5'1 x (e State Pollution Control Commission 011 Northern Rivers Study No.1 Water Qual ity in the Tweed-Terranora Estuary e State Pollution Control Commission Northern Rivers Study No. 1 Water Quality in the Tweed-Terranora Estuary • Other titles In the Northern Rivers Study are: Wa ter Quality In the Brunswick River Water Quality In the RichrJond River Water Quality In the ClareI!ce River' Water Quality In the Mac/eay River Wa ter Quality in the Bellinger'-Kalang. Rivers Water .Quality In the Hastings River Water Quality In the. Manning' River This report was prepared' by Ross Williams, State' Pollution Control Comntission, Sydney, ISBN 0 7305 0406 9 . First published January 1985 This printing Miy 1981' Summary Rapid population increase in the Tweed Heads district, particularly in Tweed Heads itself, and at Banora Point and Kingscliff, has led to the augmentation of sewage treatment works and· a review· of effluent disposal strategies. The Terranora and Cobaki Broadwaters which receive effluent from the Tweed Heads West and Banora Point sewage treatment works are a complex hydrological system of creeks, broadwaters and inlets. As part of this review and strategy development, the State Pollution Control Commission conducted an investigation into the options for effluent disposal to the Terranoni-Cobaki Broadwater systems. Water quality data for the Tweed Riv~r and associated ·canal developments were also measured during the study.· The investigation, carried out in October-November 1983 and January 1984, involved water quality determinations, estimation .of sewage treatment works effluent loads and included a study of dispersion and movement of sewage effluent discharged to the Broadwaters from the Banora Point and Tweed Heads West sewage, treatment works; A significant deterioration in water quality in the Broadwaters occurred since 1978 so that excessive floatin'g or attached algal communities could be expected to develop. During this period catchment popUlation increased causing an increase in nutrient loadings from the sewage treatment facilities. The current limiting factor for algal growth is probably high tu'rhidity in the shallow Broadwaters which· restricts light penetration. Dredging operations could decrease turbidity within the Broadwaters. Effluent dye tracing studies demonstrated the poor flushing characteristics of the Broadwater system and indicated high effluent retention under normal flow/tidal conditions. These studies enabled more desirable discharge locations to be determined together with suggested restricted disposal periods to ensure accumulation of nutrients does not occur. W,ater quality in the Tweed River is probably adversely affected by agricultural activities, particularly sugar growing, flood mitigation and urban run-off from Murwillumbah. Wastewater discharges from the Condong sugar mill have a significant localised impact on a section,' of the river. Tweed River 'water quality requires further investigation, especially the section from Tumbulgum to Murwillumbah. Water quality studies in some canal developments identified stratification and oxygen depletion at depth; further investigation· will be necessary to establish the extent of the problem. Wa ter quality in the Terranora and Cobaki Broadwaters is expected to further deteriorate unless changes are implemented in the location of points of effluent disposal or in the quality of the effluent itself. Problems may become more acu te if curren t disposal s t ra tegies and dredging con tinues, ,particularly in Cobaki Broadwater. In addition, to assist' in the reduction of nutrient inputs and sediment loads to the system, the impact of urban drainage in any new urban developments· and, where possible, in existing \. urban areas should be minimised. Thus to ensure the preservation of waterways for existing, uses: effluent from Banora Point and Tweed Heads West sewage treatment works should be discharged to Terranora Inlet no further upstream than Diy Dock and should only be permitted during the last hour of the flood-tide' and 'the first three hours of the ebb-tide to ensure it is adequately flushed ,.into' the Tweed River and t~e ocean; and' ' suspended solids, in the return waters from. dredging need to b'e controlled closely particularly in the Broadwater areas. Areas of concern on other water quality issues include the' stretch of the tweed between Tumbulgum and Murwilluinbah, ,and deep areas within canal developments. The extent of 'the localised influence of the sugar mill,' requires further inves tiga tion. " - 1 - CONTENTS 1 INTRODUCTION 5 2 BACKGROUND 7 2.1 Location 7 2.2 Cl imate 7 2.3 Land Use 7 2.4 Population and Strategic Planning 9 2.5 Local Industry- 10 2.6 Pesticides Usage in-Tweed Valley 11 2.7 Rainfall 11 "2.8 Other Environmental Studies 12 3 STUDY PROGRAMME 13 - ; 3.1 WaterQuality 13 3.2 Effluent Dye Tracing 16 4 TWEED RIVER 19 5 TERRANORA AND_ COBAKIBROADWATERS 23 5.1 Effluents Discharged to Terranora Inlet and Cobaki Broadwater 23 5.2 Stream Flows 25 5.3 Phosphorus in Broadwater Sediments 25 5.4 w.ater Quality in the Inlet/Broadwaters 26 5.5 "Effluent Transport and Retention 31 6 CANAL DEVELOPMENTS 42 7 DISCUSSION AND CONCLUSIONS 46 7.1 Limitations of the Study 46 7.2 Tweed River / 46 7.3 Population Growth and Its Influence on Terranora and Cobaki Broadwaters 47 7.4 Transport and Retention of Sewage Effluents Within the Terranora and Cobaki Broadwaters 49 7.5 Canal Developments 51 8 RE COMMEN DA TI ON S 52 9 REFERENCES 53 10 ACKNOWLEDGEMENTS 54 - .2 - CONTENTS (continued) APPENDIX A: Summary of Department of Agriculture (Division of Fisheries) ·Water-Quality Data October 1978 55 APPENDIX B: Water-QuaHty Sample Site Locations 56 APPENDIX C: Dye-Effluent Study Transect Locations 58 APPENDIX D: Bacterial Densities: Surveys 1 and 2 59 APPENDIX E: Table E.l Water-Quality Data (0.5 m) 1983/84 60 . Table E.2 Water~Quality Data (3 and 4 m) 1983/84. 62 I . - 3 - LIST OF FIGURES Page 1 Locality Map for Tweed River and Terranora/Cobaki Broadwater 6 2 Land Use in the Tweed Vall ey 8 3 Water-Quality Sample Location (Tweed River) 14 4 Water-Quality Sample Location (Broadwater)' 15 5 Location of Transect Sites 18 6 Tweed River Water-Quality Data (0.5 and 4.0 m) 20 7 Tweed River Water-Quality Data (0.5 m) 21 8 Terranora and Cobaki Broadwater Water-QualitYData (0.5 m) 27 9 Terranora Inlet and Broadwater Water Quality (.o.5m) 28 . 10 Terranora Inlet and Cobaki Broadwate~Water Quality (0.5m) 30 . .' 11 Dye Concentration with Distance, Study 1 32 12 Dye Concentration at Boyds Bay Bridge with Time, Study 1 33 ~3 Dye Concentration.with Distance, Study 2 I . 34 14 Dye Concentrations at Transects T9, Tl3 and T24 wi th Time, Study 2 35 15 Dye Concentration with Distance, Study 3 36 16 Dye Concentration Upstream of Kennedy Drive Bridge, (T24/5) with Time, Study 3 38 17 Dye Concentration with Distance, Study 4 39 18 Canal Developments Water Quality (0.5 and 3 m). 43 19 Canal Developments Water-Quality Data (0.5 m) 45 20 Summary of Dyed Effluent Movement jn Terranora/Cobaki System 50 4 - LIST OF TABLES Page 1 Monthly Rainfall in Survey Area 11 " 2 Comparisons of Calculated. Advised and Actual Dye Volumes 16 3 Dye-Tracing Study Programme ' 17 ,4 Tweed Heads West and Banora Point Sewage"'T~eatment ,Works Characteristics 23 5 Tweed Heads West and Banora Point - Prel iminary Loading Estiniates 24 6 Freshwater Inflows to Terranora and Cobaki ~roadwaters 25 7 ',Phosphorus Concentra'ti onsin Silt/Clay Fraction of the Broadwater Sediments (984) " 25 8 Tidal Discharge Volumes per Cycle (Estlmated from PWD Preliminary Data) 40 , , 9 Summary of Water-Qual ity Data in the Tweed River, 46 10 Summary of Water-Quality Data in Terranora and Cobaki Broadwaters , '48 11 Nutri erit Loadings Retai ned Wi thi n the Broadwa ters 51 - 5 - 1 INTRODUCTION The State Pollution Control Commission conducted an investigation into the water quality of the Tweed River and Cobaki/Terranora Broadwater system (Figure 1) in October/November 1983 and January 1984. The investigation also included a study of the dispersion and roovement of effl uents di scharged to these waterways from the Banora Point and Tweed Heads sewage":treatment plants. The investigation was carried out in response to a request from the New South Wales Department of Public Works s~eking the Conmission~s view on sewage-effluent disposal strategies for the Tweed Heads area. The Terranora and Cobaki Broadwaters, to which existing works discharge effluents, are a complex hydrological system of creeks, lakes and inlets. Insufficient information was known about existing quality of water in the estuary, the effects of effluents on the receiving waters and the flushing characteristics of the system. The study Was deslgned to obtain data necessary for determi.ning the impact of effluents on the receiving waters and. their ultimate fate or assimilation. Limited historical water-quality information was available from a study conduc.ted by NSW State Fisheries (now called the Division of Fisheries in the Department of Agriculture) in 1978-79, but the data collected by State Fi sheries were not adequate to determi ne an effl uent-di sposa 1 stra tegy. They did not include information about tidal dispersion and assimilation, but rather were orientated towards fish'resources, and they had. been . collected seven years ago, \totten the catchment population was lower. The investigation was developed in consultation and following discussion with other government departments and organisations, particularly the NSW Department of Publ ic Works, the Department of Agriculture (Division of Fisheries) and Tweed.Shire Council. During the study period other areas of cqncern were brought to the Commission's notice and the programme was expanded where poss·ible to incorporate these new areas.