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THE FATE OF LEAD IN SOILS CONTAMINATED WITH LEAD SHOT
submitted in partial fulfilment
of the requirements for the Degree of Doctor of Philosophy
by C.P. Rooney
Lincoln University 2002
In memory of my Grandmothers:
Vivian Dunkley (née Canning) Mima Rooney (née Kofoed) 1911 – 2001 1909 – 2001
The future is not completely beyond our control.
It is the work of our own hands.
Robert F. Kennedy
Abstract of a thesis submitted in partial fulfilment of the requirements for the Degree of Ph.D.
THE FATE OF LEAD IN SOILS CONTAMINATED WITH LEAD SHOT ABSTRACT
by C.P. Rooney
A study of the interactions between lead (Pb) shot and the soil, and the fate of Pb at shooting ranges was conducted in order to address the lack of research on these issues at the current time. Surveys of the spatial and vertical distributions of Pb in the soil were carried out at selected Canterbury clay target shooting ranges. These confirmed that maximum soil Pb concentrations at New Zealand ranges are comparable to those reported overseas, which commonly exceed 10,000 mg kg-1, while in New Zealand the guideline limit for Pb in soil is 300 mg kg-1. There was evidence of Pb movement from the contaminated surface-soil zone into underlying soil. While sufficient Pb had been solubilised from the Pb shot to cause substantially elevated soil Pb concentrations, the majority (68-99%) of the total Pb at each site was present as intact Pb shot >2 mm.
An incubation experiment was designed to assess the rate of oxidation of Pb shot and subsequent transfer of Pb to the soil under various environmental conditions. The onset of Pb shot oxidation and subsequent development of the corrosion crust surrounding individual Pb pellets was relatively rapid. Corrosion products in the corrosion crust and precipitated directly onto the soil solid phase readily dissolved over a soil pH range of approximately 4.5-7. Soil solution and fine earth (<1 mm) Pb concentrations became substantially elevated within 6 months, and there was evidence of the development of a quasi-equilibrium within 24 months. Soil pH, moisture content and temperature impacted on these processes.
The data generated by the incubation experiment was used to predict the speciation of Pb in soil solutions using GEOCHEM-PC modelling software. Mechanisms of control of Pb solubility were deduced from equilibrium solubility diagrams constructed with the aid of the speciation calculations. The solubility diagrams were strongly suggestive of Pb solubility regulation by the pH-dependent solubility of corrosion products associated with the corrosion crust or soil solid phase. The dominant mineral controlling Pb solubility appears to be PbCO3. Thus, the presence of Pb shot in the soil alters the control of solubility from
adsorption mechanisms in uncontaminated soil to precipitation-dissolution mechanisms in contaminated soil. High soil pH reduces, but does not prevent, the solubility of Pb shot corrosion products.
A lysimeter leaching experiment, using intact soil cores collected from three shooting ranges, was carried out to assess the potential for Pb mobility. Leachates from contaminated soil contained elevated Pb concentrations which were sustained over multiple leaching events.
Solubility diagrams confirmed that Pb mobilisation was caused by the dissolution of readily soluble Pb shot corrosion products. A large pool of potentially soluble Pb is generated by the readily soluble fine earth Pb and Pb minerals in the corrosion crust. There was a close relationship between leachate Pb concentrations and soluble organic carbon dynamics, indicating the importance of organic complexation on Pb mobility. The results indicate there is a high risk of chronic Pb leaching from contaminated shooting ranges. Natural attenuation by subsoil is expected to delay movement of Pb down the profile, but Pb shot deposition onto young soils with little attenuation capacity could generate substantial movement of Pb.
Management options were considered for soil contaminated with Pb shot. Phosphate addition to Pb-contaminated soil aims to precipitate Pb as relatively insoluble pyromorphite minerals. This study confirmed the effectiveness of phosphate immobilisation of Pb in a preliminary incubation study using fine earth with elevated Pb concentrations. Lysimeters were then collected from three shooting ranges and subjected to leaching following the application of phosphate. Where molar P:Pb ratios were sufficient, substantial reduction in the concentrations of Pb in leachates was observed, confirming that phosphate immobilisation could be used at shooting ranges to reduce Pb mobility. In addition to reducing the mobility of soluble Pb, phosphate addition has the potential to encourage bulk transformations of soluble Pb minerals in the corrosion crust and soil into much less soluble pyromorphite compounds. The use of phosphate immobilisation will be limited by soil vulnerability to phosphorus loss, and environmental sensitivity to the eutrophic effect of phosphorus.
The results of this thesis contribute valuable information on the fate of Pb at CTS ranges.
With this new knowledge, the effectiveness of range and contamination management techniques currently suggested in the literature is discussed.
Lead shot, shooting ranges, soil contamination, lead shot transformation, lead carbonate, lead mobility, phosphate immobilisation, range management.
The data presented in Chapter 4 has been published within:
Initial Investigation into Lead Contamination at Clay Target Clubs and Wetlands in Canterbury.
Lobb, A.J., Rooney, C.P. and Main, M. 1997.
Technical Report R97(6). Canterbury Regional Council, Christchurch.
Selected data presented in Chapter 4 has also been published as a paper:
Distribution of soil Pb contamination at clay target shooting ranges.
Rooney, C.P. and McLaren, R.G. 2001
Australasian Journal of Ecotoxicology, 6(2): 95-102.
I am grateful to the following individuals and for permission to access clay target range sites:
Mr A.S. Chaffey, of Mead; the late Mr O. Osborne, of Leeston; Mr T. Casey, for Inter-Island Horse Transport Ltd.
The cooperation of the following Clay Target Clubs is acknowledged:
Ashburton, Canterbury, Timaru, and Waihora.
Special thanks to my supervisor, Ron McLaren, for his patient guidance and constant advice throughout all phases of my PhD. I am grateful for his superb skill as a supervisor and scientist. The support given during and after illness was particularly appreciated.
Many thanks to my associate supervisor, Leo Condron, for his helpful comments and advice, especially with work for Chapter Seven, and his fantastic editing skills.
Thank you to John Adams, in particular for securing the use of a fantastic room for writing – quiet, scenic views, and a great forum for discussion with fellow writers.
I gratefully appreciate the assistance of:
Members of the department who made useful comments on experimental design after the presentation of my research proposal;
The analytical services team, Roger Cresswell, Leanne Hassall, Jason Breitmeyer and Hayley Barlow – for help with analysis and advice on methods;
Shane Clouston, Matt Ryan and (R.G.) Silva – for maintaining moisture contents in my incubation experiment during times when I was away;
Leanne Hassall, Jamuna Pamidi and Jason Watson – for assistance with analysis;
Shane Clouston, Roger McLenaghan and Andrea Lobb – for assistance with sample collection;
Alistair Campbell – for his time and XRD expertise;
Alan Wise – for developing specialised laboratory equipment;
Trevor Webb and Peter Almond – for U.S. soil classifications;
David Hollander – for lead shot photos in Chapter Five;
Meg Hunter – for help with Geochem data input;
Brett Anderson – for passing on two US Shooting Sports publications;
Steven Brown, Geology Department, Canterbury University – for help with XRD interpretation.
Funding assistance from the following organisations and individuals is recognised and appreciated:
AGMARDT and Lincoln University for scholarships
Ministry for Environment and the Canterbury Regional Council My parents.
Thank you to my many fellow post-grads, for the laughs, chats, support and advice. Special thanks must go to Aziz, and Suman, whose friendship I value greatly.
I would also like to acknowledge and thank all those from whom I have learnt, including my family; friends, especially
James and Meg; and school and academic teachers.
TABLE OF CONTENTS
List of contents vii
List of tables xi
List of figures xiii
List of plates xviii
List of frequently used abbreviations and terms xix Chapter One
1.1 Project rationale 1
1.2 Clay target shooting 5
1.3 Thesis structure 8
General Literature Review
2.1 Release of lead into the environment 10
2.1.1 Background soil lead concentrations 11
2.1.2 Elevation of soil lead concentrations 12
2.1.3 Guidelines for soil protection and risk assessment 13
2.2 Contamination of soil by lead shot 14
2.2.1 Initial transformations of lead shot 16
2.2.2 Secondary transformations of lead shot 19
2.2.3 The effects of lead shot contamination 21
2.3 Forms of soil lead 23
2.3.1 Lead in the soil solution 23
2.3.2 Lead associated with the soil solid phase 26
2.4 Sorption and desorption of lead by soils 30
2.4.1 The role of soil components in sorption and desorption 32
2.4.2 Effect of pH on lead sorption 37
2.5 Summary 38
Chapter Three Methods and materials
3.1 Soil characterisation 39
3.1.1 Soil preparation 39
3.1.2 Soil particle size analysis 39
3.1.3 Soil pH 39
3.1.4 Cation exchange capacity 40
3.1.5 Total carbon 40
3.1.6 Oxalate extractable iron 40
3.2 Soil analyses 41
3.2.1 Total fine earth lead 41
3.2.2 EDTA-extractable fine earth lead 42
3.2.3 Fractionation of fine earth lead 42
3.2.4 Atomic adsorption spectrophotometry (AAS) 44
3.3 Soil solution and leachate analyses 45
3.3.1 pH 45
3.3.2 Soluble lead 45
3.3.3 Soluble carbon 45
3.3.4 Major cations 45
3.3.5 Major anions 45
3.3.6 Aqueous phase speciation modelling 46
3.4 Statistical analysis 47
Soil lead contamination at Canterbury clay target shooting ranges
4.1 Introduction 48
4.2 Methods and materials 52
4.2.1 Study sites 52
4.2.2 Sampling design 54
4.2.3 Sample preparation and analysis 59
4.3 Results and discussion 62
4.3.1 Spatial distribution of lead (fine earth fraction) 62
4.3.2 Lead removed by 2 mm sieve 70
4.3.3 Lead in the soil profile 71
4.3.4 General discussion 74
4.4 Conclusions 75
Oxidation rate of lead shot
5.1 Introduction 76
5.2 Methods and materials 78
5.2.1 Experimental design 78
5.2.2 Experimental procedure and analysis 80
5.2.3 Analysis 82
5.3 Results and discussion 85
5.3.1 Development of crust material 85
5.3.2 Analytical precision 92
5.3.3 Effect of soil pH 93
5.3.4 Effect of moisture content 108
5.3.5 Effect of temperature 118
5.3.6 Effect of lead loading 128
5.3.7 Partitioning of Pb after 24 months incubation 133
5.4 Conclusions 137
Mobility of lead at clay target shooting ranges
6.1 Introduction 138
6.2 Methods and materials 141
6.2.1 Study soils 141
6.2.2 Lysimeter preparation 142
6.2.3 Experimental procedure and analysis 144
6.3 Results and discussion 145
6.3.1 Motukarara lysimeters 145
6.3.2 Lismore lysimeters 155
6.3.3 Waimakariri lysimeters 163
6.3.4 General discussion 171
6.4 Conclusions 178
Management of lead mobility
7.1 Introduction 179
7.1.1 Brief outline of in situ stabilisation technology 181 7.2 Phosphate immobilisation: theoretical background 184 7.2.1 Interactions of lead and phosphorus in aqueous systems 184
7.2.2 Pyromorphite formation in soil 186
7.2.3 Mechanisms controlling lead-phosphorus reaction 188 7.2.4 Use of phosphorus immobilisation technology in the field 192 7.2.5 Potential for phosphorus immobilisation of lead at shooting ranges 194
7.2.6 Summary 194
7.3 Part I: Effect of phosphorus addition on lead solubility 195
7.3.1 Objectives 195
7.3.2 Materials and methods 195
7.3.3 Results and discussion 197
7.3.4 Conclusions 201
7.4 Part II: Effect of phosphorus application on lead mobility in undisturbed
soil cores from contaminated shooting ranges 202
7.4.1 Objectives 202
7.4.2 Materials and methods 202
7.4.3 Results and discussion 205
Motukarara soil 208
Lismore and Waimakariri soils 216
Practical aspects of effective field application of phosphorus amendments 225 Potential adverse effects of phosphorus amendments 227
7.4.4 Conclusions 229
Chapter Eight General discussion
8.1 Introduction 230
8.2 General summary 230
8.3 Implications for New Zealand ranges 234
8.3.1 Canterbury clay target shooting ranges 238
8.4 Implications for current range management guidelines 242 8.4.1 Management practices requiring reconsideration 243 8.4.2 Management practices with continued potential effectiveness 246 8.4.3 Recommendations for general management practices at
New Zealand clay target shooting ranges 249
8.5 Alternatives to lead shot 250
8.6 Conclusions 251
8.7 Recommendations for future research 253
Appendix A: Personal Communications 272
Appendix B: Preliminary pH adjustment study for Chapter 5 274 Appendix C: Development of crust material on Pb shot removed from
Temuka soil 275
Appendix D: pH of Pb sorption equilibrium solutions 276 Appendix E: Data used to construct PbX solubility plots 277 Appendix F: Inputs into GEOCHEM-PC modelling software 278 CD-ROM GEOCHEM-PC software
LIST OF TABLES
Table Short title Page
1.1 Minimum number of active clay target shooting sites in New Zealand. 3
2.1 Examples of anthropogenic uses of Pb. 10
2.2 Annual Pb input from various sources to agricultural land in England and Wales. 13 2.3 Summary of the concentration of Pb and density of Pb shot at shooting ranges
as reported by various authors. 15
2.4 Content of Pb in important Pb minerals by stoichiometry. 17 2.5 Stability constants for reactions of Pb2+ in the soil solution. 24
3.1 Comparison of Pb concentrations in reference soils determined using USEPA
method 3051 with published values. 41
3.2 Summary of fine earth Pb fractionation procedure. 42
3.3 Organic acids and their reference concentrations in the mixture model. 46
4.1 Details of clay target shooting ranges in Canterbury. 50
4.2 Soil pH and carbon at the four study sites. 59
4.3 Maximum concentration of Pb shot removed from samples at each site. 70
4.4 Summary of data obtained from soil profile sampling. 71
5.1 Summary of variables in the four components of the incubation experiment for
each of the two soils. 78
5.2 Selected physical and chemical characteristics of the uncontaminated soils. 80 5.3 Relationship between Kd and solution pH after 24 months incubation at field
capacity and 25oC. 105
5.4 Relationship between Kd and solution pH after 24 months incubation at 70% of
field capacity and 25oC. 113
5.5 Relationship between Kd and temperature after 24 months incubation at field
6.1 Selected physical and chemical characteristics of the study soils. 142 6.2 Average bulk density, porosity and pore volume measured in representative
6.3 Amount of Pb contained in the Motukarara lysimeters and Pb removed during
6.4 Amount of Pb contained in the Lismore lysimeters and Pb removed during
6.5 Amount of Pb contained in the Waimakariri lysimeters and Pb removed during
6.6 Water quality guidelines for Pb. 171 6.7 Rate of Pb removal by percolating water, and estimation by extrapolation of the
rate of Pb migration from contaminated surface soil expected in the field. 175
7.1 Solubility products of some geologically important Pb minerals. 184 7.2 Reduction in exchangeable + carbonate Pb in soil by the formation of
pyromorphite minerals. 189
7.3 Impact of pH on the formation of pyromorphite minerals in contaminated soil. 189 7.4 Phosphorus addition required for molar P:Pb ratios of 3:5 to 30:5 in the three
soils studied. 196
7.5 Mean soil pH of fine earth samples incubated with diammonium phosphate. 199 7.6 PO4-P extracted by Ca(NO3)2 after the incubation period as a percentage of
added P. 200
7.7 Motukarara lysimeters: Fine earth Pb concentrations and phosphate treatment
7.8 Lismore lysimeters: Fine earth Pb concentrations and phosphate treatment
7.9 Waimakariri lysimeters: Fine earth Pb concentrations and phosphate treatment
8.1 Estimated time for complete decomposition of the Pb shot contained in the
lysimeters used in the study described in Chapter Six. 232 8.2 Climatic and soil parameters expected to accelerate the rate of release of Pb into
the environment, related to geographic areas of New Zealand. 235 8.3 Climatic and soil parameters expected to reduce the rate of release of Pb into
the environment, related to geographic areas of New Zealand. 236
LIST OF FIGURES
Figure Short title Page
1.1 Diagrammatic layout of a skeet shooting field. 6
1.2 Overall thesis structure (Fate of Lead in Soils Contaminated with Lead Shot). 9
2.1 Typical concentrations of Pb in soil contaminated according to land use. 12 2.2 The distribution of Pb in shooting range soils between pellets, corrosion crust
and Pb in soil (from Jørgensen and Willems, 1987b). 20 2.3 Speciation diagram for Pb hydroxy species in terms of pH. 24 2.4 Distribution of Pb in the presence of excess humic acids. 25 2.5 Possible reaction mechanisms which can affect the Pb2+ concentration in the
soil solution. 27
2.6 Stability relations among Pb compounds in water at 25oC. 28
4.1 Location of clay target clubs in Canterbury. 49
4.2 Site plan of Ashburton CTC showing sampling positions. 55 4.3 Site plan of Canterbury CTC (McLeans Island) showing sampling positions. 56 4.4 Site plan of Timaru CTC showing sampling positions. 57 4.5 Site plan of Waihora CTC showing sampling positions. 58 4.6 Comparison of EDTA-Pb and total Pb concentrations in the fine earth
4.7 Total fine earth Pb concentrations at the Ashburton CTC. 63 4.8 Total fine earth Pb concentrations at the Canterbury CTC (McLeans Island). 64 4.9 Total fine earth Pb concentration at the Timaru CTC. 65 4.10 Total fine earth Pb concentrations at the Waihora CTC. 66 4.11 Total fine earth Pb concentrations at each CTC on a transect perpendicular to
the line of traps in front of the most frequently used trap. 67 4.12 Spatial distribution of EDTA-extractable fine earth Pb at Ellesmere CTC. 68 4.13 Total fine earth Pb concentrations in the soil profile at each of the four sites. 72
5.1 Experimental design for incubation experiment. 79
5.2 Centrifugal apparatus for removal of soil solution from moist soil. 82
5.3 Stability relations among Pb compounds in water at 25oC. 87
5.4 Effect of solution pH on soil solution Pb concentrations - field capacity
moisture content and 25oC. 95
5.5 Solution pH data – field capacity moisture content and 25oC. 97 5.6 Effect of pH on fine earth Pb concentrations - field capacity moisture content
and 25oC. 98
5.7 Effect of pH on the proportion of fine earth Pb in nominal soil fractions - field
capacity moisture content and 25oC. 99
5.8 The solubility of important Pb minerals. 100
5.9 Waimakariri soil solution data plotted with the solubility of PbCO3. 102 5.10 Temuka soil solution data plotted with the solubility of PbCO3. 103 5.11 Diagrammatic representation of the effect of soil pH on soil Pb sorption. 104
5.12 Diagrammatic representation of the relative importance of corrosion compound solubility and soil Pb sorption capacity on equilibrium solution
concentration control. 106
5.13 Effect of soil pH on x-ray diffraction patterns of corrosion crust material - field
capacity moisture content and 25oC. 107
5.14 Effect of moisture content on soil solution Pb concentrations in soil incubated
with Pb shot at 25oC. 111
5.15 Effect of moisture content on fine earth Pb in soil incubated with Pb shot at
5.16 Effect of moisture content on the proportion of fine earth Pb in nominal
fractions of Waimakariri soil. 114
5.17 Effect of moisture content on the proportion of fine earth Pb in nominal
fractions of Temuka soil. 115
5.18 Soil solution data for each pH treatment at low moisture content plotted with
the solubility of PbCO3. 117
5.19 Henry’s Law constant, KH, for the solubility of CO2 in water as a function of
5.20 Effect of temperature on soil solution Pb concentrations in soil incubated with
Pb shot. 120
5.21 Effect of temperature on total fine earth Pb concentrations in soil incubated
with Pb shot. 121
5.22 Solution pH data – soil incubated with Pb shot at 10-30oC. 123
5.23 Effect of temperature on the proportion of fine earth Pb in individual soil
5.24 Soil solution data for both soils from samples incubated at field capacity moisture content for each temperature treatment, plotted with the solubility of
PbCO3 at 25oC. 126
5.25 Effect of temperature on x-ray diffraction patterns of corrosion crust material. 127 5.26 Effect of Pb loading on soil solution Pb concentrations in soil incubated with
Pb shot. 129
5.27 Solution pH data - 20% Pb shot. 129
5.28 Fine earth Pb concentrations during experimental period for 20% treatment. 130 5.29 The proportion of fine earth Pb in nominal soil fractions – 20% Pb shot. 131 5.30 Soil solution data for both soils plotted separately with the solubility of PbCO3
(shaded area). 132
5.31 Effect of pH on the partitioning of Pb between pellet, corrosion crust and soil. 134 5.32 Effect of temperature on the partitioning of Pb between pellet, corrosion crust
and soil. 135
5.33 Effect of Pb loading on the partitioning of Pb between pellet, corrosion crust and soil.
6.1 Cross-section of the lysimeter core prepared for the leaching experiment. 143 6.2 Fine earth Pb concentrations in representative profiles of the Motukarara soil. 145 6.3 Pb concentrations and pH of leachates from ‘background’ Motukarara
lysimeter in which Pb was detectable. 147
6.4 Pb concentrations and pH of leachates from contaminated Motukarara
6.5 Partitioning of Pb in contaminated Motukarara soil. 149 6.6 GEOCHEM-PC model predictions for Pb speciation in selected leachates
from Contaminated Motukarara lysimeter 2.
6.7 Leachate-Pb data for Motukarara lysimeters plotted with the solubility of
important Pb minerals. 151
6.8 Soluble organic carbon concentration) in selected leachates from contaminated
Motukarara lysimeters. 153
6.9 Fine earth Pb concentrations in representative profiles of the Lismore soil. 155 6.10 Pb concentrations and pH of leachates from contaminated Lismore lysimeters. 158
6.11 Partitioning of Pb in contaminated Lismore soil. 157
6.12 GEOCHEM-PC model predictions for Pb speciation in selected leachates
from Contaminated Lismore lysimeter 2. 160
6.13 Soluble organic carbon concentration in selected leachates from contaminated
Lismore lysimeters. 159
6.14 Leachate-Pb data for Lismore lysimeters plotted with the solubility of
important Pb minerals. 162
6.15 Fine earth Pb concentrations in representative profiles of the Waimakariri soil. 163 6.16 Pb concentrations and pH of leachate from background Waimakariri lysimeters
in which Pb was detectable 165
6.17 Pb concentrations and pH of leachates from contaminated Waimakariri
6.18 Partitioning of Pb in contaminated Waimakariri soil. 167 6.19 GEOCHEM-PC model predictions for Pb speciation in selected leachates
from Contaminated Waimakariri lysimeter 2. 168
6.20 Soluble organic carbon concentration in selected leachates from contaminated
Waimakariri lysimeters. 169
6.21 Leachate-Pb data for Waimakariri lysimeters plotted with the solubility of
important Pb minerals. 170
6.22 Lead sorption isotherms for the three soils studied. 174
7.1 Effect of P addition on Ca(NO3)2-extractable Pb in Motukarara fine earth. 198 7.2 Effect of P addition on Ca(NO3)2-extractable Pb in Lismore fine earth. 198 7.3 Effect of P addition on Ca(NO3)2-extractable Pb in Waimakariri fine earth. 199
7.4 Pb concentrations of leachate fractions (Ci) from Motukarara lysimeters as a proportion of the initial leachate concentration (C) for respective lysimeters.
7.5 Leachate-PbTotal data for Motukarara lysimeters plotted with the solubility of
PbCO3 and Pb5(PO4)3(OH). 210
7.6 Leachate pH and PO4-P concentrations for treated and control
Motukarara lysimeters 211
7.7 Fractionation of Motukarara lysimeters: SSP-treated lysimeters compared with
mean of the control lysimeters. 212
7.8 PO4-P concentration of leachate from Motukarara SSP-treated lysimeters
relative to P addition to respective lysimeters. 213
7.9 Sorption of P in the fine earth fraction of the three soils studied. 214 7.10 Pb concentrations of leachate fractions (Ci) from Lismore lysimeters as a
proportion of the initial leachate concentration (C) for respective lysimeters. 217 7.11 Pb concentrations of leachate fractions (Ci) from Waimakariri lysimeters as a
proportion of the initial leachate concentration (C) for respective lysimeters 217 7.12 Leachate-PbTotal data for Lismore lysimeters plotted with the solubility of
PbCO3 and Pb5(PO4)3(OH). 218
7.13 Leachate-PbTotal data for Waimakariri lysimeters plotted with the solubility of
PbCO3 and Pb5(PO4)3(OH). 219
7.14 Leachate pH and PO4-P concentrations for treated and control Lismore
7.15 Leachate pH and PO4-P concentrations for treated and control Waimakariri
7.16 Fractionation of fine earth Pb in Lismore SSP Lysimeter 1 compared with
mean of the Lismore control lysimeters. 222
LIST OF PLATES
Plate Short title Page
1.1 Pb shot deposition on the soil surface of the Motukarara CTC. 2
1.2 Layout of a trap shooting field. 6
5.1 Development of crust material on Pb shot incubated in soil over 24 months at
field capacity moisture content and 25oC. 86
5.2 Effect of pH on Pb shot corrosion crusts after 24 months incubation. 94 5.3 Effect of moisture content on Pb shot corrosion crusts after 24 months
5.4 Effect of temperature on crust development on Pb shot after 24 months
LIST OF FREQUENTLY USED ABBREVIATIONS AND TERMS
BTC(s) Breakthrough curve(s)
CTC(s) Clay Target Club(s)
CTS Clay Target Shooting
SOC Soluble organic carbon
Fine earth Soil <1mm (Pb shot removed)
Corrosion products (Pb shot) corrosion compounds associated with the corrosion crust or precipitated directly onto the soil solid phase.
Chapter One General Introduction
1.1 PROJECT RATIONALE
Lead (Pb) has been used frequently by humans from early times to the present day. The beneficial physical properties of the metal have promoted its mining, smelting and various uses, including weather-proofing of buildings, water pipelines, cooking vessels and as an additive to wine. More recent uses include ammunition, pigments for paints, lead-acid batteries and petrol additives. This anthropogenic use of Pb has led to recurring environmental contamination in developing and industrialised areas of the world.
Contamination of soil with Pb from the main recognised sources is well documented: mining and smelting, automotive emissions, Pb-based paints, and industrial activity. Recognition as an environmental contaminant has led to the removal of Pb from paint products and petrol.
Accumulation of Pb in surface soils impacts on environmental health and can affect food quality and human health. The diversity of the biological population in soils may be affected (Kabata-Pendias and Pendias, 1992), and biochemical processes including soil organic matter breakdown (Kabata-Pendias and Pendias, 1992) and nutrient cycling (Davies, 1990; Lorenz et al., 1992; Marzadori et al., 1996) have been shown to be influenced by high soil Pb concentrations. Evidence of the impact of elevated soil Pb concentrations on plant growth is inconsistent, but the contribution of plant-Pb to elevated levels of Pb in the food chain is significant. Movement of Pb from contaminated sites may occur via dust, lateral and vertical drainage into surface- and ground-water, plant uptake and uptake by animals and migratory birds (Castrale, 1989). Pica (soil ingestion) is a common cause of Pb poisoning in children.
Lead accumulates in the bones and kidneys of humans and other animals, and is carried in red blood cells (Howard, 1985). Poisoning can impact on the nervous system, affecting development and behaviour.
Relatively limited attention has been given to the contamination of soil by Pb shot deposition onto land. Clay target shooting (CTS) is a popular recreational and sporting activity around the world and Pb is the material of choice for shotgun pellets due to ballistic qualities, availability and cost (Soulsby, 1998). It has become apparent that shooting ranges in dryland environments may be highly contaminated with Pb. Soil containing elevated levels of Pb have been reported at shooting ranges in Scandinavia (Jørgensen and Willems, 1987b;
Tanskanen et al., 1991; Manninen and Tanskanen, 1993), England (Mellor and McCartney, 1994; Merrington and Alloway, 1995), Germany (Fahrenhorst and Renger, 1990; Braun et al., 1997), and the United States of America (Murray et al., 1997; Edwards et al., 1999). At CTS sites there is a definable area in which spent Pb shot is deposited, and the Pb shot is readily visible on the soil surface (Plate 1.1). Exposed to soil and atmospheric chemical processes, the Pb shot develops a crust of oxidised Pb compounds which interact with the soil, causing soil Pb concentrations to become elevated.
Plate 1.1. Lead shot deposition on the soil surface of the Motukarara CTC.
The New Zealand Clay Target Association lists 91 clay target shooting clubs in New Zealand (New Zealand Clay Target Association Incorporated, 2001). Clay target shooting is also offered as a recreational activity to clients at various luxury accommodation lodges around New Zealand. Table 1.1 summarises information available on the number of ranges in the country. In addition, a number of businesses throughout the country include CTS in commercial adventure tourism activities. The Moving Target franchises operate with mobile trap units that may be set up anywhere with suitable space.
Table 1.1 Minimum number of active clay target shooting sites in New Zealand with shooting ranges for various shooting disciplines (see Section 1.2 for discipline descriptions).
Number of clay target shooting clubs 91
Number of individual trap ranges 228
Number of individual skeet ranges 116 Number of individual sporting fields 19
Luxury lodges offering on-site clay target shooting 6 (Sources: Lobb et al., 1997; New Zealand Clay Target Association Incorporated, 2001;
Trips and Travel Limited, 2002)
The Canterbury Regional Council has identified 18 sites in Canterbury which are, or have been, used by CTS clubs for varying durations (Lobb et al., 1997). Many sites are significantly contaminated with Pb. Reports of live pigeon shooting matches in issues of The Press circa 1870 confirm the long history of pigeon and clay target shooting in Canterbury. In this region and throughout New Zealand, many clay target shooting sites are located on, and/or shoot onto, land used for agricultural production. This appears to be a continuation of historical practice, where paddocks were offered as sites to hold pigeon shooting matches (News of the Day, 1876). Decades of shooting at the present sites has led to accumulation of Pb shot, and subsequent elevation of soil Pb concentrations on or near the soil surface. The high loadings of Pb pose a risk to environmental health, especially if there is potential for movement of Pb from the sites, however little is known of the fate of the Pb at CTS ranges.
The effect Pb shot accumulation on agricultural land is illustrated by elevated plant Pb concentrations measured in plants growing at CTS ranges (Manninen and Tanskanen, 1993;
Mellor and McCartney, 1994) and growing in soil collected from a CTS range (Rooney et al., 1999). There have been cases of cattle poisoned by exposure to soil-Pb or Pb shot derived from shooting activities in Ireland (Rice et al., 1987), Germany (Braun et al., 1997) and New Zealand (The Dominion, 1978; G.B. Daivs, personal communication, 22 August 1997).
Spent Pb shot has also caused poisoning of mourning doves (upland game birds) feeding in fields used for shooting in the US (Castrale, 1989), and elevated tissue Pb concentrations in small mammals at a CTS site in the US (Stansley and Roscoe, 1996).
The high loadings of Pb present at clay target shooting sites pose a risk to environmental health and warrant further investigation, as little is known of the fate of Pb at such sites. The objectives of this experimental study were:
• To improve the understanding of the behaviour of Pb in soils;
• To quantify the rate of transformation of metallic Pb shot into ‘soil’ Pb compounds under New Zealand conditions;
• To use a fractionation technique to identify the forms in which transformed Pb exists;
• To establish the potential for leaching of Pb in relation to soil properties;
• To provide information for the development of possible management or remedial strategies for contaminated sites.
1.2 CLAY TARGET SHOOTING
Clay target shooting is an outdoor recreational and competitive sport which involves participants firing shotguns containing cartridges of spherical pellets of Pb to break moving targets launched into the air. It is different to the sport of target shooting, which involves the use of rifles and pistols to shoot at stationary, bulls-eye type targets. Clay target shooting was developed from live-bird shooting that was popular in late 18th century England and subsequently in North America, where scarcity of live pigeons and protests about the shooting of live birds prompted the creation artificial targets made from kiln-fired clay.
Modern clay targets are made from a mixture of lime and pitch (Baer et al., 1995), and the modern-day trap is a spring-loaded throwing arm which can throw targets for distances of up to 135 m. In the past, each ammunition cartridge contained a maximum of 1⅛ ounces (32 g) of spherical Pb pellets, or shot. This weight has now been reduced to 1 ounce (28 g).
Clay target shooting involves many variations of the sport in the way that targets are presented to the shooters, such as changes in the height and speed of the target, the direction of flight, and the location of stations where shooters stand. The more common disciplines are Trap, Skeet and Sporting Clays.
Trap shooting, also referred to as ‘down the line’ shooting (DTL), involves targets launched from a single traphouse within a horizontal spread of approximately 90o (Lobb et al., 1997).
A shooter successively shoots at the launched targets from different positions in five lanes positioned in an arc of about 40o behind the traphouse (Plate 1.2). Generally a minimum of 25 targets are launched in a shooter’s round. A second shot is usually fired at a target that is not broken with the first shot. The targets used commonly measure 11 cm in diameter and weigh about 100g, and the size of Pb shot is usually ‘No. 7.5’ (2.4 mm diameter). The Pb shot appears to be deposited directly in front of the trap for a distance of some 250 m (Rooney et al., 1999).
Plate 1.2. Layout of a trap shooting field; shot-fall area is over the fence, in the middle-distance
Skeet shooting, also referred to as ‘across the line’ shooting, involves shooting two clay targets launched from two separate traps in towers located about 40 m apart. The targets are released alternately or simultaneously along intersecting flight paths and shooters stand in a series of 8 shooting stations (Figure 1.1). A smaller shot size (‘No. 8’ to ‘No. 9’; 2.3 to 2.0 mm diameter) is used for skeet shooting so that there is a more dense pattern of shot dispersal in flight. There is some evidence that the Pb shot may be deposited mainly in two areas approximately 80 m beyond the skeet towers in line with the intersecting trajectories of the targets (Rooney et al., 1999).
Shooting stations Approx. 40 m
Figure 1.1. Diagrammatic layout of a skeet shooting field.
Sporting Clays is a relatively new discipline which simulates actual field hunting by combining different target flight speeds and angles and different target sizes. The targets might be crossing, climbing, incoming, outgoing, streaking high overhead, flying low, or any combination of the above. The area of Pb shot deposition from Sporting Clays is less well- defined and a predictable pattern of deposition is unlikely due to the use of mobile traps and target flight variations.
1.3 THESIS STRUCTURE
The thesis structure is summarised in Figure 1.2.
Chapter 2 presents a review of literature pertinent to Pb in soil and the current research published on Pb shot contamination of soil.
Chapter 3 details general analytical methods used throughout the experiments.
Chapter 4 describes a survey of Pb contamination at selected Canterbury clay target clubs.
Chapter 5 is concerned with the transformation rate of Pb shot in contact with soil.
Chapter 6 determines the potential for leaching of Pb from contaminated soils at CTS ranges.
Chapter 7 assesses the use of amendments for the mitigation of the effects of Pb at shooting ranges, firstly in a review of literature pertaining to the topic of phosphate amendment of Pb-contaminated soil, followed by experimental work.
Chapter 8 contains a discussion of the implications for CTS ranges and range management of the information gained from the experimental work. Issues requiring future research are identified.
Literature review Chapter 2 Thesis summary, general discussion, conclusions, and recommendations Chapter 8
Effect of Pb shot deposition on the distribution of elevated soil Pb concentrations at CTS ranges Chapter 4 Potential for Pb mobility Chapter 6
Effects of elevated soil Pb concentrations Management strategies to reduce Pb mobility Chapter 7
Nature and rate of transfer of Pb from pellets to soil Chapter 5 Figure 1.2 Overall thesis structure (Fate of Lead in Soils Contaminated with Lead Shot)
General Literature Review
2.1 RELEASE OF LEAD INTO THE ENVIRONMENT
The present amount of Pb in the environment is a result of release by natural processes and a long history of anthropogenic use of Pb. Lead is released naturally into the environment by way of weathering rocks, ash from bush fires, wind-borne soil, volcanic activity, and sea-spray (Markus and McBratney, 2000), however natural sources supply approximately only 1% of the present-day atmospheric Pb burden (Royal Society of New Zealand, 1986).
Anthropogenic use of the metal for more than 5000 yrs (Table 2.1) has caused substantial change to the natural global biogeochemical cycle of Pb, and has lead to widespread pollution of the atmosphere, soils, water bodies and sediments. The sources of anthropogenic Pb most commonly recognised to have contributed to this pollution are primary metal production (mining and smelting), industrial sites where Pb is processed or products containing Pb are synthesised, automobile exhaust emissions, Pb pigments in house-paints, and horticultural pesticides.
Table 2.1. Examples of anthropogenic uses of Pb
Ancient uses Water pipes and ducting Writing slates and pencils Lining cooking vessels Sinkers and anchors
Weather-proofing buildings Coinage
Medicinal and cosmetic uses Wine additives Shot for sling-shots used in ancient warfare
Subsequent Ammunitions Pb-based bearings
and/or present uses Paint pigments Petrol additive Pesticides, bird repellents Pb-acid batteries Colouring pottery and glassware
Telephone and power cable sheathing
Metal alloys, for example, bronze and brass, Pb-solder (Sources: Burns, 1948; Adriano, 1986; Markus and McBratney, 2000).
The current global rate of production of Pb is approximately 5 × 107 t year-1 (Markus and McBratney, 2000). Calculations and estimates of maximum anthropogenic Pb emissions have been reducing since the 1980’s, for example, 3.76 × 105 t year-1 in 1983 (Nriagu and Pacyna, 1988) and 2.09 × 105 t year-1 in 1989 (Pacyna et al, 1995 in Markus and McBratney, 2000).
Approximately 70% of the 1989 value is derived from the use of leaded petrol, and therefore Pb emissions will continue to reduce as further countries decrease the use of leaded petrol.
2.1.1 Background soil lead concentrations
Much has been published on the background concentrations of Pb in soil. A range of values are given for the background concentration of Pb in surface soils, as they are influenced by local, regional and global anthropogenic contributions as well as naturally occurring sources of Pb from parent materials.
The range of mean Pb concentrations in rocks is approximately 3-24 mg kg-1 for igneous rocks and 6 to 30 mg kg-1 for sedimentary rocks (Markus and McBratney, 2000). Similar ionic radii leads to Pb substituting isomorphically for potassium (K), barium (Ba) and calcium (Ca) in minerals, so that Pb is commonly found in feldspars, micas, phosphate minerals and plumbogummite minerals (Nriagu, 1978; McLaughlin et al., 1996).
Pristine soils probably no longer exist with respect to Pb, therefore the lowest background Pb concentrations are now found in ‘remote’ soils – soils some distance from anthropogenic sources of Pb and therefore under only slight influence from such inputs. Lead concentrations quoted for remote soils are generally in the range 10-30 mg kg-1 (Davies, 1990). The widespread low-level Pb pollution from anthropogenic activities has resulted in
‘normal’ background soil Pb concentrations in the range 10-100 mg kg-1 (Davies, 1990;
McLaughlin et al., 1996; Breckenridge and Crockett, 1998; Markus and McBratney, 2000).
Naturally elevated soil Pb concentrations up to 45,000 mg kg-1 can exist in areas containing Pb-bearing ore deposits (Zimdahl and Arvik, 1973; Royal Society of New Zealand, 1986).
In New Zealand, rural soils may be considered to be ‘remote’ soils, with background concentrations of Pb only slightly elevated by anthropogenic sources. Nriagu (1978) lists the mean soil Pb concentration in New Zealand as 16 mg kg-1. Background soil Pb concentrations of approximately 20 mg kg-1 have been measured in the Te Aroha area unaffected by ore deposits (Ward et al., 1977a). Rooney et al. (1999) determined background Pb concentration in the Leeston area to be <10 mg kg-1.
2.1.2 Elevation of soil lead concentrations
Soil is a major sink for Pb released into the environment. Approximately 60-70% of annual global atmospheric Pb emissions are estimated to be deposited onto soil (Nriagu and Pacyna, 1988; Nriagu, 1989; McGrath, 1997 in Markus and McBratney, 2000). Historic or current inputs of Pb to soil have occurred from a variety of sources including mining and smelting, the use of petrol containing Pb additives, industrial activities, paint additives, and the application of biosolids, fertilisers and agrochemicals. There is a large amount of literature covering Pb pollution from various individual sources which will not be covered in this review. Typical soil Pb concentrations that result from pollution are summarised in Figure 2.1. The divisions are somewhat arbitrary, but the abundance of literature covering instances of soil Pb contamination indicates the concentrations quoted in Figure 2.1 are common.
0 30 200 2000 20000 +
Remote Urban soil Industrial contamination
Figure 2.1. Typical concentrations of Pb in soil contaminated according to land use (adapted from Markus and McBratney, 2000).
Reductions in the use of leaded petrol have caused some reduction in atmospheric Pb deposition onto soil in recent decades. However, calculations by Nicholson et al. (1998) show that atmospheric deposition continues to be a significant source of Pb input into agricultural land in England and Wales (Table 2.2). Hird et al. (1996, in Nicholson et al., 1998) found that the average rate of atmospheric deposition for Pb at 29 rural sites in England and Wales was 33 g ha-1 yr-1. Alloway (1999) obtained a mean atmospheric deposition rate of 42.2 g Pb ha-1 yr-1 for 34 remote sites in England and Wales. A further site downwind from a large lead-zinc smelter recorded Pb deposition greater than those at the remote locations by a factor of 35. Literature compiled from European and North American studies by Bergkvist et al. (1989) showed that deposition rates over 100 g Pb ha-1 yr-1 have commonly been recorded.
Gray et al. (2001) recorded a mean atmospheric Pb deposition rate of 19.7 g ha-1 during a 12 month monitoring period at 7 rural New Zealand sites.
Table 2.2. Annual Pb input (t) from various sources to agricultural land in England and Wales (from Nicholson et al., 1998).
Source Annual Pb Input
Atmospheric deposition 365
Sewage sludge 95
Animal manures 52
Fertilisers and lime 13
Irrigation water <1
Industrial by-product wastes <1
2.1.3 Guidelines for soil protection and risk assessment
Regulatory limits and guidelines for Pb in soil are ubiquitous. The Dutch soil protection guidelines, first drawn up in 1983, are well recognised. These have been refined a number of times, and now take into account the amount of clay and organic matter in soils. The current values that relate to soil Pb contamination are: a reference/target value of 85 mg Pb kg-1 soil (for soil with 25% clay and 10% organic matter), 290 mg kg-1 for ecotoxicity intervention, and 300 mg kg-1 for intervention where risk is posed to human health (Smit 1998, in McLaughlin et al., 2000). Further refining and the consideration of soil type and land use of agricultural soils has lead to the desirable limit for Pb in agricultural soil being 150-200 mg kg-1 (Smit 1998, in McLaughlin et al., 2000).
The New Zealand Ministry for the Environment adopted the guidelines for the assessment and management of contaminated sites developed by the Australian and New Zealand Environment and Conservation Council and the National Health and Medical Research Council (ANZECC/NHMRC, 1992). These guidelines are largely derived from the Dutch regulations (Moen 1988, in McLaughlin et al., 2000), and are based on threshold levels for phytotoxicity and unacceptable residue levels. The guideline level established by ANZECC for Pb in soil is 300 mg Pb kg-1 soil. This defines the concentration of soil Pb at which further investigation of a contaminated site is recommended. Any further action such as remediation, or restrictions on use or access is to be determined after considering site-specific data including land use, and soil and contaminant characteristics.
2.2 CONTAMINATION OF SOIL BY LEAD SHOT
The risks of Pb shot deposition in wetland environments (waterfowl Pb toxicosis, elevated sediment Pb concentrations) have been recognised for some decades. It has also become apparent that the deposition of Pb shot at shooting ranges in dryland environments routinely leads to substantial soil Pb contamination. Elevated soil Pb concentrations have been reported in every study of shooting ranges that has analysed soil for Pb (Table 2.3).
Jørgensen and Willems (1987b) and Rooney et al. (1999) have shown that a relatively small proportion of the Pb shot at shooting ranges has oxidised and become associated with the soil solid phase. This indicates that the substantially elevated soil Pb concentrations, commonly >10,000 mg Pb kg-1, reported at shooting ranges (Table 2.4) are only the tip of the iceberg. Murray et al. (1997) and Rooney et al. (1999) have demonstrated that there is a definable area where elevated Pb concentrations occur as a result of Pb shot deposition at CTS ranges.
Mellor and McCartney (1994) calculated that approximately 6000 t of lead shot was deposited on the soil surface by CTS activities in the United Kingdom (UK). Based on information for Canterbury CTS ranges (Lobb et al., 1997), a conservative estimate of the proportion of UK CTS ranges situated on agricultural land would be 50%. Even at this level, input of lead shot far out-weighs all other sources of Pb quantified by Nicholson et al. (1998; Table 2.2). This is also likely to be the case in New Zealand, where annual deposition rates of Pb shot at individual Canterbury CTS ranges were estimated to be between 0.2 and 9.8 t yr-1 (Lobb et al., 1997).
New Pb pellets deposited onto the soil commonly contain approximately 97% metallic Pb, 2% antimony (Sb) and <2% arsenic (As), and sometimes contain <0.5% nickel (Ni) (Jørgensen and Willems, 1987b; Tanskanen et al., 1991). Lead shot with a mottled appearance, caused by light grey, white, and brown coatings, is invariably reported at shooting ranges (Jørgensen and Willems, 1987b; Lin et al., 1995; Uren et al., 1995; Murray et al., 1997).
These coatings effectively form a ‘weathering crust' around the pellets, and have been referred to as transformation products or crust material. Lead in soil samples collected from shooting ranges is found as metallic Pb in the pellets, as Pb minerals in the crust covering the pellets, and as extractable Pb associated with the soil solid phase. There are essentially two stages in the transformation of Pb shot into soil-Pb compounds:
(i) the initial weathering of the Pb shot to form transformation products; and
(ii) the interaction of the transformation products with the soil solution and soil colloids.
Table 2.3. Summary of the concentration of Pb (mg kg-1 ) and density of Pb shot (kg shot m-2 ) at shooting ranges as reported by various authors. Author(s) Maximum total soil Pb †Maximum EDTA- extractable soil Pb † Maximum Pb shot density Depth of soil analysed (mm) Jørgensen and Willems (1987b) – 1,000 0.4 0 - 50 Castrale (1989) – – ≈ 2.3 *– Fahrenhorst and Renger (1990) ≈ 1,000 – –– Tanskanen et al. (1991) 10,500 22.35 0 - 70 Stansley et al. (1992) – – ≈ 10.4 × 103 * 0 - 75 Engström 1993, in Lin (1996) 24,500 – – 0 ≤ 100 Manninen and Tanskanen (1993) 54,000 52,000 – 0 - 40 Mellor and McCartney (1994) 10,620 – ≈ 534 * 0 - 150 Lin et al. (1995) 3,400 475 – 0 - 60 Merrington and Alloway (1995) 7390––– Siepel (1995) – – 1.01 0 - 50 Uren et al. (1995) 31,200– 2.3 × 104 0 - 50 Stansley and Roscoe (1996) 75,000 – ≈ 10.4 × 103 * 0 - 75 Merrington and Alloway (1997) 8,100 – – – Murray et al. (1997) 987– – 50 - 150 Rooney et al. (1999) – 8,300 – 0 - 75 Bruell et al. (1999) 420– – 0 - 900 Chen et al. (2001a) 15,368 83 pellets 100 g soil-1 0 - 10 † Lead shot removed * Estimated from data provided, on the basis of 1 pellet = 28 g 15
2.2.1 Initial transformations of lead shot
Various authors have analysed the mineralogy of crust material on Pb shot by x-ray diffraction (XRD). Results consistently show a predominance of Pb3(CO3)2(OH)2
(hydrocerussite) and minor amounts (<10%) of PbCO3 (cerussite), PbSO4 (anglesite), PbO (massicot) and PbO2 (plattnerite) (Jørgensen and Willems, 1987b; Fahrenhorst and Renger, 1990; Tanskanen et al., 1991; Lin et al., 1995; Lin, 1996; Murray et al., 1997). Fahrenhorst and Renger (1990) also found pyromorphite (Pb5(PO4)5X-, where X- may be OH-, chloride (Cl-), fluoride (F-) or nitrate (NO3-) ions) in crust material.
Pellet crust morphology has only been studied to any great extent by Lin (1996). Electron microprobe analysis (EMPA) identified two concentric layers of crust material: a light grey- coloured, discontinuous inner layer of between 10 and 30 µm width, and a darker grey coloured, continuous outer layer of between 50 and 150 µm width. The Pb concentration of the corroded pellets was in the order, pellet core (≈ 97% Pb) > inner layer (88-92% Pb) >
outer layer (77-84% Pb), indicating that the crust material consisted of secondary Pb minerals with lower Pb stoichiometry. When Lin compared this data to the stoichiometry of known Pb minerals (Table 2.4), the Pb content of the two layers suggested the presence of PbO and PbO2 in the inner layer, and Pb3(CO3)2(OH)2 in the outer layer. X-ray diffraction analysis of the overall crust provided a semi-quantitative estimation of mineralogy:
75-80% Pb3(CO3)2(OH)2, ≈10% PbSO4, 5-10% PbO, 5% PbCO3 and <5% PbO2. This provided further evidence that Pb3(CO3)2(OH)2 was the dominant mineral in the outer layer of the crust, which represents about 80% of the total crust volume. Considering the Pb content of PbO.PbCO3 and PbCO3 (Table 2.4), it is possible these minerals could also have contributed to the lower Pb content of the outer layer. Jørgensen and Willems (1987b) observed a similar reduction in the Pb content of crust material. The authors measured 68-73% Pb in the crust material of pellets from four sites, which compares well with the content of a number of possible Pb minerals (Table 2.4).
Table 2.4. Content of Pb in important Pb minerals by stoichiometry (from, Jørgensen and Willems, 1987b; Lin et al., 1995).
Pb mineral Molecular formula % Pb
Plattnerite PbO2 92
Massicot PbO 86
Anglesite PbSO4 68.3
Lead oxide carbonate PbO.PbCO3 84.5
Hydrocerussite Pb3(CO3)2(OH)2 80.1
Cerussite PbCO3 77.5
Uren et al. (1995) reported much thinner coatings of transformation products (up to 20 µm thick) on Pb pellets, and scanning electron microprobe (SEM) analysis of pellets showed the presence of the soil-derived elements aluminium (Al), calcium (Ca), iron (Fe), potassium (K), silicon (Si) and titanium (Ti). Presumably these elements were associated with the crust materials, which suggests formation of various mixed-mineral precipitates or elemental substitution. SEM analysis by Merrington and Alloway (1995) revealed surface corrosion and exfoliation of pellets, with small particles of <2 µm commonly found adjacent to the pellet.
This could suggest differential dissolution of the crust material, leaving small particles of less immediately-soluble mineral, or illustrate the tendency for the crust material to disintegrate after slight disturbance, as noted by Rooney et al. (1999).
Lin (1996) concluded that the EMPA data suggest that Pb-oxides form on the pellet surface and are then replaced by Pb-carbonates. Replacement appears to first take place along Pb oxide fractures, which are filled with the darker grey Pb3(CO3)2(OH)2 material. Park and MacDiamid (1970, in Lin, 1996) hypothesised that when the temperature of a mineral is raised, a point is reached at which the atomic structure of a mineral is disordered but loosely tied together, allowing ions to diffuse through the crystal with ease. Lin (1996) lends this hypothesis to the rise in temperature of Pb pellets when they are fired from the shotgun, and that the loosely tied atomic structure would allow carbon dioxide to diffuse through fractures and form Pb-carbonates.
A few authors have found a small amount of PbSO4 in some samples of pellet crust material, particularly in pellet samples from sites with low soil pH (Jørgensen and Willems, 1987b; Lin et al., 1995; Lin, 1996). Lin (1996) showed that the PbSO4 was most likely present only in the inner rim of crust material and then only in some of the crust samples, suggesting that PbSO4
becomes completely replaced by Pb-carbonates. The solubility of PbSO4 is not affected by pH, and it is theoretically more stable than Pb3(CO3)2(OH)2 and PbCO3 at low pH, but there is generally insufficient sulphur present in soils for PbSO4 to be present in much larger proportions. Competition (for ion association with Pb2+) from the large proportion of carbonate ligands produced by the soil organic carbon pool results in the overriding precipitation of Pb3(CO3)2(OH)2 (Lin, 1996).
It appears that PbCO3 is more likely to be present in the crust material at higher pH.
Jørgensen and Willems (1987b) observed that while Pb3(CO3)2(OH)2 always dominated crust mineralogy, PbCO3 was present in the greatest amount at the site with the highest soil pH(CaCl2) (7.4). Less PbCO3 was present at the two sites with soil pH(CaCl2) of 5.5, and no PbCO3 was present at the site with soil pH(CaCl2) of 3.5. Lin et al. (1995) studied the crust material found on pellets at five shooting ranges. Less than approximately 5% of the crust material was PbCO3 at the three sites where soil pH(KCl) <4.5, but the proportion of PbCO3
increased 3- to 4-fold at the two sites where soil pH(KCl) was 6.8-7.0. These two high-pH sites were rifle backstop berms that contained corroding lead bullets, instead of the lead pellets at the 3 low-pH shooting ranges, but effectively the two types of ammunition are exposed to the same type of soil conditions. Therefore corrosion would be expected to occur by similar processes. Interestingly, crust composition was not greatly affected by organic matter content at near-neutral pH. The two high-pH sites (pH(KCl) 6.8 and 7.0) differed greatly in organic matter content (12.5 and 2.4% respectively) but the proportions of Pb minerals measured (semi-quantitatively) varied little (75-80% Pb3(CO3)2(OH)2; 15-20% PbCO3; 5%
2.2.2 Secondary transformations of lead shot
The vast majority of the Pb at shooting ranges is present as metallic Pb shot (Jørgensen and Willems, 1987b; Rooney et al., 1999). Rooney et al. (1999) found that only 11% of the total Pb burden at a site was associated with the fine earth fraction. Jørgensen and Willems (1987b) reported the distribution of Pb between pellet, crust and soil at four shooting range sites (Figure 2.2). There is a clear difference between the cultivated and uncultivated sites.
Approximately three times more Pb is associated with the crust and soil fractions at the two cultivated sites. This difference seems to be of greater influence than pH, soil type or organic matter content. The crust material is relatively brittle and the action of ploughing would easily dislodge fragments of crust material from the Pb shot. This could lead to more rapid dissolution of the fragments, and encourage further corrosion of a pellet where the crust has been removed. At most of the sites studied by Jørgensen and Willems (1987b) the distribution of Pb is in the order metallic Pb > EDTA-extractable soil Pb > crust Pb, however one site (Parup-C) presents an anomaly in that there is a build-up of crust material but less Pb has transferred into the soil (Figure 2.2). Formation of crust material at the Parup-C site may be advanced by the CO2 produced by increased mineralisation caused by the ploughing of the highly organic soil, but subsequent dissolution of the crust may be prevented by the high soil pH.
It is evident that pellet decomposition is controlled by a complex combination of environmental factors including soil pH, organic matter content, available ligands and leaching rate. The rate of Pb shot decomposition was estimated by Jørgensen and Willems (1987b) from analytical data obtained from studying the reactivity of the crust material. The authors estimated that in some Danish soils, half of the Pb shot would be transformed into soil-Pb compounds in 40 to 70 years, and that complete transformation would occur in 100 to 300 years. Lin (1996) estimated that complete transformation of Pb shot to soil Pb compounds would occur in approximately 200 years.
Stenlille, cultivated, silt loam (13) pH(CaCl2) 5.5, 2027 g m-2 of Pb
Pårup, grassland, organic (6) pH(CaCl2) 5.5, 890 g m-2 of Pb
Holstebro, heath, podzol (6) pH(CaCl2) 3.5, 539 g m-2 of Pb
Pårup-C, cultivated, organic (6) pH(CaCl2) 7.4, 1328 g m-2 of Pb
Metallic Pb in pellets
Pb compounds in crust material
Extractable Pb in soil
Figure 2.2. The distribution of Pb in shooting range soils between metallic Pb in pellets, corrosion crust material and extractable Pb in soil (from Jørgensen and Willems, 1987b). The
areas of the circles are proportional to the total amounts of Pb. The numbers in parentheses are estimated average residence time (years) of the Pb pellets in the soil.
In summary, the existing literature indicates that:
(i) pH is an important factor in crust formation and dissolution;
(ii) Pb3(CO3)2(OH)2 is the dominant mineral formed, regardless of soil pH;
(iii) The amount (thickness) of crust material is affected by pCO2, regardless of soil pH;
(iv) At high soil pH, pH is more important in the formation of PbCO3 than pCO2; (v) The presence of PbSO4 is determined by the S:C ratio.