Land -use and erosion source discrimination of soil and[Title] carbon sources to the Logan and Albert rivers using Compound Specific Isotope Analysis Gary Hancock and Andy Revill CSIRO Land and Water Science Report 2/11 January 2011 CSIRO Land and Water Science Report series ISSN: 1834-6618 Copyright and Disclaimer © 2011 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. Important Disclaimer: CSIRO 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 (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: Description: An example of channel bank erosion in the mid-catchment region of the Albert river after the January 2008 flood event. Photographer: Tim Pietsch © 2008 CSIRO CONTENTS 1 INTRODUCTION................................................................................................. 1 1.1 Background............................................................................................................... 1 1.2 Compound specific δ13 C measurements as sediment tracers.................................. 2 1.3 Study site .................................................................................................................. 2 2 METHODS ..........................................................................................................3 2.1 Sample collection...................................................................................................... 3 2.2 Sample preparation and analysis ............................................................................. 5 3 RESULTS............................................................................................................6 3.1 Characterising soil sources....................................................................................... 6 3.2 Determining sediment sources ............................................................................... 11 3.2.1 Lower Logan River.............................................................................................. 11 3.2.2 Lower Albert River .............................................................................................. 15 3.2.3 Teviot Brook........................................................................................................ 16 4 CONCLUSIONS................................................................................................ 17 4.1 CSIA as a sediment source tracing technique........................................................ 17 4.2 Enhancement of sediment tracing capabilities ....................................................... 17 4.3 Value to CSIRO and wider...................................................................................... 18 5 ACKNOWLEDGMENTS.................................................................................... 18 REFERENCES ........................................................................................................... 19 APPENDIX ................................................................................................................. 20 iii EXECUTIVE SUMMARY The Compound Specific Isotope Analysis (CSIA) technique has been used to identify the sources of soil erosion contributing sediment to the Logan-Albert catchment. Soil samples were collected in January 2010 and used to assess the ability of the CSIA technique to discriminate probable sources of soil erosion. Fatty acid and bulk carbon δ13 C were measured. This study has built on a previous sediment tracing study undertaken in 2008 using fallout radionuclides and major/minor element geochemistry. It is found that surface soil from forest, pasture and cultivated land-uses are well discriminated using CSIA. Furthermore, sub-surface soil sources associated with channel bank erosion and exposed subsoils (gullies and hillslope scalds) occurring specifically in the mid-western Logan catchment could also be discriminated. The CSIA and bulk carbon δ13 C data were used in the IsoSource mixing model to estimate the erosion sources to sediment collected during the January 2008 flood. The results of this analysis were compared with results obtained using other sediment tracers. For the lower Logan River, the CSIA tracing results are consistent with fallout and element geochemistry tracing, with channel bank erosion being confirmed as the major sediment source. However, the significant contribution to Logan River sediment of exposed subsoils originating on hillslopes and drainage lines from the mid-western region of the Logan catchment has also been confirmed by CSIA. This erosion source was not quantified by catchment modelling. In the Albert River catchment about 50% of soil comes from forest land-use, although more than half of this soil comes from sub-surface sources. These results have demonstrated that the CSIA technique has the potential to significantly enhance the ability of CLW sediment tracing studies to determine the extent that different land-uses are contributing eroded soil to rivers, thus providing a check on catchment model predictions and/or a calibration of model parameters. In particular, sources of surface soil erosion in agricultural regions can now be distinguished from pristine forest hillslopes. Likewise, sub-surface soil sources such as river bank and gully erosion may also be distinguished. iv 1 INTRODUCTION 1.1 Background This report describes work designed to provide “proof of concept” for a new technique to discriminate eroded soil (and associated carbon) sources to rivers according to land-use in Australian catchments. The study entails the collection of characteristic soil and sediment samples from the Logan and Albert River catchments and their measurement by Compound Specific Isotope Analysis (CSIA). The CSIA technique measures the δ13 C isotope signature of specific organic compounds (in this case, fatty acids) associated with the organic matter bound to the soil/sediment. It is proposed that different land-uses will provide distinctive CSIA signatures due to the different types of vegetation which, on decomposition, make up the organic matter bound to soil particles. It is well known that various plants (e.g. grasses, shrubs, trees etc.) synthesise 13 C differently, leading to different δ13 C values for the vegetation of those plants. On decomposition of this vegetation the δ13 C values of the carbon compounds are retained and transferred to the soil. Thus, different plants (e.g. “C3” and “C4” plants) associated with different land-uses may allow the discrimination of land- uses and erosion processes contributing sediment to rivers and estuaries. A sediment sourcing study was recently been undertaken in the Logan-Albert catchment funded by the South-East Queensland Healthy Water Partnership and CSIRO Land and Water (Hancock and Caitcheon, 2010). The study used traditional radionuclide and major/minor element tracers, and provided new information on sediment sources and erosion processes. However, these tracers could not discriminate between different land-uses within the same geological province, in particular between the proportions of sediment originating from pasture, forest and cultivated regions. For example, in Table 1 below the tracers used by Hancock and Caitcheon (2010) identified surface soil erosion as contributing about 20% to river sediment, with exposed subsoils on hillslopes contributing 20-30%. Based on catchment modelling (SedNet, Caitcheon et al., 2001) channel bank erosion is assumed to contribute the remaining ~50%. However, the relative contributions of sub-surface soil sources (channel banks, exposed subsoils on hillslopes) are not well distinguished by surface tracers. Nor it is known where in the catchment the subsoil sources come from, and whether they are associated with pasture, forest and/or cultivated land-uses. The work outlined in this report investigates the ability of CSIA to characterise soils associated with specific land-uses, potentially providing improved discrimination of sediment and carbon sources to the Logan estuary. The work utilises the facilities and expertise of CMAR in Hobart (A. Revill, D. Holdsworth) where fatty acid extraction techniques and CSIA procedures are well established. 1 Table 1. Proportion (as %) of various erosion sources delivered as sediment to rivers, calculated using 137 Cs, excess 210 Pb and 7Be as tracers (Hancock and Caitcheon, 2010). River reach Reach description Reach Surface Recently Channel distance soil (%) exposed Bank (%) subsoil (%) Lower Logan Josephville to estuary ~40 km 10 ±3 40 ±10 50 ±10 Lower Albert Mundoolun to estuary ~40 km 15 ±3 30 ±10 55 ±10 Lower Teviot Boondah to Logan ~40 km 10 ±3 45 ±10 45 ±10 confluence 1.2 Compound specific δδδ13 C measurements as sediment tracers The CSIA method