• No results found

Organic matter in UK aquifers

N/A
N/A
Protected

Academic year: 2020

Share "Organic matter in UK aquifers"

Copied!
416
0
0

Loading.... (view fulltext now)

Full text

(1)

ORGANIC MATTER IN UK AQUIFERS

A thesis submitted for the degree of Doctor of Philosophy

at the University of London

Alison Catherine Macleod

June 1998

Department of Geological Sciences University College London

(2)

ProQ uest Number: U 6 4 4 0 9 7

All rights reserved

INFORMATION TO ALL U SE R S

The quality of this reproduction is d ep en d en t upon the quality of the copy subm itted.

In the unlikely even t that the author did not sen d a com plete manuscript

and there are m issing p a g e s, th e se will be noted. Also, if material had to be rem oved,

a note will indicate the deletion.

uest.

ProQ uest U 6 4 4 0 9 7

Published by ProQ uest LLC(2016). Copyright of the Dissertation is held by the Author.

All rights reserved.

This work is protected against unauthorized copying under Title 17, United S ta tes C ode.

Microform Edition © ProQ uest LLC.

ProQ uest LLC

789 East E isenhow er Parkway

P.O. Box 1346

(3)

ABSTRACT

Natural Organic Matter

A robust sampling and storage method for dissolved organic carbon (DOC) at natural abundances involves acidifying to pH 3.00 and sparging with nitrogen gas for 15 minutes; this preserves samples at room temperature for up to one month. Concentrations of DOC in two major UK aquifers are:

in the Lower Greensand aquifer of Sussex and Surrey, 0.51 ± 0.05 mg/1 (range 0.459), in the Lower Greensand aquifer of Kent, 0.28 ± 0.05 mg/1 (range 0.300),

in the Chalk aquifer o f Berkshire, 0.51 ± 0.05 mg/1 (range 0.277).

Concentrations of DOC do not differ significantly between the unconfined and confined parts o f the aquifers, despite the fact that oxygen and nitrate reduction occurs in the confined part o f the aquifers. If DOC is sourced solely at outcrop, the concentrations of DOC in these aquifers is insufficient to drive microbial reduction reactions. Redox reactions may be driven in the Chalk by iron sulphide oxidation and in the Lower Greensand by oxidation of Fe(II) in glauconite.

Contaminant Organic Matter

(4)

ACKNOWLEDGMENTS

I would like express my sincere thanks to my supervisor, Dr. J. McArthur, for the use of the Wolfson Laboratory and all his guidance, support and constructive criticism throughout my time at UCL.

Special thanks must also go to all in the Hydrogeology Group, especially Jane for the chats.

Without the generous assistance of the following, this work could never have been carried out: Albury Estates, Cowdray Estates, East Surrey Water, Environment Agency, Leconsfield Estate, Mid Kent Water, South East Water, Southern Water Services, Thames Water (especially Ramon Franco at Fobney), the many other well owners in Sussex, Surrey, Kent and Berkshire that are too numerous to mention, and those who have to remain nameless.

Thanks to The Natural Environment Research Council and The Jackson Environment Institute for funding this project.

Thanks to all the staff and students at UCL for their assistance: Tony Osborn for his help and advice in the Wolfson lab; Helen Prince for doing battle with that machine; Sarah for help with the mass spectra, Tobias for the ion balances; Celine, Ron, Janet and Lisa for sorting stuff out.

Thanks to all the UCL gang for beers and moans: Simon, Nic, Dave P, Big Dave, Jon, Andy, Delphene; Zuwena, Wilson, Shawn, Hannah, Ben, Rhodri, Pim.

Gigantic thanks must go to the Streatham gang (including Wimbledon, Coventry, Norwich), Sheri, Nicki, Sue, Jo, Bobby, Steve, Wilsh, Stells, Rufus and Jazza for keeping me cheered up, beered up, sane (I think) and giving me a good ooohing whenever I lost my sense of humour.

(5)
(6)

CONTENTS

Title page...1

Abstract...2

Acknowledgements... 3

Table of contents... 5

List of figures... 10

List of tables...17

List of plates...19

Chapter 1 Introduction...20

1.1 Aims... 20

1.2 Introduction... 21

SECTION 1 NATURAL ORGANICS Chapter 2 Dissolved organic carbon...24

2.1 Introduction to dissolved organic carbon... 25

2.2 Composition...25

2.3 Abundance... 28

2.4 Sources in aquifers... 29

2.4.1 Surface sources of DOC... 29

2.4.2 Aquifer sources of DOC... 31

2.4.3 DOC sources in confining beds...32

2.4.4 Biomass and necromass sources...32

2.5 Sinks... 33

2.5.1 Oxidation/reduction... 33

2.5.2 Adsorption...38

2.5.3 Precipitation... 39

2.5.4 Volatilisation... 39

2.5.5 Complexation... 39

(7)

3.1 Introduction to DOC sampling...41

3.2 Sample containers... 42

3.3. Sample collection... 42

3.3.1 Cleanliness...43

3.4 Filtration... 43

3.5 DOC sample preservation...44

3.5.1 Acidification and sparging...44

3.5.2 Freezing... 46

3.5.2.1 Freezing using LDPE...46

3.5.2.2 Freezing using PP... 47

3.5.3 Storage... 48

3.6 Blanks...49

3.6.1 Trip blanks... 50

3.6.2 Field blanks...50

3.7 Well head sampling...51

3.8 Analysis of samples... 52

Chapter 4 Dissolved organic carbon in the Lower Greensand Aquifer...54

4.1 Lower Greensand...55

4.1.1 Abstraction and uses...55

4.1.2 Stratigraphy...55

4.1.3 Lithology and mineralogy... 58

4.1.4 Hydrogeology...60

4.1.4.1 Recharge... 65

4.1.5 Hydrochemistry...66

4.2 Results... 69

4.3 Discussion...72

4.3.1 Dissolved organic carbon in Sussex and Surrey, and Kent... 72

4.3.2 Solid organic carbon in the Lower Greensand... 77

4.3.3 Redox species, alkalinity and pH in Sussex and Surrey... 80

4.3.4 Other parameters in Sussex and Surrey... 91

4.3.4.1 Physical parameters...91

(8)

4.3.4.3 Minor elements...98

$.3.5 Redox species, alkalinity and pH in Kent... 100

$.3.6 Other parameters in Kent...109

4.3.6.1 Physical parameters... 109

4.3.6.2 Major elements... I l l 4.3.6.3 Minor elements... 115

CHAPTER 5 Dissolved organic carbon in the Chalk Aquifer 5.1 The Chalk... 116

5.1.1 Aquifer history... 117

5.1.2 Stratigraphy... 117

5.1.3 Lithology and mineralogy... 102

5.1.4 Hydrogeology...120

5.1.4.1 Recharge... 122

5.1.5 Hydrochemistry... 123

5.2 Results... 125

5.3. Discussion...125

5.3.1 Dissolved organic carbon...125

5.3.2 Redox species, alkalinity and pH...131

5.3.3 Redox migration since 1987... 144

5.3.4 Other parameters... 145

5.3.4.1 Physical parameters... 145

5.3.4.2 Major elements... 147

5.3.4.3 Minor elements... 148

SECTION 2 CONTAMINANT ORGANICS Chapter 6 The landfill site...153

6.1 Introduction...154

(9)

6.3 Geology and hydrogeology... 163

6.4 Hydrology, water balance and groundwater... 167

5.4.1 Vertical groundwater flow... 169

5.4.2 Lateral groundwater flow...170

6.5 Cortaminant movement... 171

Chapter 7 Sampling methodology, and sample treatment and analysis...173

7.1 Introduction...174

7.2 Sample containers and preservation... 174

7.2.1 Containers and preservation for dissolved inorganic carbon...174

7.2.2 Containers and preservation for dissolved organic carbon...175

7.2.3 Containers and preservation for volatile and non-volatile organics...176

7.3 Sample collection... 177

7.3.1 Well head sampling... 179

7.4 Sample analysis... 179

7.4.1 Analysis of samples for DOC and DIC... 179

7.4.2 Analysis of samples for non- volatile and volatile organics... 180

Chapter 8 Results of landfill site investigation... 186

8.1 Results... 192

8.2 Discussion...192

8.2.1 Dissolved organic carbon... 192

8.2.2 Redox Species...195

8.2.3 Contaminant compounds at the landfill site... 210

8.2.3.1 Vertical contaminant migration...227

8.2.3.2 Lateral contaminant migration... 230

8.2.3.3 Coastal migration... 233

8.2.5 Physical controls on contaminant migration rates and concentration changes... 236

8.2.6 Contaminant degradation... 237

8.2.6.1 BTEX’s... 238

8.2.6.2 PCE, TCE, chloroform...239

(10)

8.2.6.4 Contaminants identified to compound level... 245

!.2.7 Other parameters...247

8.2.7.1 Temperature, alkalinity, pH and electrical conductivity... 247

8.2.7.2 Major elements... 253

8.2.7.3 Minor elements... 261

8.2.7.4 Ion balances...265

Chapter 9 Conclusions and further work... 268

9.1 Conclusions... 269

9.2 Recommendations for further work... 272

References... 274

Appendices... 290

Appendix 1 Drinking water standards...291

Appendix 2 Major and minor hydrochemistry blanks from the Lower Greensand and Chalk aquifers, and the area surrounding the landfill site... 295

Appendix 3 Analysis of Lower Greensand core for carbon and sulphur... 299

Appendix 4 Geology of the area surrounding the landfill site... 301

Appendix 5 The reaction, equilibrium constant, and any temperature corrections, as used by W ATEQ...306

(11)

LIST OF FIGURES

Figure 2.1. The size continuum of particulate and dissolved organic carbon in natural waters...27 Figure 2.2. Approximate concentrations of dissolved and particulate organic carbon in natural waters... 27 Figure 2.3. Podzolisation and decrease of organic matter in interstitial water of

soils... 30 Figure 2.4. Pore water profiles of marine sediment core with porewater data...34 Figure 2.5. Hydrogen bonding involving natural organic matter and silica alumina minerals surfaces in sediments...38

Figure 3.1 Chart showing the results of storing acidified and sparged samples for up to six days... 49 Figure 3.2 Chart showing the results of storing acidified and sparged samples for up to twenty days...49

Figure 4.1. Map showing the location of the Lower Greensand, UK... 56 Figure 4.2. Stratigraphie column showing the Lower Greensand and adjacent beds 57 Figure 4.3. Schematic cross section through the Weald Anticline and the London

(12)

Figure 4.8a. Spatial distribution of DOC concentrations in the Lower Greensand aquifer, Sussex and Surrey...75 Figure 4.8b. Spatial distribution of DOC concentrations in the Lower Greensand aquifer, Kent...78 Figure 4.9. DOC concentrations in the Lower Greensand aquifer... 78 Figure 4.10. Weight % of carbon and sulphur in Lower Greensand core... 79 Figure 4.11. Concentrations of nitrate, dissolved oxygen, sulphate and DOC in springs and boreholes in the Lower Greensand aquifer, Sussex and Surrey...83 Figure 4.12. Dissolved oxygen versus DOC and sulphate in the oxic part of the Lower Greensand aquifer, Sussex and Surrey... 83 Figure 4.13. Manganese and total iron concentrations in the Lower Greensand aquifer, Sussex and Surrey... 86 Figure 4.14. Sulphate versus nitrate in the Lower Greensand aquifer, Sussex and

Surrey... 86 Figure 4.15. Sulphate versus calcium in the lower Greensand aquifer, Sussex and

Surrey... 87 Figure 4.16. Saturation indices of calcite and gypsum in the Lower Greensand aquifer, Sussex and

Surrey... 87 Figure 4.17. Alkalinity in the Lower Greensand aquifer, Sussex and Surrey... 89 Figure 4.18. pH in the Lower Greensand aquifer, Sussex and Surrey... 89 Figure 4.19. Alkalinity versus pH in the Lower Greensand aquifer, Sussex and Surrey.90 Figure 4.20. Temperature of groundwater in the Lower Greensand aquifer, Sussex and Surrey... 92 Figure 4.21. Electrical conductivity in the Lower Greensand aquifer, Sussex and

(13)

Figure 4.26. Potassium, calcium, magnesium and sodium in the Lower Greensand

aquifer, Sussex and Surrey...96

Figure 4.27. Chloride versus nitrate in the Lower Greensand aquifer, Sussex and Surrey... 97

Figure 4.28. Aluminium and strontium in the Lower Greensand aquifer, Sussex and Surrey... 99

Figure 4.29. Concentrations of nitrate, dissolved oxygen, sulphate and DOC in the Lower Greensand aquifer, Kent, along section 4.2a... 101

Figure 4.30. Manganese and iron in the Lower Greensand aquifer, Kent, along section 4.2a... 102

Figure 4.31 Sulphate versus nitrate in the Lower Greensand aquifer, Kent... 106

Figure 4.32. Sulphate versus oxygen in the oxic part of the Lower Greensand aquifer, Kent... 106

Figure 4.33. Alkalinity in the Lower Greensand aquifer, Kent, along section 4.2a...107

Figure 4.33b. pH in the Lower Greensand aquifer, Kent, along section 4.2a...107

Figure 4.34. Alkalinity versus pH in the Lower Greensand aquifer, Kent... 107

Figure 4.35. Saturation indices of calcite and gypsum in the Lower Greensand aquifer, Kent... 108

Figure 4.36. Temperature in the Lower Greensand aquifer, K en t, along section 4.2a... 110

Figure 4.37. Electrical conductivity in the Lower Greensand aquifer, Kent, along section 4.2a...110

Figure 4.38. Sodium:chloride ratios in groundwater samples from the Lower Greensand aquifer, Kent... 110

Figure 4.39. Piper plot of groundwater samples from the Lower Greensand aquifer, Kent...112

Figure 4.40. Potassium, calcium, magnesium and sodium in the Lower Greensand aquifer, Kent, along section 4.2a...113

Figure 4.41. (Ca+Mg)/HC0 3' ratios and (Ca+Mg+Na)/HC0 3' ratios in the Lower Greensand aquifer, Kent along section 4.2a...114

Figure 4.42. Strontium in the Lower Greensand aquifer, Kent, along section 4.2a...114

Figure 5.1. Location of the Chalk in the UK... 118

(14)

Figure 5.3. Section through the Berkshire Syncline... 120

Figure 5.4. The Chalk of the Berkshire Syncline, with groundwater contours on the Chalk... 124

Figure 5.5. Distribution of DOC concentrations in groundwater samples from the chalk aquifer...128

Figure 5.6. DOC concentrations in the Chalk aquifer... 128

Figure 5.7. DOC concentrations in the Chalk of Berkshire... 130

Figure 5.8. Concentrations of Nitrate, dissolved oxygen, sulphate, and DOC in the chalk, along with section 5.2a... 132

Figure 5.9. Dissolved oxygen versus sulphate in the Chalk aquifer... 134

Figure 5.10. Manganese and iron in the chalk aquifer, along section 5.2a...137

Figure 5.11. Sulphate versus nitrate in the Chalk aquifer... 137

Figure 5.12. Calcium versus sulphate in the Chalk aquifer...137

Figure 5.13. Sulphate versus chloride and sodium versus chloride in the Chalk aquifer...139

Figure 5.14. Alkalinity in the Chalk aquifer... 142

Figure 5.15. pH in the Chalk aquifer... 142

Figure 5.16. Saturation induces of calcite and gypsum in the Chalk aquifer, along section 5.2a... 143

Figure 5.17. Calcium versus alkalinity in the chalk aquifer...143

Figure 5.18. Temperature in the Chalk aquifer...146

Figure 5.19. Electrical conductivity in the Chalk aquifer... 146

Figure 5.20. Piper plot of the groundwater samples from the chalk aquifer... 149

Figure 5.21. Potassium, sodium, magnesium and calcium in the chalk aquifer, along section 5.2a... 150

Figure 5.22. (Ca+MgVHCOa' ratios and (Ca+Mg+Na^HCO]' ratios in the Chalk aquifer, along section 5.2a... 151

Figure 5.23. Strontium conetrations in the Chalk aquifer... 151

Figure 6.1. Map showing the location of the disposal lagoons and trenches in the landfill site... 156

(15)

showing the position of boreholes and sample numbers... 162

Figure 5.4. Stratigraphie column of the units in the area surrounding the landfill site.. 164

Figure 5.5. Stratigraphie column showing the aquifers, aquitards and adjacent beds in the area surrounding the landfill site...165

Figure 6.6. Hydrographs from sampling point 6, in the Upper Aquifer, and from sampling point 26, in the Lower Aquifer...168

Figure 7.1. Gas chromatograms of organics from a sample extracted using SPME 182 Figure 7.2. Graphs of concentration versus gas chromatograph peak area, from trials to determine the saturation point of the SPME fibre... 183

Figure 7.3. Gas chromatograms from purge and trap analyses... 184

Figure 8.1. DOC in the area surrounding the landfill site, along section 6.1...194

Figure 8.2. Idealised distribution of redox zones in the groundwaters beneath a source of organic contaminant... 196

Figure 8.3. Dissolved oxygen from the area surrounding the landfill site, along section 6.1... 196

Figure 8.4. Oxygen reduction zones along section 6.1... 198

Figure 8.5. Nitrate in the area surrounding the landfill site, along section 6.1...199

Figure 8.6. Nitrate reduction zones along section 6.1... 200

Figure 8.7. Manganese in the area surrounding the landfill site, along section 6.1...201

Figure 8.8. Manganese reduction zones along section 6.1... 202

Figure 8.9. Total iron in the area surrounding the landfill site, along section 6.1...204

Figure 8.10. Iron reduction zones along section 6.1... 205

Figure 8.11. Sulphate in the area surrounding the landfill site, along section 6.1... 206

Figure 8.12. Sulphate reduction zones along section 6.1...207

Figure 8.13. Comparative concentrations of benzene in the area surrounding the landfill site, along section 6.1... 211

Figure 8.14. Comparative concentrations of chloroform in the area surrounding the landfill site, along section 6.1... 212

(16)

surrounding the landfill site, along section 6.1... 214 Figure 8.17. Comparative concentrations of trichloroethylene (TCE) in the area

surrounding the landfill site, along section 6.1... 215 Figure 8.18. Comparative concentrations of toluene in the area surrounding the landfill site, along section 6.1... 216 Figure 8.19. Comparative concentrations of o-xylene in the area surrounding the landfill site, along section 6.1... 217 Figure 8.20. Comparative concentrations of a butyl methyl ketone in the area

surrounding the landfill site, along section 6.1... 218 Figure 8.21. Comparative concentrations of an aldehyde in the area surrounding the landfill site, along section 6.1... 219 Figure 8.22. Comparative concentrations of /?-chloro-m-cresol in the area surrounding the landfill site, along section 6.1... 220 Figure 8.23. Comparative concentrations of bromophenol in the area surrounding the landfill site, along section 6.1... 221 Figure 8.24. Comparative concentrations of a phenol in the area surrounding the landfill site, along section 6.1... 222 Figure 8.25. Comparative concentrations of an acetate in the area surrounding the landfill site, along section 6.1... 223 Figure 8.26. Comparative concentrations of a chloro methyl phenyl ether in the area surrounding the landfill site, along section 6.1... 224 Figure 8.27. Comparative concentrations of an alkyl amine in the area surrounding the landfill site, along section 6.1... 225 Figure 8.28 Comparative concentrations of a phenyl ether in the area surrounding the landfill site, along section 6.1... 226 Figure 8.29 Map showing coastal erosion of the coastline to the east of the landfill

site... 235 Figure 8.30. PCE and TCE in the area surrounding the landfill site, along section

6.1 241

Figure 8.31. Bromide conetrations along section 6.1...244 Figure 8.32. Temperature in the area surrounding the landfill site, along section 6.1..249 Figure 8.33. Electrical conductivity in the area surrounding the landfill site, along

(17)

carbon concentrations, in the Upper/Middle Aquifer in the area surrounding the landfill site, along section 6.1... 252 Figure 8.35. Alkalinity in the area surrounding the landfill site, along section 6.1...252 Figure 8.36. pH in the area surrounding the landfill site, along section 6.1... 252 Figure 8.37a. Calcium, sodium, potassium and magnesium in the Upper/Middle aquifer in the area surrounding the landfill site, along section 6.1...254 Figure 8.37b. Calcium, sodium, potassium and magnesium in the Middle aquifer in the area surrounding the landfill site, along section 6.1...255 Figure 8.37c. Calcium, sodium, potassium and magnesium in the Lower Aquifer in the area surrounding the landfill site, along section 6.1...256 Figure 8.38. (Ca+Mg)/HC0 3' ratios and (Ca+Mg+Na)/HC0 3' ratios in the Upper/Middle Aquifer, from the area surrounding the landfill site, along section 6.1... 257 Figure 8.39. Chloride in the area surrounding the landfill site, along section 6.1... 258 Figure 8.40. Chloride concentrations in the Upper/Middle Aquifer, Middle Aquifer and Lower Aquifer in the area surrounding the landfill site, contoured using SURFER

(18)

LIST OF TABLES

Table 2.1. Processes and reactions for the bacterially mediated oxidation of organic

m atter...35

Table 3.1.DOC in field and laboratory blanks untreated, and treated in various ways...45

Table 3.2.DOC using LDPE Nalgene bottles...47

Table 3.3. Results of trails to determine whether the transferring of samples between vials in a hminar flow hood contributes any DOC contamination... 47

Table 3.4. Results of freezing trials using polypropylene (PP) centrifuge tubes... 48

Table 3.5. Results of laboratory blank storage trials... 50

Table 3.6. Results of the field trails on contamination from exposure to the air... 51

Table 4.1a Major element hydrochemistry of the Lower Greensand aquifer, Sussex and Surrey... 70

Table 4.2b Major element hydrochemistry of the Lower Greensand aquifer, Kent...71

Table 4.2a. Minor element hydrochemistry of the Lower Greensand aquifer, Sussex and Surrey... 71

Table 4.2b. Minor element hydrochemistry of the Lower Greensand aquifer, Kent...71

Table 4.3. DOC concentrations in the Lower Greensand aquifer... 72

Table 4.4. Redox species in the Lower Greensand aquifer, Sussex and Surrey... 81

Table 4.5. Physical parameters in the Lower Greensand aquifer, Sussex and Surrey...91

Table 4.6. Redox species in the Lower Greensand aquifer, Kent...100

Table 4.7. Physical parameters in the Lower Greensand aquifer, Kent...109

Table 5.1 Major element hydrochemistry of the Chalk aquifer... 126

Table 5.2 Minor element of the Chalk aquifer...127

Table 5.3 DOC concentrations in the Chalk aquifer... 125

Table 5.4 Redox species in the Chalk aquifer...131

Table 5.5 The dissolved oxygen, nitrate and sulphate concentrations in the oxic part of the Chalk aquifer, in 1987 and today... 144

Table 5.6 Physical parameters in the Chalk aquifer of Berkshire... 145

(19)

Table 6.2. Borehole identifications and monitoring point numbers...163

Table 6.3. Water levels in sampling point 6, based on Fig. 6.6... 167

Table 7.1. Results of DIC preservation trials using mercury chloride...175

Table 7.2. Results of preserving samples with high DOC concentrations... 175

Table 7.3. DIC and TDC concentrations in filtered and unfiltered laboratory blanks... 180

Table 8.1. Major element hydrochemistry of the landfill site... 188

Table 8.2. Minor element hydrochemistry of the area surrounding the landfill site 189 Table 8.3a. Volatile organic compound identified in samples from the area surrounding the landfill site...190

Table 8.3b. Non-volatile organic compounds identified in samples from the area surrounding the landfill site... 191

Table 8.4. Maximum and minimum concentrations of DOC in the area surrounding the landfill site... 193

Table 8.5. Zones of oxygen reduction in the area surrounding the landfill site...197

Table 8.6. Distance of peak manganese concentrations along section 6.1... 197

Table 8.7. Concentration of organic compounds in the area surrounding the landfill site in 1987...227

Table 8.8. Aquifer where peak concentration of the organic compounds occurs, and organic compound physical properties...230

Table 8.9 Lateral migration rates of organic compounds in the area surrounding the landfill site, which show a clear maximum concentration peak...232

Table 8.10. Half lives of some organic contaminants... 246

Table 8.11. Minimum and Maximum temperature in the area surrounding the landfill site...247

Table 8.12. Minimum and maximum electrical conductivity in the area surrounding the landfill site... 250

Table 8.13. Minimum and maximum pH in the area surrounding the landfill site...251

Table 8.14. Minimum and maximum chloride concentrations in the area surrounding the landfill site... 253

Table 8.15. Major element hydrochemistry of the landfill site, with ion balances...266

(20)

LIST OF PLATES

Plate 5.1 Electron micrograph of gypsum being formed from a groundmass of marcasite/pyrite... 134

Plate 6.1. Photograph of the landfill site...159 Plate 6.2. Photograph of the chffs to the east of the landfill site, showing the Lowestoft Till and the Cromerian sands... 166

(21)

CHAPTER 1

AIMS AND INTRODUCTION

1.1 Aims of this thesis

(22)

1.1 Aims of thesis

The aims of this thesis were:

1) To develop a sampling methodology for dissolved organic carbon (DOC) in groundwaters.

2) To determine DOC concentrations in two aquifers with different flow regimes, mineralogies and lithologies, viz. the Lower Greensand Aquifer, and the Chalk Aquifer, and to assess the effects of DOC on the redox chemistry in these natural aquifers.

3) To determine the concentrations and composition of DOC beneath a landfill site, and the effects of that DOC on the redox chemistry, to determine whether biodégradation and migration of organic contaminants beneath the landfill site poses a future threat to the environment.

1.2 Introduction

Microbially-driven reduction-oxidation (redox) reactions are ubiquitous in groundwaters, and can be both beneficial to water quality, through the removal of contaminant nitrate and the degradation of organic pollutants, and detrimental to water quality, through the production of iron and poisonous hydrogen sulphide gas.

(23)

Despite the fact that the amount of organic matter in aquifers may limit bacterial redox activities, basic information on organic matter in aquifers in limited. For example, how much DOC is in groundwaters, where it comes from, what its composition is, how is it degraded during redox reactions, how it controls redox reactions and even how to sample for DOC, is limited. Measurements of DOC in groundwaters have been reported in Europe and America (Belin et a l, 1993; Murphy et al., 1989; Wassenaar et a l , 1991) but only a handful of measurements of DOC in UK aquifers have been made (Edmunds et a l, 1982; McArthur a/., 1996; Turrell, 1994).

There have been many investigations evaluating different methods of sample collection and preservation of DOC in waters (Norman, 1993; Fukushima, 1996; Tupas, 1994; Sugimura and Suzuki, 1988; Leenheer et a l, 1974), but there is little consensus of opinion and much controversy. This thesis presents an effective method for sample collection and preservation of DOC in groundwaters.

Also, while the pollution of groundwaters from landfill leachates and industrial chemicals unquestionably occurs (Rivett, et al., 1990; Konopka, et a l, 1991; Wilhams et a l , 1991; Zoetman, et a l, 1981), the processes that degrade contaminant organic matter, details of its migration, and how both control the development of redox reactions around landfill sites is poorly understood. While many organic contaminants have been degraded in laboratory experiments (Lovley et al., 1994; Kawai, 1995; Keck et al., 1989), whether individual organic contaminants will degrade in-situ, or at what rates they will degrade, is difficult to predict (Montgomery et al., 1994).

(24)

SECTION 1

(25)

CHAPTER 2

DISSOLVED ORGANIC CARBON

2.1 Introduction to dissolved organic carbon

2.2 Composition

2.3 Abundance

2.4 Sources in aquifers

2.4.1 Surface sources of DOC 2.4.2 Aquifer sources of DOC 2.4.3 Aquitard sources of DOC 2.4.4 Biomass and necromass sources

2.5 Sinks

2.5.1 Oxidation/reduction 2.5.2 Adsorption

(26)

2.1 Introduction to dissolved organic carbon

Microbial activity may drive redox reactions in aquifers. An outline of such activity, and the redox reactions it drives, is given here for two reasons. In later discussion of the natural dissolved organic carbon (DOC) distributions in aquifers, the relation of DOC to redox zonations will be discussed and the effect of bacterial redox reactions on DOC concentrations will be examined (see Chapters 4 and 5).

In the second part of this thesis (see Section 2) microbially driven redox reactions are shown to very active beneath a landfill site where, because of the many poisons dumped in the site, bacterial activity would not be expected to occur.

A discussion of bacterial redox reactions is therefore given here to serve as a guide to later chapters.

2.2 Composition

In this thesis, measurements of DOC are reported as total DOC. An abundant literature describes how DOC may be subdivided into compound classes by a variety of methods based principally on the method used to extract it from groundwater. A brief survey of these compound classes is given below; it is brief because these distinctions are not made in this thesis.

Dissolved organic carbon (DOC) is any organic carbon molecule • in natural waters that passes through a 0.45 pm filter. This fraction is part of a size continuum of organic carbon found in natural waters (Fig. 2.1). In most groundwaters DOC constitutes >95% (Fig. 2.2) of the total organic carbon (Thurman, 1989), in the Triassic sandstone aquifer there is no difference between TOC and DOC (J. McArthur, pers. comm.), and therefore, in this study all organic carbon in groundwaters is presumed to be DOC.

DOC consists of:

(27)

• hydrophilic acids (10-30% of DOC) (Murphy et a l , 1989); • simple compounds, viruses and bacteria. (35-72% of DOC).

Humic substances and hydrophilic acids are the most abundant fractions of DOC in

groundwater. These classes of compounds are operationally defined; humic substances are extracted from waters by adsorption onto XAD resins, whereas hydrophilic acids are not retained by XAD resins, and have to extracted from waters using Silicalite, a silica- based molecular sieve (Murphy et a i, 1989). XAD resins are ion exchange resins^

Humic substances are non-volatile and have molecular weights of between 500-5000 Thurman (1985); they are often referred to as high molecular weight (HMW) acids or hydrophobic acids. Humic substances can be further sub-divided into fulvic acids (the most abundant fraction), small water-soluble molecules that are approximately 65% aliphatic (Thurman, 1985), and humic acids which form aggregates with clays, amorphous iron, and aluminium oxides; humic acids are less soluble than fulvic acids as they contain fewer carboxyl groups (Thurman, 1985b; Murphy et a l, 1989).

Hydrophilic acids, or low molecular weight (LMW) acids, have only recently been identified in natural waters and their composition is not yet well known. Hydrophilic acids may be mixtures of simple organic compounds (such as volatile fatty acids and hydroxy acids) and complex polyelectrolytic acids containing many hydroxyl and carboxyl functional groups (Leenheer, 1981; Thurman, 1985).

Simple organic compounds, viruses and bacteria comprise the remainder of DOC.

Simple organic compounds, such as amino acids and short chain carbohydrates, are produced by the decomposition of larger organic molecules. Viruses are the only living organisms that pass into the DOC range, although they are not found in measurable amounts in natural waters (Thurman, 1985).

(28)

10 1 0' 10 10' 10 10'

Molecule radius (m)

10 10-10

Zooplankton

T

0.45

micron

boundary FA

Phytoplankton

Fulvic Acid

Bacteria AA

Viruses HC

Clay-humic-metal complexes

POC

FA Fatty acids CHO C arbohydrates

DOC

10® 10' 10* 10* 10" 10* 10* 10’

Molecular Weight |4 Colloidal r a n g e ►

Amino Acids

AA I HC| H ydrocarbons

Figure 2.1 The size continuum of particulate and dissolved organic carbon in natural waters (After Thurman, 1985).

Total Organic Carbon, mg/l

0 10 15 2 0 25 30 35

s e a w a ter

ground w ater

precipitation

oligotrophic lake

river

eutrophic la k e s

m arsh

b og

0:5

]

0.7

]

1.1

i

2.2

7lO

17

33

P a rtic u la te o r g a n ic c a rb o n D isso lv e d o r g a n ic c a rb o n

(29)

The systems used to classify DOC fractions are based on the methods used to extract each fraction. There are many methods of extracting DOC from water, and separating the fractions, such as molecular sieves, XAD resins and tangential ultra-flow filtration, and no guarantee that the same fractions are being extracted by the different methods. While Thurman (1985)’s classification of DOC in groundwaters, described above, is one of the most comprehensive, a standardized method for the extraction and naming of DOC is needed to avoid confusion when comparing the work of different authors.

2.3 Abundance

Dissolved organic carbon concentrations in natural groundwaters commonly range between 0.1-15 mg/l, with a median concentration of 0.7 mg/l, but the majority of groundwaters have concentrations below 2 mg/l (Thurman, 1985; Turrell, 1994). Concentrations of DOC in groundwater follow a log-normal distribution, so the median (Leenheer et a i, 1974) or the geometric mean are better estimates of the average concentration than the mean DOC concentration (Erricker, 1981).

Groundwater in tropical or semi-tropical regions tend to have higher DOC concentrations, between 6-15 mg/l, due to the recharge waters being heavily laden with decomposing organic matter (Thurman, 1985). Groundwaters associated with coal deposits and oil-shales also contain large amounts of dissolved organic matter (5-10 mg/l). Further more, trona water, which contains large amounts of sodium bicarbonate, leaches kerogen from oil-shales and may have concentrations of DOC as high as 40,000 mg/l (Thurman, 1985). Median DOC concentrations of 0.7 mg/l, for sandstone, limestone and sand, and gravel aquifers, and 0.5 mg/l for crystalline rock aquifers were reported for aquifers in the USA (Leenheer, 1974). It is possible that the different methods used to collect and preserve groundwaters for DOC analysis have contributed to the variation in DOC concentrations reported by different authors (see Chapter 3).

(30)

with DOC concentrations of <0.10 mg/l in the confined part of the aquifer (Edmunds, 1982; Turrell, 1994). In the Lincolnshire Limestone aquifer DOC concentrations ranged between 0.7-1.5 mg/l (McArthur gf a/., 1996).

Implicit in writings on DOC in groundwater is a belief that DOC concentrations in groundwater are low probably because the long residence time of groundwater in aquifers (<10^ years) provides time for the DOC to be consumed by redox reactions and bacterial metabolism (see Chapters 2.5.1), and/or because after the DOC has passed through the soil zone, the DOC is refractory and of no use for bacterial metabolism (see Chapter 4 and 5).

2.4 Sources of DOC in aquifers

There are four possible sources of DOC in groundwater:

• surface sources, e.g. from degrading organics in soils in recharge areas;

• aquifer sources, e.g. the fermentation of solid organic matter (SOM) contained within aquifer rocks;

• aquitard sources, e.g. the fermentation of SOM in aquitards followed by its diffusion into the aquifer (Murphy et a l , 1989).

• bacterial biomass and necromass.

These sources are discussed below.

2.4.1 Surface sources of DOC

Organic matter in soils comprises dead plants and animals and their degradation products. The amount of organic matter is controlled by the type of soil and the land use. Both, in turn, control the soil’s bacterial population (Thurman, 1985).

(31)

may arise from this source (Cronan and Aitken, 1985; Murphy et al., 1989; Wassenaar et al., 1991). Rivers and streams can also contain high concentrations of organic matter (Fig. 2.2) and some aquifers are recharged directly by rivers and streams entering the aquifers via swallow holes (Fetter, 1994). For example, the Lincolnshire Limestone locally derives up to 50% of its recharge, and 15% of its overall recharge, from river and stream flow into swallow holes (Kim er a i, 1995).

The unsaturated zone acts as a barrier to the passage of soluble organic matter from soils to groundwater (Cronan and Aitken, 1985). DOC concentrations in soil waters decrease with depth from the upper O/A horizons to the lower B/C horizons. The upper horizons also show a greater seasonal variation in DOC concentrations that do the lower horizons (Cronan and Aitken, 1985; Thurman, 1985) (Fig 2.3). Thus, it is assumed that the influx of DOC from the soil zone into the groundwater will be relatively stable and can be approximated by steady state conditions.

j ÿ p p i rs t , Interstitiahflater dissolves Pp c # A 2 Then, DOC complexes Fe and

1' ‘ DOC-metal complexes i '

c oat i ng

15 20

m g/l

(32)

The major processes that remove DOC from the pore waters in soils are the utilization of DOC by microbes, adsorption, and metal co-precipitation (see Chapter 2.5). It is estimated that these processes can account for the recycling/retention of 80% of the initial flux of DOC into soils (Wassenaar, 1991). The relative importance of these processes depends upon factors such as temperature, recharge rates, soil moisture content and seasonal variability.

Comparisons of the ‘"^C activities of HMW and LMW DOC in groundwater, soil interstitial waters and aquifer kerogens suggest that the HMW DOC in the groundwater originates from organic carbon sourced at the surface and the LMW DOC in the groundwater originates from the degradation of solid organic carbon in aquifers and aquitards (Murphy et a i, 1989).

2.4.2 Aquifer sources of DOC

Solid organic carbon (SOC), remnant in aquifers from the time of deposition, can source DOC (Wassenaar et a l, 1991). In anaerobic environments heterotrophic bacteria (see Chapter 2.5.1) can utilize this solid organic carbon as a source of carbon and energy via two fermentation pathways (Eq. 2.1 and Bq. 2.2).

ferm entation

SOC --- > LMW DOC + CO] Eq. 2.1

(Wassenaar er rz/., 1991)

ferm entation aceta te form ate

SOC --- > CO] + 2H^ + CHCOO + HCOO Eq. 2.2 (Chapelle, 1993)

Wassenaar et al. (1991) also suggested that DOC in aquifers produced by fermentation may subsequently undergo condensation, resulting in an autochthonous production of humic substances (Krom and Sholkovitz, 1977) (Equation 2.3).

con d en sation

(33)

2.4.3 Aquitard sources of DOC

Another potential source of DOC in groundwaters is the diffusion of organic acids from the aquitard sediments that confine aquifers, such as shales and clay. In aquitards the rates of microbial fermentation, i.e. the production of simple organic compounds such as formate, acetate and CO?, exceeds the rates of the organic acid consumption, allowing organic acids to accumulate in the aquitard (Chapelle and Bradley, 1996; McMahon and Chapelle, 1991). The LMW DOC in aquitard sediments from the Atlantic coastal plain contained 55)LLM-5mM of formate, acetate and propionate, whereas the aquifer sediments contained lesser amounts, 30-55|LiM, and ground water in the aquifer contained only

trace amounts of formate (3 pM) and acetate (<3pM) (Chapelle and Bradley, 1996; McMahon and Chapelle, 1991).

The concentration gradient between the aquitard and aquifer results in a net diffusive flux of organic acids from the aquitard into the aquifer (Chapelle and Bradley, 1996). The diffusivity of formate in the Atlantic coastal plain aquifer was 1.4 x 10'^ m“/year and acetate 8.7 x 10 " mVyear (McMahon and Chapelle, 1991). DOC from aquitards could be sufficient to maintain bacterial respiration in aquifers with low autochthonous DOC concentrations (McMahon and Chapelle, 1991). Therefore, it is possible that this flux of organic acids from the aquitard may be the limiting factor controlling microbial respiration in aquifers which contain low concentrations of organic matter. It was also suggested by Murphy et al. (1989) that the primary source of organic matter in the Milk River aquifer, Canada, was organic-rich shale layers present in the aquifer. In some aquifers a proportion of recharge can occur as leakage from aquitards (Egerton, 1994; Fetter, 1994; Schwarz, 1996) and the diffusive flux of organic acids from aquitards into aquifers must be enhanced by this leakage.

2.4.4 Biomass and necromass sources

(34)

(Chapelle, 1993)), and therefore the necromass, consists mainly of bacterial cells (Chapelle, 1993). Although necromass is a possible source of DOC in aquifers no data are available to date of how much organic matter in groundwaters and aquifer and aquitard sediments might be necromass.

2.5 Sinks

The processes that remove DOC from aquifers are oxidation-reduction reactions, sorption, precipitation, volatilization and complexation; these are discussed below.

2.5.1 Oxidation/reduction

The most important processes that consume organic matter in aquifers are reduction/oxidation, or ‘redox’, reactions. Studies of redox reactions in aquifers developed from studies of diagenetics in marine sediments (Lynn and Bonatti, 1965; Presley et a i, 1967; Froelich et a i, 1979), where the high flux of organic carbon and minerals into the sediment causes chemical changes to pore water chemistry (e.g. Fig. 2.4). The same reactions, or changes, influence groundwater quality (e.g. Champ et a l,

1979; Edmunds et a i, 1982), although at slower rates, due to the lower concentrations of carbon, and perhaps other nutrients, in aquifers than in ocean sediments.

Redox reactions can be described as by the transfer of electrons, oxygen or hydrogen from one species to another, e.g. Eq. 2.4, which shows the reaction of pyrite with Fe (III), where the pyrite is oxidized and the iron is reduced from ferrous to ferric iron . (Appelo and Postma, 1994).

FeSi + 14Fe’"^ + 8H2O — > 15Fe^* + 2 5 0 4^' + 16H+ Eq. 2.4

Redox reactions (such as nitrate reduction) occur in groundwaters at measurable rates because they are catalysed by bacterial enzymes. Bacteria require energy and a source of cellulai' carbon and they derive both by redox reactions in groundwaters (Schlegal,

(35)

20

-'2

40 60

n o;

10___ 20 30

M n

1 2

*

+

O x id izin g

con d itio n s o

1001

R e d u c in g c o nd itio ns

2001

Fe"

O x id izin g

c ond itio ns

n

100 K

SO:

10 20 30

+ ' * + + + +

< = a n a ly tic a l b la n k v a lu e s

R e d u c in g c ond itio ns

+

+

*

200

Figure 2.4 Pore water profiles o f marine sediment core with porewater data. All units are in micromoles/litre except dissolved oxygen which

is expressed as % saturation (After Thomson et al, 1997).

Bacteria are the predominant form of life in subsurface environments (Balkwill and Ghiorse, 1985) and substantial numbers of viable bacteria are found in a wide range ' of aquifer sediments (Wilson et a i, 1983; Balkwill and Ghiorse, 1985; Chapelle et a l,

1988; Balkwill, 1989; Fredrickson et a l, 1989; Hicks and Fredrickson, 1989; Balkwill, 1990; Konopka and Turco, 1991; Severson et a l, 1991). Up to 95% of bacteria are found attached to sediment particles (Mitchell, 1978; Parker and James, ^

1985; Fredrickson et a l, 1989, Balkwill, 1990; Chapelle, 1993), possibly because the ^ nutrients required by bacteria are concentrated at the solid-liquid interface (Mitchell,

1978).

(36)

This is probably as a result of the smaller pore throat diameters found in clays, which severely restrict the movement of bacteria in the clays (Thurman, 1985; Chapelle and Lovley, 1990; Fontes et a l, 1991; Chapelle, 1993). The porosity and hydraulic conductivity of clays are also much smaller and this reduction in the flow of water in clays would restrict the supply of nutrients to the bacteria, again reducing their activity.

In theory, bacterially-catalysed redox reactions occur in aquifers sequentially in order of energy yield (Table 2.1).

Table 2.1 Processes and reactions for the bacterially mediated oxidation of organic

Process Reaction

(kJ/mol C H2O) I. Aerobic respiration C H2O + 0 2 - > C O2 + H2O -475 ii. Denitrification 5CH2O + 4NO_r - > 2N2 + 4HCO’' + CO2

+ 3H2O -448

hi. Manganese reduction C H2O + 3COt+ H2O + Mn02

2Mn^* + 4HCO’- -349

iv. Iron reduction CH2O +7CO2 + 4Fe(OH)3

- 4 4Fe"* +8H C O /' + 3H2O -114

V. Sulphate reduction 2CH2O + SO4' ^ H2S + 2HCO’- -77

vi. Methanogenesis 4H2 + C O2 —^ C H4 + 2H2O -32.4

2C H3O O H C H4 + C O2 -31

Despite this theoretical sequence, separate zones of oxygen, nitrate, manganese and sulphate reduction are rarely reported to occur. Three redox zones only (oxygen-nitrate, iron-manganese, and sulphide) were reported by Champ et al. (1979) and Thomson et at. (1993), whilst Lovley et al. (1994) reported 5 redox zones: the oxic zone, nitrate- manganese reduction zone, iron reduction zone, sulphate reduction zone, and the methanogenic zone. Only two zones of reduction, nitrate-oxygen and iron-manganese, are seen in the Chalk aquifer (Edmunds et al., 1987 and see Chapter 5) and the Lower Greensand aquifer (see Chapter 4).

(37)

t t

develop in the aquifer sediments, surrounded by oxygen-rich groundwater, allowing nitrate reduction to proceed at the same aquifer depths as oxygen reduction (Appelo and Postma, 1994 and Hiscock^etal 1991). Bacterial populations may occur as multi-functional populations that can metabolise different redox species at the same locality in the aquifer. Whilst some bacteria metabolise only one redox species, e.g. obligate anaerobes and Paracoccus denitrificans (Schlegel, 1985), other co-existing bacteria are physiologically flexible (Wellsbury et al, 1997) and may metabolise more than one species, e.g. both oxygen and nitrate.

The redox reactions shown in Table 2.1 are discussed below.

Oxygen reduction is the most energetically favourable mechanism for bacteria to

oxidize organic carbon (Table 2.1). Wassenaar et al. (1991) attributed nearly all the DOC losses between 1 and 6.5m depth of an aquifer to bacterial oxygen reduction. Inorganic, non-microbial oxygen reduction can also occur in aquifers when oxygen reacts with any reduced species, such as Fe(II) (e.g. Eq. 2.6) and pyrite, as occurs in the Lower Greensand and Chalk aquifers (see Chapters 4 and 5).

^ 2 + , T T + , . r 3 +

Fe'+ + 2H+ + 0 2 -^ Fe'^ -h H2O Eq. 2.6

Denitrification (Table 2.1) is a microbial process important to the water supply

industry, as nitrate in drinking water is thought to be associated with adverse health affects, such as stomach cancer, and methaemoglobinemia ( ‘blue baby syndrome’) which has occurred when the nitrate concentrations in drinking water were > 1 0 0 mg/l (Addiscott et al, 1992; Schlegel, 1986). Autotrophic denitrification (Eq. 2.7) may also occur in aquifers (Gayle et al., 1989), although is not common (Hiscock et al. , 1991). Nitrate can also be reduced by the bacterially catalyzed reaction with pyrite and Fe(II) (Appelo and Postma, 1994).

(38)

Manganese reduction in aquifers (Table 2.1) can improve water quality as the maximum admissible concentration of manganese in the UK is 50 jig/1 (Appendix 1), although manganese is usually present in natural groundwaters in concentrations approaching the detection limit of most analytical methods, <0.01 mg/l (See Chapters 4 and 5). In groundwater manganese reduction is usually associated with iron oxidation (Eq. 2.8) (Appelo and Postma, 1994).

2Fe^^ + MnOi + 4IT ^ 2Fe’* + 2H2O Eq. 2.8

Iron reduction (Table 2.1), as mediated by bacteria, is a process not well understood,

although it is thought that the bacteria responsible for iron reduction are heterotrophic (Chapelle, 1993). In oxic groundwaters Fe^"^ is oxidized by bacteria to Fe'^‘^(Eq. 2.9), precipitating as iron-oxyhydroxides (Eq. 2.10) (Chapelle, 1993). Possible sources of iron in groundwater are the dissolution of iron-bearing silicates (e.g. amphiboles and pyroxenes), pyrite, magnetite, clay minerals or the reduction of iron-oxyhydroxides.

Fe^"" + 4H"' + O2 —> Fe’* + 2 H :0 Eq. 2.9

Fe’* + OH— > FeOOH Eq. 2.10

Sulphate reduction (Table 2.1 ) by sulphate reducing bacteria can only occur in anoxic

conditions and is usually found in deeper parts of confined aquifer systems (Raiswell and Berner, 1986). Possible sources of sulphate in groundwater can be from the oxidation of pyrite, dissolution of gypsum and mixing with seawater or formation water (Appelo and Postma, 1994).

Methanogenesis (Table 2.1) usually only occurs in the deepest confined aquifers

(Chapelle, 1993).

2.5.2 Adsorption

(39)

adsorption prevents organic matter migrating to groundwater from the soil zone. In soil waters. Wassenaar et al. (1991 ) measured DOC concentrations of 32.9 mg C/1 at the top of a sod zone, and only 3.2 mg C/1 at its base.

Hydrophobic sorption occurs as non-polar organic molecules partition onto non-polar organic solids from a polar water phase (Allen-King et al., 1996). Polar sorption occurs through the sharing of a hydrogen atom between the DOC and a mineral surface and is particularly important for adsorbing organic matter onto silica, alumina and iron- hydroxide surfaces (Fig. 2.5). It is particularly important in the B horizon in soils where DOC-metal complexes are bonded to the surface of clays particles (Fig. 2.3).

The ion exchange capacity of sediments depends on the mineralogy and surface area of aquifer minerals, and possibly the area comprised of organic coatings on minerals (Thurman. 1985). The cation exchange capacity of sediments is usually 10-40 meq per 100 grams of sediment. Organic matter has a high cation exchange capacity (approximately 150-300 meq of cation-exchange capacity per 100 grams of organic matter) because it contains carboxyl groups which ionize at the pH of normal water (Thurman. 1985). Silt, clay and/or organic-rich aquifers have high cation exchange capacities. Anion exchange is only important with alumina surfaces, as the anion exchange capacity of nearly all other geologic materials is zero (Thurman, 1985).

Silica

'Or-a

N H — O —

Si—o

àrdiM

Alum ina

o

J

H OoC —c —N

A d so r b e n ts

/ H

H

O rgan ic b a s e s

H— O —

IS '

% / )

O rgan ic a c id s

(40)

2.5.3 Precipitation

Precipitation may also contribute to the removal of organics from groundwater. This is a minor process in most parts of the aquifer, because it only occurs when low ionic strength waters mix with those of a greater strength: if there is a 10 to 100 fold increase in the ionic strength of the resulting water, organic acids of low water solubility may precipitate with salts, ‘salting out’(Thurman, 1985). Precipitation of DOC may occur in the most confined parts of the aquifer, where meteoric waters mix with formation waters, and at saline incursions near the coast.

2.5.4 Volatilization

The volatilization of DOC is a very minor process, only occurring in the unconfined part of the aquifer, because less than 1% of the natural dissolved organic matter in rivers and streams is volatile at the temperatures normally found in natural waters (Thurman, 1985). Volatilization is a more important process in contaminated groundwaters, as many organic contaminants are volatile at much lower temperatures than the DOC found in natural waters (see Section 2),

2.5.5 Complexation

(41)

CHAPTER 3

SAMPLING METHODOLOGY AND ANALYSIS OF SAMPLES

3.1 Introduction to DOC sampling

3.2 Sample containers

3.3 Sample collection

3.3.1 Cleanliness

3.4 Filtration

3.5 DOC sample preservation

3.5.1 Acidification and sparging 3.5.2 Freezing

3.5.2.1 Freezing using LDPE 3.5.2.2 Freezing using PP 3.5.3 Storage

3.6 Blanks

3.6.1 Trip blanks 3.6.2 Field blanks

3.7 Well head sampling

(42)

3.1 Introduction to DOC sampling

When sampling groundwater for DOC, or any other constituent, the need to obtain accurate data is absolute. The problems of obtaining DOC samples representative of the groundwater in the aquifer have been widely discussed (Sharp et al., 1993; Turrell, 1994; Tupas, 1994, Sugimura and Suzuki, 1988; Wilhams et a l, 1993). During the sampling and storage of water new surfaces, pressures, temperatures, light levels and contamination can alter the DOC concentrations on a time scale of hours (Sharp et al.,

1993). As much as 20% of the DOC in unpreserved samples may disappear in less than one day (Kirchman et al., 1991), so groundwater samples must be preserved as soon as possible after sampling, in order to minimise change.

In this study, a sampling strategy was used that allowed the collection of water samples that were representative of the DOC concentration in the groundwater, and that prevented the DOC concentration from changing before analysis was complete. Following initial studies at University College London (McArthur, 1995, pers. comm.; Turrell, 1994) a sampling strategy was developed by the author, specifically to overcome the problem of obtaining low DOC field blanks and ensuring sample integrity at low (<1 mg/l) DOC concentrations. Field blanks are blanks filled in the field with ultra-pure water from a Winchester, and laboratory blanks are blanks filled in the laboratory with ultra-pure water.

For this sampling strategy development several aspects of sampling were examined: • sample containers: how the nature of the container influenced DOC concentrations; • sample collection: how the physical act of sampling influenced DOC concentrations; • cleanliness: what standards were necessary to avoid contaminating samples;

• filtration: how filtration influenced DOC concentrations;

• acidification and sparging: how bacterial alteration can be minimised by removing a source of metabolite COo.

(43)

3.2 Sample containers

Containers used to store DOC samples must be rendered free from any contaminant organics (Sharp et al., 1993). Samples were therefore collected in glass containers, as they are more easily cleaned of organics (Sharp et al., 1993). Results from this thesis suggest that organics leaching from plastic containers contaminate samples with DOC (see Chapter 3.5.1). Glass containers of 50 ml volume, made by Shimadzu, were used to collect samples because this minimised sample manipulation; vials are placed in the Shimadzu 5000 auto-sampler for analysis, rather than being transferred to other sample bottles, a process that can lead to contamination (Turrell, 1994). Shimadzu glass was preferred to Pyrex glass, as the latter had been shown to result in DOC in samples increasing with time (McArthur, 1993. Pers. comm.).

The Shimadzu vials were cleaned for 24 hours with chromic acid, a strong oxidising agent, rather than baking as the latter method might activate the surface of the glass and lead to the sorption or organics onto the container walls (Sharp et al., 1993). Following cleaning, vials were rinsed with ultra-pure water and filled with 1% sodium metabisulphite, a strong reducing agent and bacterial poison, which prevents bacterial growth. The vials were sealed with aluminium foil, kept in place with a Teflon cap.

3.3 Sample Collection

(44)

3.3.1 Cleanliness

Because of the low DOC concentrations measured, a rigorously clean sampling strategy had to be employed. Fingerprints contain high concentrations of organic compounds (Sharp et a i, 1993); a fingerprint on the inside of a Shimadzu vial can contribute 1 mg/l DOC (Turrell, 1994), so powder-free, surgical gloves were worn at all times during the sampling, acidification and sparging procedures, to prevent contamination from skin. DOC samples from boreholes were taken directly from the sampling tap or outlet valve and DOC samples from springs were taken as close to the spring source as possible. Sampling taps were washed with sodium metabisulphite and then allowed to run for about five minutes prior to sampling. The boreholes were either pumping constantly or were purged for approximately 15 minutes prior to sampling.

3.4 Filtration

DOC samples collected for this study were not filtered, and therefore the DOC concentration in the sample is a measure of the total non-volatile organic carbon, rather than the non-particulate DOC. As particulate DOC usually comprises <5% of the total organic carbon in groundwater it contributes very little to the overall DOC concentration in unfiltered groundwater samples (Sharp et al., 1993).

Filtration removes particulate organic matter (molecules with a diameter >0.45 |im) and sterilises samples; bacteria alter the DOC concentration in a sample by metabolising the DOC (see Chapter 2). There are approximately 100-1000 bacteria per ml of groundwater (Chapelle, 1993) and each bacterium has a mass in the order of 10'^ mg (Schlegal, 1985), so bacterial mass does not form a significant part of the DOC concentration in unfiltered groundwater samples. Bacterial growth and metabolism of DOC can be prevented in unfiltered samples, by acidification and sparging of the sample which renders all the bacteria in the sample inactive (see Chapter 3.5).

(45)

the filter (Sharp et a l, 1993; Norrman, 1993; Turrell, 1994). This was another reason for not filtering DOC samples.

3. 5 Sample preservation

3.5.1 Acidification and sparging

Samples for DOC analysis were acidified to pH 3.0 in the field and sparged with nitrogen gas for 15 minutes in order to remove dissolved inorganic carbon (DIC) and to preserve samples. A low pH minimises heterotrophic activity but converts bicarbonate (HCO3 ) to carbon dioxide, which promotes autotrophic bacterial activity. Sparging with nitrogen gas removes this carbon dioxide. The downside of sparging after acidification is that it removes any acid-volatile DOC. Until a method is developed for DOC analysis in the presence of DIC, this problem cannot be overcome, although the volatile organic carbon fraction in groundwaters is usually less than 0.05 mg/l (Thurman, 1985).

Acidification was done with 1.6 M sulphuric acid from a glass syringe with a hypodermic needle. Acid was added through the aluminium foil of the sample vial during sparging in order to minimise exposure of the sample to the atmosphere and to maintain positive pressure in the vial during addition, further minimising the potential for contamination. Sparging was done by hypodermic needles inserted through the aluminium foil caps, using nitrogen from a portable nitrogen cylinder.

(46)

Table 3.1. DOC in field and laboratory blanks untreated, and treated in various ways. All concentrations in mg/l.

LA

Field blanks Laboratory blanks

Untreated Acidified (Hach) and

sparged

Acidified (syringe) and sparged

Untreated Acidified (Hach)

0.043 0.081 0.035 0.050 0.042

0.071 0.055 0.041 0.033 0.070

0 .0 1 1 0.131 0.048 0.049 0.041

0.053 0.068 0.036 0.024 0.041

(47)

Acidification of ultra-pure water laboratory blanks using a Hach titrator did not introduce any significant source of DOC contamination (Table 3.1), but acidification of ultra-pure water blanks in the field using a Hach titrator introduced slight contamination (Table 3.1). Acidification and sparging of ultra-pure water blanks in the field using acid from a glass syringe gave lower blanks with a lower range (Table 3.1). This suggests that acidifying blanks using a syringe gives more reproducible results than acidifying blanks using a Hach titrator.

3.5.2 Freezing

Sparging and acidifying groundwater samples is effective in preserving DOC samples, but is time consuming (15 minutes per sample), and is impractical for remote areas due to the need for bottled nitrogen. Freezing may preserve DOC samples but Sugimura and Suzuki (1988) noted a 15-20% loss of DOC in freezing. However, they gave no details of sample handling nor freezing method, nor why the DOC loss occurred. Sharp et al. (1993) suggested that quick freezing may prevent this loss.

In order to test this hypothesis, and so develop a more practical preservation methodology, field trials were carried out to determine the effectiveness of freezing. Shimadzu vials proved fragile during freezing in dry ice, whether the vials were full or half full, so alternative containers were therefore sought. Plastic vials were tried, although previously avoided because of the risk of contamination from plasticisers. Using ultra-pure water and freezing using dry ice (-78 "C), comparisons were made of laboratory and field blanks after freezing in acid washed, 30 ml low density polyethylene (LDPE) vials and freezing in acid washed 50 ml, polypropylene vials. The results are discussed below.

3.5.2.1 Freezing using LDPE

(48)

Table 3.2. DOC using LDPE Nalgene bottles.

DOC (m g /l) in field blanks, a cid ified and

sparged

DOC (m g /l) in field blan k s, frozen

0.035 0.053

0.017 0.066

0.081 0.080

0.027 0.034

0.038 0.058

M ean 0.040 ±0.049 M e a n 0.058 ±0.034

The samples were transferred in a laminar flow hood, containing an activated carbon filter, which provides a contamination free environment. There was no significant increase in the DOC concentration in blanks transferred between vials (Table 3.3).

Table 3.3. Results of trails to determine whether the transferring of samples between

DOC (mg/l) in lab blank not transferred between

vials

DOC (mg/l) in lab blank transferred with hood off

DOC (mg/l) in lab blank transferred with hood on

0.006 0.003 0.029

0.006 0 .0 2 1 0 .0 1 0

0.033 0.078 0.030

0.140 0.016 0.015

0.006 0.033 0 .0 1 0

Mean 0.013 ±0.049 Mean 0.030 ±0.034 Mean 0.019 ±0.020

3.52.2 Freezing using PP

(49)

analysed after 1 hour, and 10 vials placed in a freezer and analysed for DOC 24 hours later, after thawing by two separate methods (natural thawing at room temperature, hot thawing under hot water) (Table 3.4).

Table 3.4. Resu ts of freezing trials using polypropy ene (PP) centrifuge tubes

DOC (m g/l) in blank in glass

vials

DOC (m g/l) in PP blank analysed

im m ediately

D OC (m g/l) in PP blank after 1

hour

D O C (m g /l) in PP blank, frozen, and

hot thawed'

D O C (m g/l) in PP blank, frozen, and

thawed^

0 . 0 1 2 0.049 0.061 0.024 0.055

0.009 0.024 0.015 1.54 0.104

0.005 0.037 0.014 1.57 0.107

0.024 0.078 0.040 0.391 0.027

0 . 0 1 2 0.024 0.022 0.205 0.103

Mean 0.012

±0.014

Mean 0.042

±0.045

M ean 0 .0 3 0

±0.040

M ean 0.746

±1.50

Mean 0.079

±0.073

= the blanks were frozen in the PP vials and then thawed quickly by running the bottom half of the vial under hot running water, ensuring that the top of the vial did not come into contact with the hot water. The thawing took approximately 4 minutes.

" = the blanks were frozen in the PP vials and then thawed at room temperature. The thawing took approximately three hours.

There was a considerable difference between the DOC in the blanks in the glass vials and the DOC in blanks in PP tubes frozen and then hot thawed, although the hot thawed blanks had a very large range. These results show that freezing blanks in PP gives elevated DOC concentrations and bad reproducibility, suggesting that during the freezing and thawing process some breakdown of the plastic occurs and leaches into the blank.

It is concluded that, until more trials can be carried out to determine which plastics can withstand the freezing process and can be used for DOC collection, that DOC samples be preserved by acidification and sparging.

3.5.3 Storage

(50)

acidified and sp a rg e d can be s to re d at ro o m tem p eratu re, for up to o n e m o n th , with no significant cha nge in the D O C co n c e n tra tio n (Fig. 3.1 and Fig. 3.2) (T u rrell, 1994; M c A r th u r , 1995. pers. c o m m .).

2.9

2.74

2.54

Error bar

2.3

40 60 80 100 120 140 160

storage time (hours)

Figure 3.1. Chart showing the results of storing acidified and sparged samples for up to six days. The error bar was calculated from the mean deviation about the mean of 28 DOC samples (after Turrell, 1993).

1.3

1 '-2- E O Q

LlJ

E rro r i b a r I

10

s to r a g e tim e (d a y s)

(51)

3.6 Blanks

3.6.1 Trip blanks

The effects of storage of blanks in cleaned Shimadzu vials was assessed. DOC concentrations in lab blanks analysed for DOC immediately, were compared with DOC concentrations in trip blanks (blanks made in the laboratory, taken into the field and returned unopened) stored in cleaned Shimadzu vials for four days (Table 3.5). The results suggest that storage in cleaned vials does not contribute any significant DOC contamination.

Table 3.5. Results of laboratory blank storage trials.

DOC (mg/L) lab blanks DOC (mg/L) in lab blanks stored for 4 days

0.056 0.043

0.065 0.036

0 .0 1 0 0.063

0.036 0.041

Mean 0.042 ±0.049 Mean 0.046 ±0.024

3.6.2 Field blanks

Figure

Figure 2.1 The size continuum of particulate and dissolved organic carbon in naturalwaters (After Thurman, 1985).
Figure 2.3 Podzolisation and decrease of organic matter in interstitial water of soils (After Thurman, 1985).
Figure 2.4 Pore water profiles of marine sediment core with porewater data. All units are in micromoles/litre except dissolved oxygen which is expressed as % saturation (After Thomson et al, 1997).
Figure 4.4b The Lower Greensand of Sussex and Surrey with groundwater contours on the Folkestone Beds (After Hydrogeological Map of the South West Chilterns and the Berkshire and Marlborough Downs, 1978 and Morgan-Jones, 1985).
+7

References

Related documents

[r]

Rezakazemi M, Maghami M, Mohammadi T (2018) High Loaded synthetic hazardous wastewater treatment using lab-scale submerged ceramic membrane bioreactor. Li Y, Wang C (2008)

The DIFC courts' jurisdiction was initially established in Article 5(A)(1) of Dubai Law 12/2004 (as amended), which states that the Court of First Instance shall have

To protect your information, use Windows backup and restore utilities to back up individual files and folders, back up your entire hard drive, create system repair media (select

The Frankfurt School provides top-end investment banking and quantitative finance education – which is why we encourage employees to study their programmes.. ACI – The

Field experiments were conducted at Ebonyi State University Research Farm during 2009 and 2010 farming seasons to evaluate the effect of intercropping maize with

To the finest of our information, this is the first report of the use of PEG-400 and DMAP, DIPEA catalyst for the protection of secondary amine of pyrazole nucleus using Boc..

Preparation, characterization, and immunological properties in mice of Escherichia coli O157 O-Specific polysaccharide- protein conjugate vaccines. Konadu EY, Parke JC, Tran