Pauline English and Heinz Buettikofer (CSIRO) prepared Figure 1. Paul Rustomji (CSIRO), Bala ...... material after severe bush fires. IAHS Publ. 292, 223-230 ...
Impacts on water quality by sediments and nutrients released during extreme bushfires: Report 4: Impacts on Lake Burragorang Scott Wilkinson, Peter Wallbrink, Gary Hancock, William Blake, Rick Shakesby and Vicky Farwig
Report for the Sydney Catchment Authority
CSIRO Land and Water Science Report 6/07 February 2007
Copyright and Disclaimer © 2006 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 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: River delta in the Wollondilly arm of Lake Burragorang, February 2003 Photographer: Gary Caitcheon © 2006 CSIRO ISSN: 1883-4563
Impacts on water quality by sediments and nutrients released during extreme bushfires: Report 4: Impacts on Lake Burragorang Scott Wilkinson1, Peter Wallbrink1, Gary Hancock1, William Blake2, Rick Shakesby3 and Vicky Farwig3 1
CSIRO Land and Water
2
University of Plymouth, UK
3
University of Wales at Swansea, UK
CSIRO Land and Water Science Report 6/07 February 2007
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Acknowledgements This research was part-funded by the Sydney Catchment Authority (SCA) and the UK Natural Environment Research Council (Grant Code: NER/A/S/2002/0043). Danny Hunt (CSIRO) assisted with field sampling and preparation. Chris Leslie (CSIRO) undertook the radionuclide analysis. Pauline English and Heinz Buettikofer (CSIRO) prepared Figure 1. Paul Rustomji (CSIRO), Bala Vigneswaran and Rob Mann (SCA) provided constructive reviews of this report.
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Executive Summary This report describes the final stage of a collaborative research project between CSIRO and the Sydney Catchment Authority (SCA) on the potential impacts on water quality of extreme bushfires in the Sydney water supply catchments. The project has investigated the transfer of sediments and nutrients between different components of the Nattai River catchment slopes to Lake Burragorang reservoir continuum, following extensive wildfire in the Nattai River catchment in December 2001, to develop an understanding of the impacts of fire induced erosion on water quality. The fire burnt 225,000 ha mainly in the Nattai River catchment, and over a large proportion of this area the fire was previously found to be of high to extreme severity as measured by remote sensing and ground inspections. Phase one of the research reviewed available data and process understanding of the impact of forest fires on erosion rates. Phase two of the research demonstrated extensive redistribution of surface soil and nutrients on a burnt hillslope in the Nattai River catchment, with some storage in foot-slope locations. Phase three of the research found that sediment delivery to the river network from burnt areas in 2002 was approximately six times the mean-annual pre-fire rate, and was dominated by surface sources high in phosphorus, in contrast to predominantly sub-surface sources prefire. In 2002–2003, the yields of suspended sediment and phosphorus for individual runoff events were up to two orders of magnitude greater than those pre-fire due to their greatly increased availability for transport. The fourth and final phase of the research focussed on the downstream impacts of postfire sediment and nutrient yield to Lake Burragorang. Radionuclide and geochemistry tracers were used to investigate the long term contribution of fire to reservoir sedimentation relative to non-fire causes of catchment erosion. The results show that sediment yield to Lake Burragorang in the 13 months following the 2001 fires was below the long-term annual average rate since the reservoir was completed in 1960. This low sediment yield was unexpected given the visibility of post-fire erosion. However, 2002 was drier than average, with the annual discharge from the Nattai River being 34% of mean-annual discharge, and subsequent years were similarly dry. The dry conditions limited the post-fire erosion from the much higher rates that could have occurred in wetter conditions. Sediment delivery to the river network, and delivery through the river network to the reservoir were also limited by the below-average post-fire rainfall and low river discharge. The actual annual post-fire sediment yields to Lake Burragorang from the Nattai River were several times higher than what would have occurred without fire in the dry post-fire conditions. If post-fire rainfall had been higher, the post-fire delivery of sediment to the reservoir may have had a larger impact on reservoir water quality, including the possibility of algal blooms.
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Table of Contents 1. 2.
Introduction..................................................................................................................... 1 Reservoir sediment delivery - Methods........................................................................ 2 2.1. Sediment coring and grab sampling........................................................................................ 2 2.2. Using radionuclides to determine reservoir sediment sources and deposition chronology .... 3 2.3. Laboratory sample analysis..................................................................................................... 4 2.3.1. Core sampling ................................................................................................................. 4 2.3.2. Preparation of soil samples ............................................................................................. 4 2.3.3. Particle size analysis ....................................................................................................... 4 2.3.4. Gamma spectrometry methods for analysis of low level radioactivity............................. 4 2.3.5. Alpha spectrometry methods for analysis of plutonium activity....................................... 5 2.4. Estimating pre and post-fire sediment yield to the reservoir ................................................... 5
3.
Reservoir sediment delivery - Results and interpretation.......................................... 6 3.1. The effect of fire on the radionuclide label of sediment delivered to Lake Burragorang......... 6 3.2. Reservoir sediment data ......................................................................................................... 6 3.2.1. Nattai arm surface grabs ................................................................................................. 6 3.2.2. Nattai arm sediment core ................................................................................................ 7 3.2.3. Wollondilly arm sediment cores..................................................................................... 10 3.2.4. Summary of reservoir sediment results......................................................................... 11
4.
Discussion .................................................................................................................... 12 4.1. 4.2. 4.3.
Attenuation of post-fire sediment delivery to the reservoir .................................................... 12 The sensitivity of post-fire sediment delivery to rainfall......................................................... 13 Long-term impacts of fire on Lake Burragorang.................................................................... 15
5. Conclusions .................................................................................................................. 16 References ........................................................................................................................... 17
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1. Introduction A collaborative three-year project between CSIRO and the Sydney Catchment Authority (SCA) on the impacts on water quality of extreme bushfires aimed to investigate the rates and amounts of transfers of sediments and nutrients between different components of the Nattai catchment slope to reservoir continuum. The 2001 fire covered 225,000 ha, mainly in the Nattai River catchment, and was the largest in the catchment in over 30 years. A field investigation of the combustion of vegetation, coupled with analysis of remote sensing data, indicated that the fire was of high to extreme severity over much of the burnt area (Shakesby et al., 2003; Chafer et al., 2004). There were several significant rainfall events in the weeks following the fire that delivered a pulse of black post-fire sediment to the river network draining the burnt area (Shakesby et al., 2003). This report presents the findings of the fourth and final stage of the project regarding the downstream impacts of post-fire sediment and nutrient losses on Lake Burragorang and the long term contribution of fire to reservoir sedimentation relative to non-fire causes of catchment erosion. The first phase of the project reviewed the literature pertaining to the effect of fire on erosion and erosion rates, with emphasis on the Nattai catchment, NSW, following the 2001 bushfires (Wallbrink et al., 2004; Shakesby et al., 2007). The second phase of the project used fallout radionuclides as sediment tracers to quantify the post-fire redistribution of soil and sediment and attached nutrients on burnt hillslopes (English et al., 2005; Wallbrink et al., 2005). The third phase of the project investigated the immediate postfire impacts of sediment and nutrient redistribution to the river network (Wilkinson et al., 2006). Thus fourth and final stage investigates sediment deposition in the downstream receiving water body of Lake Burragorang, through analysis of reservoir sediment cores and samples. Figure 1 shows the study area.
Figure 1: A map of Lake Burragorang catchment showing the location of the Blue Gum Creek study area in which the hillslope process and sediment redistribution studies were undertaken. The river network study encompassed the Little River and Nattai River and adjacent tributaries, and the impact on reservoir sedimentation focussed on the Nattai and adjacent Wollondilly arms of the reservoir (English et al., 2005).
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2. Reservoir sediment delivery - Methods 2.1. Sediment coring and grab sampling Reservoir sediment in the region of the Nattai River arm of the reservoir was sampled from a boat on 5–7 February 2003. Surface sediment samples were collected using an Eckman grab sampler and sediment cores were taken using a 50 mm diameter percussion device. The sample locations were selected with the aim of assessing the extent of post-fire deposition relative to previous non-fire related deposition. Five sub-aqueous sediment grabs from the Nattai Arm (numbered LG03005–LG03009) and three sub-aqueous sediment cores were analysed; LG03010 from the Nattai arm and LG03014 and LG03017 from the Wollondilly arm of the reservoir. A sediment profile LG03003 was also sampled from just above the waterline in the Nattai delta. Sample locations are shown in Figure 2. Given the extent of the 2001 fire was mostly in the Nattai arm of the reservoir, but also along the eastern side of the Wollondilly arm (Wilkinson et al., 2006), it was expected that the samples from the Wollondilly arm would be affected by post-fire sedimentation to a lesser degree than samples from the Nattai arm.
Figure 2: Location of the Lake Burragorang sediment samples. At full supply level the lake water surface in the Nattai arm extends approximately to the road crossing (red dashed line) but at the time of sampling it extended to near the bend in the arm downstream of the road crossing.
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The cores LG03010 and LG03017 were extracted in approximately 4 m water; LG03014 in 1.5 m. The minimum water level in 1982 (the last major drought prior to sampling) was approximately 5 m below the level at the time of coring, thus the core locations were exposed to the atmosphere for approximately 1–2 months in 1982 as shown in Figure 3. As described in the results, it appears that minimal erosion of the sediment record occurred during this period. Coring date 1999 1995 1991
5m
1987 1983 1979 1975 1971 1967 1963 1960 5
0
-5
-10
-15
-20
Water level (m below spillway)
Figure 3: Water level in Lake Burragorang since dam completion in 1960 (modified from Blake et al., 2006).
2.2. Using radionuclides to determine reservoir sediment sources and deposition chronology Fallout radionuclides were used for determining the sources and history of reservoir sedimentation by comparing the radionuclide activity of different catchment and river sources with the activity of reservoir sediment: Fallout Caesium-137 (137Cs) has a half life of 30.23 years and is a product of atmospheric nuclear weapons testing that occurred during the 1950–70s. The bulk of the activity of this nuclide is retained within the top 100 mm of the soil profile in Australian soils and thus it labels sediment that is derived from surface soil erosion since fallout occurred. Fallout Lead-210 (210Pb) has a half life of 22.3 years and is formed through the decay of 222 Rn gas via a series of short lived daughters. The parent of 222Rn is the lithogenic nuclide 226 Ra, part of the 238U decay series. In most soils it is expected that 210Pb will be in approximately secular equilibrium with 226Ra. However, some 222Rn diffuses into the atmosphere where it decays to 210Pb, which then reaches the earth’s surface by wet and dry precipitation (Wise, 1980). In this way maximum concentrations of fallout 210Pb (also known as ‘unsupported’ or ‘excess’, i.e. 210Pbex) are generally found at the soil surface. Concentrations then decrease to detection limits at about 100 mm depth. Because the 210 Pbex activity is being continually replenished in surface soil, rather than having been delivered as a pulse like 137Cs, the 210Pbex activity of sediment can also be used as an approximate indicator of the time since sediment deposition in the reservoir (Wise, 1980; Smith, 1982).
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Fallout of Plutonium isotopes 239Pu and 240Pu resulted from atmospheric nuclear weapons testing during the 1950–70s. A spike in fallout of a third isotope 238Pu occurred in 1965, particularly in the southern hemisphere, due to atmospheric re-entry and destruction of a satellite SNAP-9A. The ratio of Plutonium 238Pu/239+240Pu radio-isotopes in fallout thus increased markedly in the southern hemisphere in 1965. Prior to 1965 the ratio in the southern hemisphere was approximately 0.03 corrected for decay (Koide et al., 1979), and after 1965 the ratio was approximately 0.15 in the southern hemisphere. Thus, the ratio provides a useful marker of AD 1965 in sediment cores. The Beryllium-7 cosmogenic radionuclide used to investigate catchment sediment sources (English et al., 2005; Wilkinson et al., 2006) was not used in examining reservoir sedimentation because it has a very short half life (53 days) relative to the period of sedimentation.
2.3. Laboratory sample analysis 2.3.1.
Core sampling
The Nattai arm core LG03010 had a length of 1.48 m and was the longest of all the cores analysed. The core was dissected into 10 cm increments to obtain sufficient material in the
