Government of Western Australia Department of Water Drainage for salinity control at Pithara Department of Water 168 St Georges Terrace, Perth, Western Australia PO Box K822 Perth Western Australia 6842 Phone: (08) 6364 7600 Fax: (08) 6364 7601 www.water.wa.gov.au ter needs all our wa g after Lookin Salinity and land use impacts series Report no. SLUI 46 November 2010 Drainage for salinity control at Pithara by NM Cox Department of Water Salinity and land use impacts series Report no. SLUI 46 November 2010 Drainage to control salinity at Pithara SLUI 46 Salinity and land use impacts series Department of Water 168 St Georges Terrace Perth Western Australia 6000 Telephone +61 8 6364 7600 Facsimile +61 8 6364 7601 www.water.wa.gov.au © Government of Western Australia 2010 November 2010 This work is copyright. You may download, display, print and reproduce this material in unaltered form only (retaining this notice) for your personal, non-commercial use or use within your organisation. Apart from any use as permitted under the Copyright Act 1968, all other rights are reserved. Requests and enquiries concerning reproduction and rights should be addressed to the Department of Water. ISSN 1447-7394 (print) 1449-5252 (online) ISBN 978-1-921637-18-6 (print) 978-1-921637-19-3 (online) 978-1-921789-75-5 (Appendix CD) Recommended reference Cox, NM 2010, Drainage for salinity control at Pithara, Salinity and land use impacts series, Report no. SLUI 46, Department of Water, Perth. Acknowledgements The Department of Water thanks the following for their contributions to this case study and publication: Kingsley and Paula Roach, owners of Petrador Farms, Pithara; Sylvia Tetlow for groundwater monitoring and data management; the Department of Water Swan-Goldfields hydrographical group for water monitoring; Water Information for data collation and management and Peter Geste for water sampling for chemical analysis and Alex Waterhouse for editing. For more information about this report, contact: Nick M Cox Salinity and Water Resource Recovery Branch Department of Water Telephone +61 8 6364 7804. Disclaimer This document has been published by the Department of Water. Any representation, statement, opinion or advice expressed or implied in this publication is made in good faith and on the basis that the Department of Water and its employees are not liable for any damage or loss whatsoever which may occur as a result of action taken or not taken, as the case may be in respect of any representation, statement, opinion or advice referred to herein. Professional advice should be obtained before applying the information contained in this document to particular circumstances. This publication is available at our website <www.water.wa.gov.au>. Contact the department for alternative formats such as audio, large print or Braille for those with special needs. Cover photograph: The downstream drain gauging station at the Pithara site Photographer: Nick Cox Salinity and land use impacts series Drainage for salinity control at Pithara SLUI 46 Contents Summary ................................................................................................................... vii 1 Introduction.............................................................................................................. 1 1.1 Objectives ........................................................................................................................ 1 1.2 Supporting information ..................................................................................................... 2 1.3 Errors and accuracy ......................................................................................................... 2 2 Pithara drainage site ............................................................................................... 4 2.1 Climate ............................................................................................................................. 5 2.2 Land use .......................................................................................................................... 8 2.3 Topography and natural drainage .................................................................................... 8 2.4 Soils ................................................................................................................................. 9 2.5 Geology and hydrogeology ............................................................................................ 11 2.6 Watertables and salinity ................................................................................................. 13 3 Pithara project design and methods ...................................................................... 15 3.1 Drain design and construction ........................................................................................ 15 3.2 Monitoring and measurements ....................................................................................... 17 4 Results .................................................................................................................. 20 4.1 Groundwater level responses to the drain ...................................................................... 20 4.2 Groundwater salinity ...................................................................................................... 21 4.3 Groundwater pH ............................................................................................................. 24 4.4 Groundwater chemistry .................................................................................................. 26 4.5 Upstream drain discharge .............................................................................................. 27 4.6 Downstream drain discharge ......................................................................................... 32 5 Water and salt balances ........................................................................................ 36 5.1 Drainage site .................................................................................................................. 37 5.2 Water balance for the upstream drainage site ............................................................... 38 5.3 Water balances for the drains ........................................................................................ 41 5.4 Salt balance for the upstream drainage site ................................................................... 42 5.5 Salt balances for the drains ............................................................................................ 43 6 Discussion ............................................................................................................. 45 6.1 Groundwater inflows ...................................................................................................... 45 6.2 Total drain discharges .................................................................................................... 46 6.3 Groundwater response ................................................................................................... 47 6.4 Salt land recovery .......................................................................................................... 49 6.5 Drain performance simulation ........................................................................................ 51 6.6 Construction costs .......................................................................................................... 56 7 Conclusions ........................................................................................................... 57 Department of Water iii Drainage to control salinity at Pithara SLUI 46 Salinity and land use impacts series Appendix A Watertable responses ...................................................................... 58 A.1 Comparison bores ......................................................................................................... 58 A.2 Transect 1 ..................................................................................................................... 59 A.3 Transect 2 ..................................................................................................................... 61 A.4 Transect 3 ..................................................................................................................... 65 A.5 Transect 4 ..................................................................................................................... 67 Appendix B Water balance methodology ............................................................. 70 B.1 Bounded and unbounded drains ................................................................................... 70 B.2 Drain inflows, discharges and loads .............................................................................. 72 B.3 Groundwater heads and drain discharge ....................................................................... 75 B.4 Aquifer storage change ................................................................................................. 77 B.5 Rainfall recharge ........................................................................................................... 80 B.6 Aquifer discharge .......................................................................................................... 81 B.7 Capillary discharge and groundwater evaporation ......................................................... 84 Glossary and abbreviations ......................................................................................