Special thanks go to my father, Eoin Lawrence, for all his support during my ... This thesis provides the measurements of 222Rn exhalation rates, 210Pb ...
QUEENSLAND UNIVERSITY OF TECHNOLOGY SCHOOL OF PHYSICAL AND CHEMICAL SCIENCES
MEASUREMENT OF 222Rn EXHALATION RATES AND 210
Pb DEPOSITION RATES IN A TROPICAL ENVIRONMENT
Submitted by Cameron Lawrence (B. App. Sc., M. App. Sc.) to the School of Physical and Chemical Sciences, Queensland University of Technology, in partial fulfilment of the requirements of the degree of Doctor of Philosophy. March 2005
Key Words Radon, exhalation, emission, Lead-210, deposition, excess, redistribution, budget, Kakadu, Ranger, uranium, mining, radionuclides, isotopes, soil moisture, radium, activity concentration, land application, soil erosion, atmospheric transport, geomorphic
landscapes,
tropics,
Alligator
Rivers
Region,
environmental
radioactivity, Jabiru, atmospheric dispersion, soil profile, diurnal, seasonality, wet season, dry season, precipitation scavenging, aerosol transport, aerosol removal, Hadley circulation, water inundation
i
Acknowledgements I owe great thanks to my supervisor Dr. Riaz Akber for all his support and effort during the course of this project, his assistance and direction has been invaluable. Many thanks also go to my external supervisor and the other support staff of the Enrad group at eriss, Dr. Paul Martin, Dr. Andreas Bolhöfer, Mr. Bruce Ryan, Mrs. Therese Fox and Mr. Peter Medley, for their countless hours of assistance, sample analysis and data retrieval. I also owe many thanks to the remainder of the eriss team, especially the Jabiru Field Station, for their support during my time in Jabiru. Eriss provided my accommodation and all work facilities for the 20 months of my stay at Jabiru and for that I am gratefully appreciative. Major parts of this project would not have been possible without the assistance of ERA personnel, specifically Mr. Ian Marshman, for arranging access to the Ranger sampling locations. Special thanks go to my father, Eoin Lawrence, for all his support during my studies over the years. His support has fantastic providing me with sound advice in all the major decisions I’ve had to make. I only hope that I can continue to live up to his expectations as I enter the next phase of this life. Most of all I am pleased to have the support and love of my partner Saski who has a direct understanding of the personal commitments required to complete this work. Her support over the last year in all things has been phenomenal and I look forward to providing her the same support in all matters in her life.
ii
Abstract This thesis provides the measurements of
222
Rn exhalation rates,
210
Pb deposition
rates and excess 210Pb inventories for locations in and around Ranger Uranium Mine and Jabiru located within Kakadu National Park, Australia. Radon-222 is part of the natural
238
U series decay chain and the only gas to be found in the series under
normal conditions. Part of the natural redistribution of
222
Rn in the environment is a
portion exhales from the ground and disperses into the atmosphere. Here it decays via a series of short-lived progeny, that attach themselves to aerosol particles, to the long lived isotope
210
Pb (T1/2 = 22.3 y). Attached and unattached
210
Pb is removed
from the atmosphere through wet and dry deposition and deposited on the surface of the earth, the fraction deposited on soils is gradually transported through the soil and can create a depth profile of completing the
238
210
Pb. Here it decays to the stable isotope
206
Pb
U series.
Measurements of
222
Rn exhalation rates and
210
Pb deposition rates were
performed over complete seasonal cycles, August 2002 – July 2003 and May 2003 – May 2004 respectively. The area is categorised as wet and dry tropics and it experiences two distinct seasonal patterns, a dry season (May-October) with little or no precipitation events and a wet season (December-March) with almost daily precipitation and monsoonal troughs. November and April are regarded as transitional months. As the natural processes of 222Rn exhalation and 210Pb deposition are heavily influenced by soil moisture and precipitation respectively, seasonal variations in the exhalation and deposition rates were expected. It was observed that 222
Rn exhalation rates decreased throughout the wet season when the increase in soil
moisture retarded exhalation. Lead-210 deposition peaked throughout the wet season as precipitation is the major scavenging process of this isotope from the atmosphere. Radon-222 is influenced by other parameters such as
226
Ra activity
concentration and distribution, soil porosity and grain size. With the removal of the influence of soil moisture during the dry season it was possible to examine the effect of these other variables in a more comprehensive manner. This resulted in categorisation of geomorphic landscapes from which the 226
222
Rn exhalation rate to
Ra activity concentration ratios were similar during the dry season. These results
can be extended to estimate dry season 222Rn exhalation rates from tropical locations from a measurement of 226Ra activity concentration.
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Through modelling the
210
Pb budget on local and regional scales it was
observed that there is a net loss of
210
Pb from the region, the majority of which
occurs during the dry season. This has been attributed to the fact that
210
Pb attached
to aerosols is transported great distance with the prevailing trade winds created by a Hadley Circulation cell predominant during the dry season (winter) months. By including the influence of factors such as water inundation and natural redistribution in the soil wet season budgeting of gave very good results.
iv
210
210
Pb
Pb on local and regional scales
Contents Chapter 1: Introduction 1.1 Overview 1.2 Alligator Rivers Region 1.3 Project objectives
1 1 6 9
Chapter 2: Literature Review: Previous research in relation to radon emanation, migration, exhalation and 210Pb deposition 2.1 Overview 2.2 Radon emanation
10 10 10
2.2.1
Introduction
10
2.2.2
Radon emanation and radium distribution
12
2.2.3
Radon emanation and soil moisture
14
2.2.4
Radon emanation, soil porosity and grain size
17
2.2.5
Radon emanation, pore size and number
18
2.2.6
Radon emanation and soil temperature
20
2.2.7
Variations in emanation coefficients for radon isotopes
21
2.3 Radon migration, exhalation and soil gas concentration
22
2.3.1
Introduction
22
2.3.2
Radon exhalation measurements techniques
24
2.3.3
Radon exhalation surveys
24
2.3.4
Radon migration, exhalation, soil gas concentration and soil moisture
29
Radon exhalation, soil gas concentration and atmospheric pressure
31
2.3.6
Radon exhalation, soil gas concentration and temperature
32
2.3.7
Radon exhalation, soil gas concentration and wind speed
33
2.3.8
Radon diffusion theory
33
2.3.9
Radon exhalation temporal variations
36
2.3.5
2.3.10 Radon migration, exhalation and soil gas concentration summary
2.4 Pb-210 deposition
38
39
2.4.1
Introduction
39
2.4.2
Pb-210 depositional rate studies
41
2.4.3
Pb-210 soil studies
45
2.4.4
Pb-210 deposition and geographical location
49
2.4.5
Pb-210 atmospheric concentration studies
50
2.4.6
Pb-210 summary
51
v
2.5 Chapter summary
52
Chapter 3: Project location, site selection and measurement schedules 3.1 Overview 3.2 Exhalation from open ground – Investigation of physical parameters [226Ra activity concentration, distribution in grains, grain size and porosity]
55 55
55
3.2.1
Ranger operations
55
3.2.2
Ranger site selection
60
3.2.3
Ranger measurement schedule
63
3.3 Seasonal and diurnal radon exhalation [moisture, pressure and temperature]
66
3.3.1
Site selection
66
3.3.2
Seasonal site measurement schedule
70
3.3.3
Diurnal measurement schedule
71
210
3.4 Excess Pb soil sampling 3.5 Pb-210 deposition sampling Chapter 4: Methodology 4.1 Overview 4.2 Available techniques for radon exhalation measurements 4.3 Radon exhalation measurement with charcoal canisters 4.3.1
Charcoal canister counting system, calibration & efficiency
4.4 Radon emanometers
72 76
77 77 78 79 82
83
4.4.1
Emanometer calibration
87
4.4.2
Associated emanometer measurements
88
4.5 Soil moisture readings 4.6 Soil activity concentration measurements 4.6.1
Geofizika GS-512 portable gamma detector 226
4.6.2
Determination of
4.6.3
Soil sampling and preparation
4.6.4
Excess
Ra from gamma dose rates
210
Pb analysis of soil samples
4.7 Pb-210 deposition measurement 4.8 HPGe gamma spectroscopic system 4.8.1
Calibration of spectroscopy system for project samples
Chapter 5: Radon sources
89 91 91 93 94 97
97 99 102
107
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5.1 5.2 5.3 5.4 5.5
Overview Rn-222 exhalation rate and 226Ra activity Diurnal measurements of radon exhalation Seasonal measurements of radon exhalation Chapter summary
107 107 128 135 143
Chapter 6: Lead-210 deposition and excess 6.1 Overview 6.2 The 210Pb story 6.3 Pb-210 deposition
145 145 145 147
210
6.3.1
Seasonal
Pb results
147
6.3.2
Annual depositional rate, average values and residency time
153
6.3.3
Pb-210 deposition summary
156
6.4 Pb-210 excess in soil samples
156
6.4.1
Pb-210 inventories
156
6.4.2
Penetration half depth
161
6.4.3
Excess
210
Pb summary
163
6.5 Magela Land Application Area 6.5.1
164
Introduction
164 226
Ra and
210
6.5.2
Uranium-238,
Pb depth profile inventories
6.5.3
Experimental plot inventories
164 168
210
6.5.4
Radium-226 and
Pb distribution
6.5.5
Magela Land Application Area summary
170 172
6.6 Chapter summary
173
Chapter 7: Lead-210 budget 7.1 Introduction 7.2 Hadley circulation 7.3 Local area 210Pb budget 7.3.1 7.3.2 7.3.3 7.3.4 7.3.5
7.4.2
175 175 176
Fate of Ranger 222Rn
177
222
Determination of Rn exhalation rates from and excess 210Pb inventories Determination of excess Determination of exhalation rates Local area
7.4 Regional 7.4.1
175
210
210
210
Pb inventories from
210
Pb deposition 177 210
Pb deposition
Pb deposition and inventories from
Rn 182
210
Pb budget summary
Pb budget
183
184
Kakadu dry season 222Rn emission Kakadu wet season
179
222
210
Pb budget
7.5 Chapter Summary
184 188
191
vii
Chapter 8: Conclusions and future directions 8.1 Project outcomes 8.2 Future directions 8.3 Conclusions
viii
193 193 197 198
List of Figures Figure 1.1: The uranium and actinium natural decay series with radon isotopes highlighted ....................................................................................................3 Figure 1.2: Global population weighted average of human exposure to natural sources of radiation, total 2.4 mSv.y-1 (UNSCEAR 2000) .......................4 Figure 1.3: Alligator Rivers Region, Northern Territory, Australia, curtesy Supervising Scientists Division .................................................................................7 Figure 2.1: Fate of 222Rn nucleus just after 226Ra-222Rn transmutation, R is the recoil range of 222Rn nucleus in solid material. A: Recoil and embedding in same grain. B: Recoil, ejection from grain and stopping in interstitial space C: Recoil, ejection from grain, crossing air gap and embedding in neighbouring grain. D: Recoil, ejection from grain and embedding in neighbouring grain. E: Recoil and stopping in water in the interstitial space. .12 Figure 2.2: On right scanning electron micrograph of monazite (top) and zircon (bottom). On left thorium distribution in the same grain (Holdsworth and Akber 2004) ........................................................................................................14 Figure 2.3: Emanation coefficient as function of increasing water content for sample of till sieved into various grain sizes. (Adapted from Markkanen and Arvela (1992)) ............................................................................................................16 Figure 2.4: The ratio of the saturated emanation coefficient to dry emanation coefficient for increasing moisture (Emission ratio). (Adapted from Sun and Furbish (1995)) ..........................................................................................................16 Figure 2.5: Emanation coefficient for increasing grain size and differences between radium distribution for natural samples. Surface Ra has thickness equal to the recoil range (40 nm). (Adapted from Greeman and Rose (1995))18 Figure 2.6: Radon emanation, migration and exhalation ...................................23 Figure 2.7: Typical 210Pb soil profiles for various soil uses (adapted from Walling et al. (2003)).................................................................................................48 Figure 3.1: Ranger Uranium Mine, numbers indicate approximate sampling locations used for this project..................................................................................57 Figure 3.2: Flow chart of Ranger processing (ERA 2005)..................................58 Figure 3.3: Original Magela Land Application Area (MLAA)...............................64 Figure 3.4: Map of region displaying seasonal sites............................................69 Figure 3.5: Dry season (April-October) wind rose for Jabiru East (data courtesy of Australian Bureau of Meteorology) [26 years averaged]................73 Figure 3.6: Map of Jabiru and Ranger, numbers indicate approximate locations of selected sites for soil samples ...........................................................74 Figure 4.1: Charcoal canister ..................................................................................80 Figure 4.2: Radon emanometer ..............................................................................84 Figure 4.3: Cutter used to accurately place emanometer saucer......................85 Figure 4.4: Schematic of radon/thoron emanometer ...........................................86 Figure 4.5: Set up for emanometer calibration .....................................................88 Figure 4.6: Default calibration curve for Diviner 2000 soil moisture probe ......90 Figure 4.7: Geofizika Brno NaI(Tl) GS-512 gamma spectrometer in use at Rangers waste rock dump .......................................................................................93 Figure 4.8: Base of discs used for soil samples ...................................................96 Figure 4.9: 210Pb deposition collector deployed at Oenpelli ...............................98 Figure 4.10: eriss detector room, Darwin (Photograph by Bruce Ryan).........100
ix
Figure 4.11: Calibration curves and equations for pressed disc soil samples, energy unit is keV....................................................................................................104 Figure 4.12: Efficiency calibration for resin samples .........................................105 Figure 5.1: Plot of 222Rn exhalation rates vs. 226Ra activity concentrations for all sampling sites .....................................................................................................122 Figure 5.2: Ratio of 222Rn exhalation rate to 226Ra activity concentration (RE-R) for locations during dry conditions ........................................................................124 Figure 5.3: Plot of 222Rn exhalation rate vs. 226Ra activity concentration for all sites categorised by geomorphic groups .............................................................127 Figure 5.4: Diurnal variations of atmospheric pressure observed at Jabiru East (Data courtesy of Australian Bureau of Meteorology) ..............................129 Figure 5.5: Diurnal variations in atmospheric and soil temperatures at Jabiru East (Data courtesy of Australian Bureau of Meteorology) ..............................129 Figure 5.6: Normalised 222Rn exhalation rate for all sites vs. time of day of measurement ...........................................................................................................131 Figure 5.7: Normalised 220Rn exhalation rate for all sites vs. time of day of measurement ...........................................................................................................131 Figure 5.8: 222Rn exhalation rate vs. soil temperature.......................................132 Figure 5.9: 220Rn exhalation rate vs. soil temperature.......................................132 Figure 5.10: 222Rn exhalation rate vs. change in atmospheric pressure ........133 Figure 5.11: 220Rn exhalation rate vs. change in atmospheric pressure ........133 Figure 5.12: Seasonal variations of 222Rn exhalation rates and cumulative rainfall ........................................................................................................................136 Figure 5.13: Seasonal variations of 222Rn exhalation rates and cumulative rainfall continued .....................................................................................................137 Figure 5.14: Averaged 222Rn exhalation rates and atmospheric concentrations for sampling periods at Mudginberri..........................................139 Figure 5.15: 2002-2003 wet season moisture profiles for Jabiru East ...........141 Figure 5.16: 2003 dry season soil moisture profiles for Jabiru East ...............141 Figure 5.17: Mirray soil moisture profiles, all readings......................................143 Figure 6.1: Jabiru East 210Pb deposition and cumulative rainfall.....................148 Figure 6.2: Oenpelli 210Pb deposition and cumulative rainfall ..........................149 Figure 6.3: Relationship between excess 210Pb and 40K...................................160 Figure 6.4: Relative cumulative excess 210Pb versus depth for soil scrapes.162 Figure 6.5: Relative cumulative excess 210Pb versus depth for soil cores .....162 Figure 6.6: Inventory depth profile for 2cm sectioned cores from irrigated TM1 and non-irrigated TM2 ...................................................................................165 Figure 6.7: Inventory depth profile for 5cm sectioned cores, irrigated (core 2) and averaged non-irrigated cores.........................................................................165 Figure 6.8: U-238 inventory depth profile for irrigated core TM1.....................166 Figure 6.9: U-238 inventory depth profile for irrigated core 1 and core 2.......166 Figure 6.10: Inventory depth profile for scrape 1 from experimental plot.......169 Figure 6.11: Inventory depth profile for scrape 2 from the experimental plot 169 Figure 6.12: Activity concentration depth profile for core collected by J. Storm 1994 (fine grains,
