regulations, controls on discharges of livestock waste and industrial effluent were in operation in the early 1990s. Industrial effluents were kept under strict control through the licensing procedures of the Water Pollution Control Ordinance ŽHong Kong Environmental Protection Department, 1991.. High nutrient loading has caused increased frequency of algal blooms and red tides occurrences. To alleviate this problem, the treated sewage effluent from both Sha Tin and Tai Po sewage treatment works were piped to other less sensitive waters. Construction of the Sha Tin effluent export system was completed in 1994. Flows from the Tai Po sewage treatment works were connected into the scheme in 1995 ŽHong Kong Government Information Services, 1994.. The discharge of alum sludge from the Sha Tin Water treatment works was ceased with the completion of the sludge transfer and handling system in 1991 ŽHong Kong Environmental Protection Department, 1992.. Implementation of the Tolo Harbour Action Plan has already produced results to a certain extent. An increase in the compliance rates with the water quality objectives, most of them from 90 to 100%, has been noted for water column average dissolved oxygen ŽG 4 mgrl ., surface chlorophyll-a ŽF 10 mgrm3 . and E. coli ŽF 50 cfur100 ml. levels in the
137
last few years. The substantial decrease in the frequency of red tides in the last few years is also indicative of the improvement in water quality ŽHong Kong Environmental Protection Department, 1999.. However, the improvement in the levels of bottom dissolved oxygen is less obvious. There are many factors contributing to the low bottom DO, including substantial nutrient loads from non-point sources, freshwater runoff and fish-farm wastes, and also the effect of water temperature in the unusually warm recent El Nino year. The SOD and sediment nutrient fluxes are also contributing factors. It is necessary to study the SOD and nutrient release from the sediment.
3. Measurement technique 3.1. Choice of method There are generally two categories of techniques for measuring SOD and sediment nutrient fluxes: laboratory measurement of sediment core samples, or in situ field measurement. Both methods have their own pros and cons. Laboratory measurements are generally more accurate than field measurements as they are carried out in a more controlled environment. However, transportation of sediment from the site to the laboratory inevitably involves a certain degree of disturbance to it, no matter how careful the process is. The extent of reproducibility of ambient field conditions in the laboratory may also affect the accuracy of the measurements. The delayed measurement of sediment samples may also have an effect. In situ field measurements can minimise manipulation of sediment and can reflect ambient field conditions well. But the measurement is carried out on site, usually under a more dynamic ambient environment, which affects the degree of accuracy to a certain extent. Moreover, depending on the actual configuration details and design of the equipment, the process of in situ measurement may also disturb the ambient conditions. Murphy and Hicks Ž1986. concluded that current techniques of in situ SOD measurement are still far from satisfactory and that a universally accepted or standard method has not yet been developed. The present work reports laboratory SOD and nutrient flux measurements made on grab sediment core samples. These measurements complement previous in-situ respirometric SOD measurements ŽLee et al., 1991; Wu, 1990; Yung, 1994; Yung and Lee, 1995..
3.2. Core and water sampling By employing a Phlege Core Sediment Sampler ŽKahl Scientific Instrument Corporation, U.S.A., Model No.
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217WA200., sediment samples were acquired from Tolo Harbour and Tolo Channel on a weekly and continuous basis from June to November in 1995. At each sampling station ŽSS1 to SS5 with locations as shown in Fig. 1., three undisturbed sediment cores were gathered Žone for SOD measurement and two for nutrient release measurements.. In order to ensure the minimum disturbance, the core samples were well protected in an ice chest for delivery to the laboratory. To avoid the formation of an oxidised layer, tests for SOD and release of nutrients were immediately performed once they were transported back to the laboratory, with travel time usually less than 1 h. In addition, by employing an Acrylic Horizontal Water Sampler ŽWildlife Supply Company, U.S.A.., raw water samples were collected at 2 m below the water surface at each of the stations.
3.3. SOD measurements Prior to the measurements, the raw water samples were pre-treated by filtering with glass microfibre filter paper ŽWhatman Company, England, Model: GFrC. and aerated for more than 8 h in a dark room. The core sediment sample was then placed into several BOD bottles. The bottles were then filled to the rim with the pre-treated water samples. Extreme care had to be exercised in these steps, so as to ensure the absence of formation of air bubbles and also to avoid disturbance to the sediment sample. The set-up was initially allowed to stay for 20 min. Continuous measurement of the amount of dissolved oxygen in the overlying water was commenced every 30 min for a
total duration of 4᎐5 h with a Dissolved Oxygen Meter ŽYellow Spring Instrument Company, USA., Model 58..
3.4. Nutrient release measurements For measuring the nutrient release fluxes, settling columns with a height of 50 cm and diameter of 9.6 cm were used. Two core sediments acquired from each sampling station were first placed in the columns. A pre-filtered water sample Ž1800 ml. was slowly added with great care to each settling column. The overlying water was continuously and mechanically stirred at a rotational speed of 4 rev.rmin. This set-up was placed in a dark, air-conditioned room and the temperature was maintained at 20⬚C. The initial nutrient concentrations of the overlying water in the settling column were first determined. The overlying water was then periodically analysed for nutrient concentrations for a total duration of 4᎐5 days. Prior to analysing for nutrient concentration, the water samples were filtered. The procedures for all these nutrient analyses were determined in accordance with standard methods ŽGreenberg et al., 1992.. Phosphorus concentration was determined using the ascorbic acid reduction technique catalysed by antimony for ortho-phosphate phosphorus and total phosphorus. Nitrate and nitrite nitrogen was determined by using the cadmium reduction method with a Spectronic 601 spectrophotometer ŽMilon Roy Company, USA... Ammonia nitrogen and kjeldahl nitrogen were determined by the ammonia selective electrode method. An Ion Analyser EA940 ŽOrion Research Incorporation, U.S.A.. and automatic TKN analyser ŽTecator Company, Sweden, System 6 1007
Fig. 1. Sampling stations in Tolo Harbour.
K.W. Chau r Ad¨ ances in En¨ ironmental Research 6 (2002) 135᎐142
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Table 1 Statistical analysis of SOD measurements at station SS1᎐SS5 Sampling station SOD mgO2rm2-h
No. of samples Mean S.D. Range
SS1
SS2
SS3
SS4
SS5
96 37.9 3.4 32.5᎐44.6
96 33.3 2.8 25.8᎐39.6
96 40.8 3.6 32.1᎐51.7
96 40.3 3.5 19.6᎐53.2
96 35.4 2.9 25.8᎐41.2
Digester and Kjeltec Auto 1030 Analyser. were employed, respectively.
4. Results and discussions 4.1. Sediment oxygen demand The sediment oxygen demand ŽSOD. is the rate of removal of dissolved oxygen by the bottom sediment from the overlying water due to the decomposition of settled organic matter. It includes both the respiration rates of benthic communities or benthic animals, i.e. living organisms in the sediment, and chemical oxidation of reduced substances in the sediment such as divalent iron and manganese, sulfide, etc. Hence, the mechanisms of SOD incorporate numerous complicated physical, chemical as well as biological processes. Fig. 2 shows the oxygen consumption by the sediment in Tolo Harbour at different stations. By using the slope of the curve in Fig. 2, the volume of the overlying water and the surface area of sediment, SOD could be computed with the amount of oxygen consumption per unit interfacial area per unit time ŽmgO 2rm2-h.. Table 1 shows the summary of statistical analysis results of SOD measurements taken at all the five sampling stations SS1 to SS5. The number of samples, mean values, S.D. values as well as the ranges of data are all listed. SOD values at each station did not differ too much. From the locations of the five stations, it was expected that water pollution at stations SS1 and SS2, which are located in the inner harbour where the
Fig. 2. Consumption rate of dissolved oxygen by sediment in SOD measurement.
flushing effect is extremely poor, would be more serious than at the other three stations. However, the measured SOD values at these two stations were not particularly higher than their counterparts at the other three stations. This can be explained by the different nature of sediments at these stations. Sediments at stations SS1 and SS2 were mainly sandy while those at stations SS3 to SS5 were fine silty-clay, which are more vulnerable to absorption and accumulation of pollutants. It is also noted that the measured SOD values at stations SS3 and SS4 were quite high, which may be related to the fish-farm activities in the vicinity of Yim Tin Tsai, and inside Three Fathoms Cove, respectively. The SOD values agreed closely with those of Spies and Merle Ž1984., who reported a few mgO 2rm2-h for sand and larger than 100 mgO 2rm2-h for organically enriched mud. They were also comparable with those reported in Seiki et al. Ž1989.. Wu Ž1990. measured the SOD of different bottoms Žsandy, muddy and organically enriched muddy. at a fish culture site and obtained values from 21 to 516 mgO 2rm2-h by employing a respirometer. Wu et al. Ž1994. reported that, by using the same respirometer, the average SOD value measured in 1990 at the fish culture zone in Three Fathoms Cove was 400 mgO 2rm2-h. Lee et al. Ž1991. listed the measured SOD values during 1987᎐1989 at the same fish culture zone and found that the values could vary from 33 to 800 mgO 2rm2-h. Yung and Lee Ž1995., by employing a twin-jet cylindrical SOD chamber, obtained a SOD value of 105 mgO 2rm2-h for an organic sediment surface prepared by mixing dewatered sludge with fine sand at a ratio of approximately 3 to 1 and a very high SOD value of approximately 500 mgO 2rm2-h at the tail of an algal bloom that occurred in 1991. After comparison with the above values, it can be stated that the SOD values obtained in this study are within the usual range. Since an increase in temperature will result in increased rates of bacterial respiration and biochemical reaction, higher SOD values are expected at higher temperature. McDonnell and Hall Ž1969. reported that biological processes increased two-fold for each 10⬚C rise in temperature. Hence tests were also performed in this study to determine the relationship of SOD values with different temperatures. Fig. 3 shows the measured SOD values at 12, 22 and 32⬚C. It was
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K.W. Chau r Ad¨ ances in En¨ ironmental Research 6 (2002) 135᎐142
quite a significant amount of phosphorus and nitrogen existing in different forms was released from sediments to the overlying waters. During the whole measurement period, it was ensured that aerobic conditions were maintained. It is logical that sediments in eutrophic water may contain enormous amounts of phosphorus existing in both organic and inorganic forms. Under aerobic conditions, a thin aerobic layer with a thickness of a few millimetres covering the sediments exists, which has been determined to be one of the factors contributing to the assimilation capacity of phosphorus. ŽPromeroy et al., 1965. When the condition changes to anaerobic, the ferric compounds are reduced and the sorption capacity substantially decreases. A free exchange of dissolved substances between the sediments and the overlying water takes place. Under such conditions, phosphorus will be gradually released into the overlying water. Compared with phosphorus, the process of nitrogen release from sediments is more complicated since it involves the inter-conversion of a larger number of nitrogen species. It was noticed that ammonia nitrogen was, among others, the key form of nitrogen released from the sediment, which agreed well with results reported by Boynton et al. Ž1980.. The release of a high concentration of ammonia nitrogen from the sediment is the result of the decomposition of organic nitrogen, which previously accumulated continuously in the sediment. The concentration of nitrate-nitrite nitrogen was found to be low since it can be released from or
Fig. 3. Graph showing relationship between SOD values and temperature.
verified that as temperature rises, SOD value increases. It was also noted that the effect of temperature is larger at lower temperature than at higher temperature, i.e. with the same temperature increment, the change in SOD value was larger at low temperature than at higher temperature. It was also found that SOD value was independent of the degree of salinity of the overlying water. This was confirmed by experimenting with artificially mixing one-third seawater and two-thirds freshwater to form the overlying water. When comparison was made with the results obtained using raw seawater, no differences were noticed.
4.2. Sediment nutrient release Table 2 lists the measurement results on release of nutrients from the sediment, which demonstrate that Table 2 Mean values and S.D. of release of nutrients from sediment Item
SS1 Žmgrm2-d.
SS2
SS3
SS4
SS5
Ortho-phosphate phosphorus
No. of samples mean S.D.
24 3.13 0.29
24 2.39 0.21
24 3.08 0.28
24 2.40 0.21
24 2.25 0.20
Total phosphorus
No. of samples mean S.D.
24 3.50 0.31
24 2.90 0.27
24 3.35 0.30
24 3.50 0.30
24 2.60 0.22
Nitrate-nitrite nitrogen
No. of samples mean S.D.
24 0.23 0.02
24 0.35 0.02
24 0.20 0.01
24 0.10 0.01
24 0.18 0.02
Ammonia nitrogen
No. of samples Mean S.D.
24 82 7
24 46 3
24 65 5
24 57 6
24 62 5
Kjeldahl nitrogen
No. of samples Mean S.D.
24 103 9
24 55 4
24 81 7
24 86 7
24 71 6
K.W. Chau r Ad¨ ances in En¨ ironmental Research 6 (2002) 135᎐142
absorbed into the sediment, depending on the concentration gradient across the interface between sediment and water. When the external nutrient loadings or sources were gradually decreased and removed from Tolo Harbour, sediment previously enriched with nitrogen could still release sufficient nitrogen quantities to support the growth of plankton and hence improvement of water quality could not be achieved immediately. It is also noted that the sediment release rate measurements are of the same order as those computed independently from a diagenesis model ŽLee and Feleke, 1999..
141
ery time will be required for the previously contaminated sediment to have no adverse effect on the water quality in Tolo Harbour.
Acknowledgements This study was supported by a research grant from the Hong Kong Research Grants Council. The assistance of F.L. Hua and H. Chua in the field work and laboratory measurements is gratefully acknowledged. References
5. Conclusion In the present study, laboratory measurements of sediments showed that the average SOD was 38 mgO 2rm2-h in the Tolo Harbour. The SOD value appeared to be reasonable and agreed closely with those reported in other literature, bearing in mind the different conditions reported in the previous literature such as nature of sediment, location, year, technique of measurement, etc. From the laboratory measurement results, it was also noticed that the release of nutrients from the sediment was quite serious, especially for ammonia nitrogen and ortho-phosphate phosphorus with average release rates of 62 and 3 mgrm2-d, respectively. The sediment release rate measurements are also of the same order as those computed independently in the literature. These measured field data will be useful in understanding the water quality of the water body and also provide parameters that may be used when developing numerical water quality models for Tolo Harbour. Over the last decade, the Hong Kong Government has exerted continuous effort in the Tolo Harbour Water Control Zone and has imposed strict controls on discharging domestic, poultry and industrial sewage into the practically land-locked embayment. Implementation of the Tolo Harbour Action Plan has already produced results in improving the water quality. An increase in the compliance rates with the water quality objectives, most of them from 90 to 100%, has been noted for water column average dissolved oxygen, surface chlorophyll-a and E. coli levels in the last few years. Though the external nutrient sources were reduced and removed as far as possible from Tolo Harbour by the Government, SOD and nutrients released from previously contaminated sediments are among the factors that delay the improvement of water quality in the overlying water. At the present moment, it still does not fully comply with all the water quality objectives in the Water Control Zone. A much longer recov-
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