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  • Groundwater–surface water interactions between streams and alluvial aquifers: Results from the Wollombi Brook (NSW) study (Part II – Biogeochemical processes)

    Sébastien Lamontagne, Andrew L. Herczeg, John C. Dighton, Jodie L. Pritchard CSIRO Land & Water Jaswant S. Jiwan NSW Department of Infrastructure, Planning and Natural Resources William J. Ullman College of Marine Studies, University of Delaware

    Technical Report 42/03, July 2003

  • i

    © 2003 CSIRO To the extent permitted by law, all rights are reserved and no part of this publication covered by copyright may be reproduced or copied in any form or by any means except with the written permission of CSIRO Land and Water.

    Important Disclaimer

    CSIRO Land and Water advises that the information contained in this publication comprises general statements based on scientific research. The reader is advised and needs to be aware that such information may be incomplete or unable to be used in any specific situation. No reliance or actions must therefore be made on that information without seeking prior expert professional, scientific and technical advice.

    To the extent permitted by law, CSIRO Land and Water (including its employees and consultants) excludes all liability to any person for any consequences, including but not limited to all losses, damages, costs, expenses and any other compensation, arising directly or indirectly from using this publication (in part or in whole) and any information or material contained in it.

    Cover Photograph

    Other images available from CSIRO Land and Water Image Gallery: www.clw.csiro.au/ImageGallery/ Description: Fordwich site Photographer: Sébastien Lamontagne © 2003 CSIRO

  • Groundwater – surface water interactions at Wollombi Brook: Biogeochemical processes i

    Summary The biogeochemistry of groundwater–surface water interactions in a subtropical sand- bed stream (Wollombi Brook, NSW) was studied over a two-year period using networks of piezometers and stream porewater profiles at three sites with different geomorphological properties. Several key findings emerged from the study: • While the lower section of the brook should be a discharge area for brackish regional

    groundwater, the brook and the alluvial aquifer remained relatively fresh. Locally, dewatering of coal seams in active mines in the region has lowered the water table in the regional aquifer and reduced or reversed hydraulic gradients.

    • Groundwater–surface water interactions were primarily controlled by lateral exchange processes between the brook and the alluvial aquifer, including bank recharge– discharge cycles during floods and hyporheic exchange during low flows.

    • The alluvial aquifer generally had reducing conditions and tended to be enriched in chemical species more stable or more soluble in anoxic environments, including NH4+, filterable reactive phosphorus (FRP), Fe2+, and Si.

    • The redox status of the alluvial aquifer may be related to the depth to the water table in the floodplain. Reducing conditions were usually associated with shallower water tables.

    • The presence of several redox interfaces throughout the alluvial aquifer probably fosters several cycles of nutrient transformations during a lateral exchange cycle. These would tend to promote the loss of some elements (such as nitrogen) relative to others (such as phosphorus).

    • Under low flows, surface water is probably exchanged with streambed porewater every several hundred metres by hyporheic processes.

    • Nutrient concentration in Wollombi Brook remained low throughout the study period. It is proposed that the large rates of lateral exchange with the sandy alluvial aquifer (especially hyporheic exchange) are fostering the maintenance of high water quality in the brook under low flows.

    It is proposed that future research on the significance of groundwater–surface water interaction on nutrient cycles in streams and rivers should focus on: • The further development of measurement and modelling techniques to characterise

    hyporheic exchange and associated biogeochemical transformations. • The integration of groundwater discharge and hyporheic exchange modelling to

    provide more accurate estimates of nutrient fluxes from groundwater. • The use of hydrograph separation techniques to estimate the contribution of

    groundwater discharge to stream nutrient budgets during flood pulses.

  • Groundwater – surface water interactions at Wollombi Brook: Biogeochemical processes ii

    Acknowledgements Many people in the Wollombi Brook catchment gave us enormous assistance and information during the course of our field studies. We especially thank Wendy and Bill Lawson, who were always helpful in supplying information, data and sometimes sustaining us with their first class semillon or chardonnay. Jim Maher enthusiastically collected water samples on a regular basis, often well beyond the call of duty when the brook was nearly dry, or during occasional flooding rains. The following people generously allowed us access to their properties to install piezometers and groundwater boreholes; Professor Tony Palfreeman at Wollombi, Mr. Andrew Furnance at Fordwich and Mrs. Heather Kannar at Warkworth. We acknowledge assistance provided by Peter Smith and Michael Williams from the NSW Department of Sustainable Natural Resources who helped get the project off the ground. This project was funded by Land and Water Australia, CSIRO Land and Water, and the NSW Department of Natural Resources.

  • Groundwater – surface water interactions at Wollombi Brook: Biogeochemical processes iii

    Table of Contents

    RESULTS FROM THE WOLLOMBI BROOK (NSW) STUDY (PART II – BIOGEOCHEMICAL PROCESSES) ..................................................................................I

    SUMMARY..........................................................................................................................I

    ACKNOWLEDGEMENTS..................................................................................................II

    TABLE OF CONTENTS ...................................................................................................III

    INTRODUCTION................................................................................................................1

    Literature Review ......................................................................................................... 2

    Methods........................................................................................................................ 4

    Site description............................................................................................................. 4

    Hydrology ..................................................................................................................... 4

    Riparian transects ........................................................................................................ 5

    Sampling Techniques................................................................................................... 6

    Field Trips..................................................................................................................... 9

    RESULTS ..........................................................................................................................9

    General patterns in water quality.................................................................................. 9

    Spatial trends across the alluvial plain ....................................................................... 10

    Patterns in nutrient concentration............................................................................... 13

    Water Table profiles ................................................................................................... 17

    Streambed porewater profiles .................................................................................... 18

    Comparison of potential water fluxes from alluvial groundwater discharge and hyporheic exchange ................................................................................................... 23

    Nitrogen mineralisation rates across the floodplain.................................................... 25

    DISCUSSION...................................................................................................................26

    Nutrient recycling in groundwater during lateral exchange......................................... 26

    Dissolved nutrients in surface water........................................................................... 29

  • Groundwater – surface water interactions at Wollombi Brook: Biogeochemical processes iv

    Future Research......................................................................................................... 31

    CONCLUSION.................................................................................................................34

    REFERENCES.................................................................................................................35

    APPENDIX 1. ANALYTICAL METHODS........................................................................40

    APPENDIX 2. DATA SUMMARY ...................................................................................42

    APPENDIX 3. SOME USEFUL TOOLS FOR GROUNDWATER–SURFACE WATER

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