Estimating bioaccessibility, phytoavailability and phytotoxicity of contaminant arsenic in soils at

257 Pages·2009·8.7 MB·English

by  O. Mojsilovic

Estimating bioaccessibility, phytoavailability and
phytotoxicity of contaminant arsenic in soils
at former sheep dip sites

A thesis
submitted in partial fulfilment
of the requirements for the Degree of
Master of Science
at
Lincoln University
by
O. Mojsilovic

Lincoln University
2009
Abstract of a thesis submitted in partial fulfilment of the requirements for
the Degree of M.Sc.
Estimating bioaccessibility, phytoavailability and
phytotoxicity of contaminant arsenic in soils
at former sheep dip sites
by
O. Mojsilovic

Recognition that the bioavailability of soil As (As) is influenced by its soil dynamics has
initiated research into development of more accurate, site-specific soil guideline values,
departing from the assumption that the total soil As content is bioavailable. With the aim of
deriving predictive models, the relationship between soil properties and As bioavailability
(bioaccessibility and phytotoxicity) was examined on a set of naturally contaminated sheep
dip soils (n = 30). Sampled soils were extensively characterised, bioaccessibility was
estimated through an in vitro procedure, and soil As toxicity and availability to plants were
evaluated using an early growth wheat bioassay.
The in vitro bioaccessibility was consistently less than the total soil As content. Arsenic
bioaccessibility was negatively correlated to soil iron (Fe), manganese (Mn) and aluminium
(Al) contents, and it was positively related to the soil As loading. The in vitro extractable soil
As concentrations were successfully modelled using linear combinations of soil As content,
soil Fe and Mn determinations and soil pH.
Differences in As phytotoxicity, expressed in terms of effective toxic concentration (EC50),
between soils were directly related to soil Fe, Mn and Al contents. Available soil phosphorous
(P) exerted an ameliorating effect on As toxicity, with the available soil As/P ratio
representing the single best predictor of plant growth suppression. Plant P nutrition appeared
to influence the relative selectivity for As and P by wheat, with greater selectivity for P
demonstrated under P deficient conditions.
Plant As uptake, its distribution, and also the plant nutrient status were all adversely affected
by increasing soil As exposure. Co-contamination by Zn corresponded to a substantial
elevation in proportion of the plant As allocated in shoots. Plant As levels exhibited a

ii
saturation-dependent relationship with increasing soil As. The best linear predictors of plant
As levels in the non-toxic range were RHIZO-extractable and effective soil As concentrations,
the latter based on the diffusive gradients in thin films (DGT) technique.
Despite the complexity of soil As dynamics, large proportions in the variances exhibited by
the two measures of bioavailability were explained using a small set of readily-available soil
properties.

Keywords: sheep dips, As bioavailability, As bioaccessibility, As phytotoxicity, wheat, As
availability indices, diffusive gradients in thin films (DGT).

iii
Contents
Abstract..............................................................................................................................ii
Figures................................................................................................................................v
Tables................................................................................................................................ix
1 Introduction................................................................................................................1
1.1 Project rationale
1.2 Project hypothesis
1.3 Project objectives
2 Literature Review.......................................................................................................3
2.1 Arsenic in Sheep Dips: History of use and current concerns
2.2 Soil arsenic dynamics
2.3 Arsenic bioaccessibility
2.4 Arsenic phytoavailability and phytotoxicity
3 Methodology............................................................................................................31
3.1 Sample inventory and site selection
3.2 Soil sampling, preparation and analysis
3.3 Arsenic bioaccessibility
3.4 Arsenic phytoavailability & phytotoxicity
3.5 Post-harvest analysis: Soil Solution and Diffusive Gradients in Thin Films
3.6 Data analysis
4 Results......................................................................................................................45
4.1 Soil Characterisation
4.2 In vitro arsenic bioaccessibility
4.3 Arsenic phytotoxicity
4.4 Plant arsenic uptake
5 Discussion..............................................................................................................182
5.1 Sample Characterisation
5.2 In vitro arsenic bioaccessibility
5.3 Arsenic phytotoxicity
5.4 Plant arsenic uptake
6 Conclusions...........................................................................................................220
7 Acknowledgments.................................................................................................223
8 References .............................................................................................................224
9 Appendices ............................................................................................................235
9.1 Appendix A: Site Plans
9.2 Appendix B: Speciation of soil pore water arsenic
9.3 Appendix C: Bioassay plates

iv
Figures
Figure 2.1.1. A simple schematic showing a basic fate of As in soil coupled with pathways of As exposure to humans, plants
and animals........................................................................................................................................................................5
Figure 2.2.1.Simplified schematic of As dynamics in soil (Adopted from Zhang and Selim (2008b)).........................................6
Figure 2.4.1. Total yield of thee grass species versus the available soil P/As ratios following application of As (0 - 3.53
kg/m2) and/or phosphorous (0 - 5 kg/m2). Solid lines represent the fitted 3 parameter sigmoid curves. Data was
published in Carrow et al. (1975)....................................................................................................................................25
Figure 2.4.2. Conceptual schematic of soil and plant controls on the availability of As in aerobic soils. Reproduced from Fitz
and Wenzel (2006)............................................................................................................................................................30
Figure 3.5.1. Cross-sectional view of a DGT device deployed in soil. A linear concentration gradient is shown across the
diffusive gel, the flux introducing a localised sink in interfacial solute concentration (Cdgt) due to diffusional and
kinetic limitations to supply. The solute concentration at the interface is lower than in bulk soil pore space (Cpw),
indicating a patrial resupply from the soil solid phase....................................................................................................39
Figure 3.5.2. Cross-sectional view of an assembled DGT device, the two gels and a filter membrane are laid over a flat
piston-like surface, and fixed using a ring with a fixed exposure window........................................................................41
Figure 4.1.1. Histogram of total soil arsenic concentrations (n = 30) for a set of surface samples collected from sheep dip
sites, highlighting the log-normal distribution of the dataset...........................................................................................48
Figure 4.1.2. Comparison of total soil arsenic content (mg/kg) as determined by the microwave-assisted aqua regia digestion
and ICP-OES analysis and the block aqua regia digestion and FAAS analysis (mg/kg)..................................................49
Figure 4.1.3. Comparison in select soil properties between a dataset of sheep dip surface soils (n = 30) verus New Zealand
Soils Database’s surface (A) horizon data. Dots represent the 5th and the 95th percentiles...........................................52
Figure 4.1.4. The association between total soil arsenic (As) and antimony (Sb) concentrations, plotted on a log-log scale.
Groups identify different contaminant signatures............................................................................................................53
Figure 4.1.5. Log-log association between the total soil arsenic (As, mg/kg) and amorphous iron (Fe) oxide content (g/kg).54
Figure 4.1.6. The distribution of soil arsenic (As) between sheep dip soil samples (upper) and between different fractions
steps of the sequential extraction (lower), displaying the uniformity and the relative importance of different sequential
extraction steps.................................................................................................................................................................59
Figure 4.1.7. The recovery of total soil arsenic (As) released by the sequential extraction procedure: total soil As,
determined by the microwave-assisted aqua regia digestion, versus the sum of five As sequential extraction fractions.60
Figure 4.1.8. Association between the soil pore water arsenic (As, mg/l) and non-specifically adsorbed As, represented by
(NH )SO-extractable As (mg/kg)...................................................................................................................................66
42 4
Figure 4.1.9. Association between the fraction of soil pore water arsenic (mg/l) versus the non-specifically adsorbed soil
arsenic ((NH4)SO-extractable As; mg/kg) for soils from site G (n=6)..........................................................................68
2 4
Figure 4.1.10. Relationship between the mass of arsenic (As) accumulated on the Fe oxide gel of DGT devices and As
concentrations in the external solution. Time of exposure = 24 h. Values represent means, error bars are a single
standard deviation (n = 3)................................................................................................................................................70
Figure 4.1.11. Relationship between mass of arsenic (As) accumulated on the Fe oxide gel of DGT devices and increasing
exposure time of the DGT devices. External concentration was constant at 100 µg/l As. Values represent means, error
bars are a single standard deviation (n = 3.)...................................................................................................................70
Figure 4.1.12. Relationship between the mass of phosphorous (P) accumulated on the Fe oxide gel of DGT devices and
increasing DGT exposure times. External concentration were constant at 100µg/l P. Values represent means, error
bars are a single standard deviation (n = 3)....................................................................................................................71
Figure 4.1.13. The association between the mass of soil arsenic (As) and phosphorous (P) accumulated by the DGT Fe oxide
gel (moles, 24 hour exposure) and the soluble concentration of As and P (M) for sheep dip soils. The upper reference
line (long dashes) represents the Fe oxide capacity as reported by Panther et al. (2008) for As. The lower reference line
represents the Fe oxide capacity for P as reported by Zhang et al. (1998a) for P...........................................................76
Figure 4.1.14. Log-log association between arsenic diffusive gradients in thin films concentrations (As , mg/l) and total
DGT
soluble As concentrations (mg/l)......................................................................................................................................78
Figure 4.1.15. Log-log association between phosphorous diffusive gradients in thin films concentrations (P , mg/l) and
DGT
total soluble P concentrations (mg/l)...............................................................................................................................78
Figure 4.1.16. The association between the ratio of DGT to soluble arsenic concentrations (As ) and (A) soluble
R
phosphorous concentrations, and (B) the arsenic labile solid phase distribution coefficient (As ). Independent
Kd
variables are plotted on a log scale..................................................................................................................................79
Figure 4.1.17. Association between the effective soil arsenic concentrations (As ; mg/l) and the DGT arsenic concentrations
E
(As ; mg/l) of sheep dip soils........................................................................................................................................83
DGT

v
Figure 4.1.18. Association between the effective soil arsenic concentrations (As ; mg/l) and the total soil pore water arsenic
E
concentrations (As , mg/l) of sheep dip samples...........................................................................................................83
SOL
Figure 4.2.1. Variation in the proportion of total soil arsenic extracted by the in vitro procedure across the sheep dip soils.
The proportion bioaccessible is shown in black...............................................................................................................85
Figure 4.2.2. Log-log association between soil arsenic (As) and iron (Fe) concentrations extracted by the in vitro procedure.
.........................................................................................................................................................................................86
Figure 4.2.3. The association between the relative bioaccessibility of iron (Fe, %) verus the proportion of Fe oxide content
accounted by the amorphous Fe fraction.........................................................................................................................86
Figure 4.2.4. Individual value plots of arsenic (As) bioaccessibility between different sheep dip locations, and the box and
whisker plot of the distribution of relative As bioaccessibility for the entire dataset (n = 30).........................................87
Figure 4.2.5. In vitro extractable soil arsenic (As, mg/kg) versus NHOH.HCl extractable soil As (mg/kg) concentrations..88
2
Figure 4.2.6. Log-log association between the in vitro extractable soil arsenic (As) and soil As associated with amorphous
Fe/Al oxides, the latter determined as part of a sequential As fractionation....................................................................90
Figure 4.2.7. Log-log association between in vitro soil extractable arsenic (As, mg/kg) and soil As extracted by the Olsen P
reagent (mg/kg)................................................................................................................................................................90
Figure 4.2.8. Log-log plot of in vitro extractable soil arsenic (As, mg/kg) and the soil amorphous iron oxide fraction (Fe,
g/kg).................................................................................................................................................................................91
Figure 4.2.9. The association between the relative arsenic bioaccessibility (%) and log-transformed soil crystalline
aluminium oxide content (g/kg)........................................................................................................................................92
Figure 4.2.10. The association between the arsenic relative bioaccessibility (%) and log-transformed crystalline Fe oxide
content (g/kg)...................................................................................................................................................................93
Figure 4.2.11. The association between the relative arsenic bioaccessibility (%) and the soil amorphous Mn oxide content
(g/kg)................................................................................................................................................................................93
Figure 4.2.12. Associations between the relative arsenic bioaccessibility (%) and cation exchange capacity (a), clay
percentage (b), soil pH (c), and base saturation percentage (d)......................................................................................95
Figure 4.2.13. The association between the relative arsenic bioaccessibility (%) and the proportion of total soil arsenic
extracted by NH2OC.HCl (a), and the proportion of total soil arsenic associated with Step 4 of the sequential extraction
procedure (NH4-oxalate + ascorbic acid) (b)..................................................................................................................97
Figure 4.2.14. The association between relative arsenic bioaccessibility (%) and the secondary variable TRANS1 (crystalline
iron oxide concentrations adjusted by the inverse logarithm of total soil arsenic)..........................................................99
Figure 4.2.15. Model predicted versus observed in vitro extractable soil arsenic concentrations (mg/kg) for a sample of sheep
dip soils. Axes were constrained to 600 mg/kg...............................................................................................................106
Figure 4.2.16. Model predicted versus observed soil arsenic bioaccessibility (%) values for a sample of sheep dip soils.....106
Figure 4.2.17. Matrix graph showing the agreement between the Juhasz, et al. (2007b) observed relative arsenic
bioaccessibility (%) and that predicted by models developed within the current study..................................................112
Figure 4.2.18. Matrix graph showing the agreement between the Yang et al. (2002) observed relative arsenic bioaccessibility
(%) and that predicted by models developed within the current study...........................................................................112
Figure 4.3.1. Correlations between the three ecological endpoints monitored in the early growth bioassay: shoot biomass,
root biomass and germination success rate....................................................................................................................114
Figure 4.3.2. Relationships between shoot and root biomass of wheat plants at different sheep dip sites. Lines represent
linear regression fits for individual groups, with the slope coefficient listed (b)............................................................115
Figure 4.3.3. Growth of wheat plants across different soil arsenic concentrations. From left to right the soil samples and
their respective 0.05 M (NH )SO-extrctable arsenic concentrations are: S7 (below detection limits), H6 (2.2 mg/kg),
42 4
A3D (4.8 mg/kg), S4 (40 mg/kg), and A2D (160 mg/kg).................................................................................................116
Figure 4.3.4. Influence and interaction of zinc (soil pore water concentrations, mg/l) and arsenic (ratios of DGT effective
arsenic to phosphorous concentrations) on wheat shoot biomass (g dry weight per pot) for soil samples from site E (n =
4)....................................................................................................................................................................................118
Figure 4.3.5. Influence and interaction between lead (Pb, oxalate extractable soil concentrations, mg/kg), and arsenic (ratio
of DGT effective arsenic to phosphorous concentrations) on wheat shoot biomass (g dry weight per pot) for samples
from site G (n=6)............................................................................................................................................................118
Figure 4.3.6. Association between wheat shoot biomass (grams dry weight per pot) and the proportion of total soil arsenic
associated with crystalline Al/Fe oxides (%), determined as the fraction extracted in the step 4 of the sequential
extraction procedure......................................................................................................................................................119
Figure 4.3.7. Associations between wheat shoot biomass (g dry weight per pot) and solution arsenic and phosphorous (mg/l),
positioned on the left and the right respectively, for soil samples collected from site G (n = 6)....................................121

vi
Figure 4.3.8. Dose - response relationships between the wheat shoot biomass (g dry weight per pot, error bars equal one
standard deviation of a duplicate mean) and different determinations of available soil arsenic (n=28). Sigmoidal curves
were fitted to the data; summary table of results for each non-linear regression is shown in tables opposite the figures.
.......................................................................................................................................................................................124
Figure 4.3.9. Dose-response relationships between wheat shoot biomass (g dry weight) and the sum of non-specifically and
specifically sorbed soil arsenic (NH HPO-extractable, mg/kg) for different sheep dip sites. The sigmoid curves for Site
4 2 4
G and Waikato (Site A + B) samples were fitted by constraining the maximum shoot biomass to 0.80 g......................130
Figure 4.3.10. Association between the fitted wheat EC50 levels for four sheep dip sites, based on NHHPO-extractable soil
4 2 4
As (mg/kg), and the median sum of amorphous and crystalline Fe oxides (g/kg)...........................................................131
Figure 4.3.11. Relationship between the wheat shoot biomass (g dry weight), NHHPO-extractable soil As (mg/kg) and
4 2 4
amorphous and crystalline iron (graph A; g/kg) and aluminium (graph B; g/kg)..........................................................134
Figure 4.3.12. Dose response relationships between the wheat shoot biomass (g dry weight) and the RHIZO-extractable soil
Zn (mg/kg) and total soluble Zn (mg/l) for soil samples collected from site E (n = 4). The fitted line represents a three-
parameter sigmoid function............................................................................................................................................136
Figure 4.3.13. Model predicted EC50 concentrations (shown in solid line) for the range in predictive variables in sample
from site E, cation exchange capacity (left) and soil pH (right) (Warne et al., 2008a, Warne et al., 2008b). The total
recoverable Zn concentrations for the 4 soils from site E are also shown.....................................................................137
Figure 4.3.14. The interaction between (a) total soil Zn and soil pH and (b) total soil Zn and cation exchange capacity on 21
day wheat shoot biomass................................................................................................................................................138
Figure 4.4.1. Wheat root iron concentrations versus soil soluble iron concentrations. The select group of soil duplicates
identified as having large discrepancies in root As and Fe concentrations is identified separately..............................141
Figure 4.4.2. Association between wheat shoot biomass (g dry weight) and log-transformed root As concentrations (mg/kg
per dry matter)...............................................................................................................................................................147
Figure 4.4.3. Association between wheat root biomass (g dry weight) and log-transformed root As concentrations (mg/kg per
dry matter)......................................................................................................................................................................147
Figure 4.4.4. Log-log plot of effective arsenic concentrations (As , mg/l) versus wheat root arsenic concentration (mg/kg per
E
dry matter)......................................................................................................................................................................148
Figure 4.4.5. Log-log plot of the total plant accrued As (µg per plot) versus the soil effective As/P concentration ratio......148
Figure 4.4.6. Log-log plot of shoot versus root arsenic concentrations (mg/kg of dry matter) for wheat grown on sheep dip
soils................................................................................................................................................................................152
Figure 4.4.7. Log-log plot of the molar proportion of arsenic, relative to phosphorous, contained in shoot versus root
compartments of wheat plants in sheep dip soils............................................................................................................152
Figure 4.4.8. Shoot to root As concentration ratio versus root arsenic concentrations in wheat plants grown on sheep dip
impacted soils.................................................................................................................................................................153
Figure 4.4.9. Changes in the wheat shoot-root As ratio versus the molar ratio of effective soil As /P . The dashed reference
E E
line indicates the EC50 value fitted based on the As /P ratio.......................................................................................153
E E
Figure 4.4.10. Plant growth inhibition, indirectly represented by the shoot biomass (g dry weight), versus the molar arsenic
to phosphorous ratio in wheat shoots.............................................................................................................................154
Figure 4.4.11. Relationship between the molar arsenic to phosphorous effective soil concentration ratio versus the As/P ratio
in the plant shoot matter. Fitted EC50 value is shown as a reference line.....................................................................154
Figure 4.4.12. The proportion of effective soil As pool (As × water volume during the plant growth) taken up by the plants
E
verus wheat root As concentrations (mg/kg)..................................................................................................................155
Figure 4.4.13. The changes in wheat concentrations of potassium, sulphur, phosphorous, calcium and cadmium with
increasing root arsenic concentrations. The dataset is split into samples below and above the EC50 value, fitted based
on the As /P molar ratio. Where shown, the straight lines represent the linear regression fits....................................158
E E
Figure 4.4.14. Changes in wheat lead (left) and shoot sodium (right) concentrations with increasing root arsenic
concentrations. The dataset is split into samples below and above the EC50 value, fitted based on the As /P molar
E E
ratio. Where shown, the straight lines represent the linear regression fits....................................................................159
Figure 4.4.15. Root wheat calcium, sodium and sulphur concentrations against increasing root arsenic concentrations. The
dataset is split into samples below and above the EC50 value, fitted based on the As /P molar ratio. Where shown, the
E E
straight lines represent the linear regression fits...........................................................................................................160
Figure 4.4.16. Wheat shoot arsenic content in plants against the respective shoot magnesium, cadmium and phosphorous
content. Figures display a subset of soils below the fitted EC50 values.........................................................................161
Figure 4.4.17. The percentage of total plant As assimilated in the shoot compartment against root calcium concentrations.
The dataset is split into samples below and above the EC50 value, fitted based on the AsE/PE molar ratio.................162

vii
Figure 4.4.18. The arsenic soil-plant concentration transport factor (left) and the arsenic shoot-root concentration transport
factor (right) against increasing shoot Zn content of plants grown on soils from Site E. Sample E5 has been excluded
due to high growth inhibition.........................................................................................................................................163
Figure 4.4.19. Log-log relationships between wheat plant shoot arsenic (left column) and root arsenic (right column)
concentrations and various soil arsenic determinations: total soil As (top row), 0.2M NH-oxalate buffer extractable As
4
(middle row) and 0.05M NHHPO-extractable As (bottom row). Solid line represents a linear regression fit, dashed
4 2 4
lines represent the 95% prediction interval....................................................................................................................167
Figure 4.4.20. Log-log relationships between wheat plant shoot arsenic (left column) and root arsenic (right column)
concentrations and various soil arsenic determinations: RHIZO-extractable soil As (top row), 0.05M (NH)SO-
42 4
extractable soil As (middle row) and the effective soil As (bottom row). Solid line represents a linear regression fit,
dashed lines represent the 95% prediction interval.......................................................................................................168
Figure 4.4.21. Log-log relationships between wheat plant shoot arsenic (left column) and root arsenic (right column)
concentrations and various soil arsenic determinations: total soluble As (top) and the effective soil As/P ratio (bottom).
Solid line represents a linear regression fit, dashed lines represent the 95% prediction interval..................................169
Figure 4.4.22. Shoot As concentrations (A) and shoot As/P molar ratio (B) against effective soil As/P molar ratio. Samples
beneath the EC50 threshold (As /P = 0.24) shown only (n = 15)................................................................................170
E E
Figure 4.4.23. Association between the ratio of arsenic to phosphorous molar concentrations in plant roots versus the square
root of the ratio of effective soil arsenic to phosphorous concentration (Plot A). Plot B shows the ratio of Root As/P to
Soil As /P versus the effective soil P concentrations....................................................................................................171
E E
Figure 4.4.24. Log-log relationships between the total arsenic content of wheat plants (µg per pot) versus various soil
arsenic determinations: (A) total soluble As, (B) 0.2M NH-oxalate buffer extractable As, (C) 0.05M NHHPO-
4 4 2 4
extractable As, (D) RHIZO-extractable soil As, (E) effective soil As , and(F), the total soluble As...............................172
Figure 4.4.25. Observed versus predicted wheat shoot arsenic concentrations for a select group of multiple linear models for
predicting untransformed shoot As concentrations: Model 1 (Soluble As, Zn, & P, & pH), Model 6 (Soluble As, Zn &
P), Model 2 (RHIZO-As, Soluble Zn, Total soil P, & pH), Model 3 (RHIZO-As, Soluble Zn & total soil P), Model 11
(As /P & Soluble Zn), and Model 12 (Total soil As and Fe). Solid line represents a 1:1 relationship.........................179
E E
Figure 4.4.26. Observed versus predicted wheat shoot arsenic concentrations for a select group of multiple linear models for
predicting log transformed shoot As concentrations: Model 3 (As /P & Soluble Zn), Model 1 (As /P , Soluble Zn &
e E E E E
Total soil Fe), Model 6 (Total soil As/P ratio & Fe), Model 4 (Total soil As/P ratio, total soil Fe & RHIZO-Zn), Model
5 (RHIZO-As, RHIZO-Zn & total soil P), and Model 12 (Total soil As & Fe). Solid line represents a 1:1 relationship.
Solid line represents a 1:1 relationship..........................................................................................................................181
Figure 9.1.1. Soil sampling plan and the resulting total soil As concentrations for a Canterbury sheep dip, identified in text
as site C. Diagram is not to scale...................................................................................................................................236
Figure 9.1.2. Soil sampling plan and the resulting total soil As concentrations for foot rot bath, identified in text as Site D.
Diagram is not to scale..................................................................................................................................................237
Figure 9.1.3. Soil sampling plan and the resulting total soil As, Zn concentrations for a Marlborough spray dip, identified in
text as Site E. Results of the organochlorine pesticide screen are included where available. Diagram is not to scale..238
Figure 9.1.4. Soil sampling plan and the resulting total soil As and Cu concentrations for a Marlborough foot rot bath,
identified in text as Site F. Diagram is not to scale........................................................................................................239
Figure 9.1.5. Soil sampling plan and the resulting total soil As and Pb concentrations for a Marlborough sheep dip bath,
identified in text as Site G. Results of the organochlorine pesticide screen are included where available. Diagram is not
to scale...........................................................................................................................................................................240
Figure 9.1.6. Soil sampling plan and the resulting total soil As concentrations for a Marlborough spray dip, identified in text
as Site E. Results of the organochlorine pesticide screen are included where available. Diagram is not to scale........241
Figure 9.2.1. The pH dependency of two major arsenate species (HAsO2- and HAsO2-) in soil pore water of sheep dip soils,
2 4 4
as determined by speciation modelling (WHAM VI)......................................................................................................244
Figure 9.3.1. Pictures of wheat plants at the end of the 21 day bioassay experiment done using soil samples collected from
sheep dip sites. Slides are arranged from highest to lowest shoot biomass (dry weight) from left to right....................246

viii
Tables
Table 2.1.1. Average As concentrations in composite soil samples in and around cattle-dip sites around Australia (Adapted
from Smith (1998)).............................................................................................................................................................4
Table 3.2.1. Sequential extraction procedure for arsenic (Wenzel et al., 2001).......................................................................33
Table 3.4.1. Salt composition of macronutrient stocks used for preparing Ruakura nutrient solution.....................................36
Table 4.1.1. Total soil arsenic (As), antimony (Sb), calcium (Ca), phosphorus (P), lead (Pb), and sulphur (S) concentrations
for a set of surface samples from sheep dip sites. For each element a duplicate mean and a single standard deviation is
shown, the latter given in brackets...................................................................................................................................46
Table 4.1.2. Total and oxide soil concentrations of iron (Fe), aluminium (Al) and manganese (Mn) in surface samples
collected from sheep dip sites. For each element a duplicate mean and a single standard deviation is shown, the latter
given in brackets...............................................................................................................................................................47
Table 4.1.3. Physical and chemical soil properties of surface samples collected from sheep dip sites. Where applicable a
duplicate mean and a single standard deviation are shown, the latter is bracketed........................................................51
Table 4.1.4. Soil arsenic (As) concentrations determined in a sequential extraction procedure, along with the soil As
concentrations extracted by Olsen and NHOH.HCl reagents. Values represent a mean and a standard deviation, based
2
on a duplicate measurement. Standard deviations are bracketed....................................................................................56
Table 4.1.5. Soil concentrations of arsenic (As), phosphorous (P), copper (Cu), lead (Pb), and zinc (Zn) determined by the
RHIZO procedure. Values represent a mean and a standard deviation, based on a duplicate measurement. Standard
deviations are bracketed. All units are in mg/kg..............................................................................................................57
Table 4.1.6. The average proportion of total soil arsenic (As) levels accounted by different extractions: the sequential
extraction procedure steps, Olsen, NHOH.HCl, and RHIZO reagents. All proportions are expressed as a percentages.
2
.........................................................................................................................................................................................58
Table 4.1.7. Total soluble arsenic (As), phosphorus (P), and phosphate (PO-P) concentrations in pore water of sheep dip
4
soil samples. Total soluble concentrations are also expressed as the percentage of total soil content. All concentrations
are in mg/l, values represent duplicate means with a single standard deviation produced in brackets...........................62
Table 4.1.8. Total soluble concentrations (mg/l) of cations (Ca, Mg, K, and Na) and selected metals (Al, Fe, Mn, Zn, and Cu)
in pore water of sheep dip samples. Values represent duplicate means, with a single standard deviation displayed in
brackets............................................................................................................................................................................63
Table 4.1.9. Total soluble sulphur (S) concentrations, soluble concentrations of selected anions (SO-S, Cl, NO-NNO-N),
4 2 , 3
and total organic carbon level in pore water from sheep dip soil samples. All concentrations are in mg/l. Values
represent duplicate means, with a single standard deviation displayed in brackets........................................................64
Table 4.1.10. Pearson correlation matrix of log-transformed soil pore water concentrations.................................................65
Table 4.1.11. List-wise pearson correlation coefficients between log-transformed soil arsenic (As) determinations, with
emphasis placed on soil pore water and DGT As concentrations. All associations were significant at the 99 %
confidence level................................................................................................................................................................65
Table 4.1.12. Pearson correlation matrix for the associations between the proportion of non-specifically adsorbed soil
arsenic (As) fraction accounted by the soil pore water concentrations (% As ) and a select group of soil properties.67
SOL
Table 4.1.13. Pearson correlation matrix for a dataset subset representing samples collected from the Site G (n = 6),
displaying associations between the soluble arsenic (As) concentrations, the proportion of non-specifically adsorbed As
fraction accounted by the total soil pore water concentrations (% As ) and a select group of soil properties.............68
SOL
Table 4.1.14. A set of linear regression models for estimating log-transformed total soluble soil As concentrations. Four
samples were excluded from analysis due to non-detectable 0.05 M (NH )SO-extractable As concentrations. Each
42 4
model is summarised using predictor coefficients and their standard errors, and includes the adjusted coefficient of
determination, standard error and Akaike Information Criteria summary statistics........................................................69
Table 4.1.15. Differences between a microwave-assisted digestion in aqua regia and the elution in 0.01M HNO in the
3
recovery of arsenic (As) applied to Fe oxide gels.............................................................................................................72
Table 4.1.16. Phosphorus and arsenic diffusion coefficients ( × 10-6 cm2 s-1) calculated from the DGT aqueous solution
experiments compared against the published values from DGT device experiments, diffusion cell experiments and
values predicted for free ion diffusion in water. Where available, the experiment temperature is cited..........................73
Table 4.1.17. Diffusive gradients in thin films (X ) and effective concentrations (X ) of arsenic (As) and phosphorus (P) in
DGT E
sheep dip soils, along with estimated labile-solid phase distribution coefficients (X ) and resupply rate constants (X ).
Kd K
Values represent a replicate mean and a single standard deviation (bracketed). Table includes soil porosity values, the
only non-constant variable in the prediction of effective concentrations.........................................................................77
Table 4.1.18. Pearson correlation matrix between the DGT and total soluble arsenic (As) and phosphorous (P)
concentrations, their ratios (X ), estimated solid phase distribution coefficients (X ), and a select number of soil
R Kd

ix
properties. All variables were log-transformed to normalise their distribution, with exception of organic carbon and
soil pH..............................................................................................................................................................................80
Table 4.1.19. Set of multiple linear regression models for estimating log-transformed arsenic DGT concentrations (As ).
DGT
Models of different complexity included to illustrate differences in model likelihood......................................................81
Table 4.1.20. Pearson correlation coefficients between the DGT phosphorus concentrations (P ) and other soil phosphorus
DGT
extractions of sheep dip soils. All variables were log-transformed..................................................................................82
Table 4.1.21. Set of multiple linear regression models for estimating log-transformed phosphorus DGT concentrations
(P ). Models of different complexity included to illustrate differences in model likelihood.........................................82
DGT
Table 4.2.1. In vitro extractable arsenic concentrations (As , mg/kg) of sheep dip soil samples, along with the fraction of
SBET
the total soil As extracted by the in vitro procedure (Bioaccessibility, %).......................................................................85
Table 4.2.2. Pearson correlation matrix for different soil arsenic extractions. Concentrations have been log-transformed. All
correlations are significant at p-value of 0.01.................................................................................................................89
Table 4.2.3. Matrix of pearson correlation coefficients between arsenic bioaccessibility and select soil properties of sheep
dip samples (n = 30).........................................................................................................................................................94
Table 4.2.4. Pearson correlation matrix for the proportions of total soil arsenic accounted by different soil extractions for a
set of surface soil samples from sheep dip sites................................................................................................................96
Table 4.2.5. Pearson correlation coefficients between log-transformed in vitro extractable arsenic concentrations and log-
transformed soil crystalline iron, crystalline aluminium and amorphous manganese oxide concentrations...................98
Table 4.2.6. Set of multiple linear regression models for predicting log-transformed SBET extractable concentrations. For
each predictor the table includes the coefficient (b) and its standard error (se). Model statistics includes the adjusted
coefficient of determination (r2 adj.), root mean square error (RMSE) and Akaike and Bayesian information criteria
(AIC and BIC). The relative differences in information criteria are shown with respect to the highest score of the model
subset..............................................................................................................................................................................101
Table 4.2.7. Set of multiple linear regression models for predicting log-transformed SBET extractable concentrations,
derived following removal of samples containing total soil arsenic at concentrations below 15 mg/kg. For each
predictor the table includes the coefficient (b) and its standard error (se). Model statistics includes the adjusted
coefficient of determination (r2 adj.), root mean square error (RMSE) and Akaike and Bayesian information criteria
(AIC and BIC)................................................................................................................................................................102
Table 4.2.8. Set of multiple linear regression models for predicting the relative arsenic bioaccessibility (%). For each
predictor the table includes the coefficient (b) and its standard error (se). Model statistics includes the adjusted
coefficient of determination (r2 adj.), root mean square error (RMSE) and Akaike and Bayesian information criteria
(AIC and BIC)................................................................................................................................................................104
Table 4.2.9. Set of multiple linear regression models for predicting the relative arsenic bioaccessibility (%), derived
following removal of samples containing total soil arsenic at concentrations below 15 mg/kg. For each predictor the
table includes the coefficient (b) and its standard error (se). Model statistics include the adjusted coefficient of
determination (r2 adj.), root mean square error (RMSE) and Akaike and Bayesian information criteria (AIC and BIC).
.......................................................................................................................................................................................105
Table 4.2.10. Summary of two published studies used in validation of the developed multiple linear regression models. The
table also contains the parameters for linear models reported by the two experiments.................................................108
Table 4.2.11. Cross-validation results: agreement between the observed and the model predicted in vitro extractable soil
arsenic concentrations and the relative arsenic bioaccessibility values using the four external models summarized in
Table 4.2.10....................................................................................................................................................................109
Table 4.2.12. Multiple linear regression models for predicting the log-transformed in vitro extractable arsenic
concentrations, developed on the sheep dip soil data using the independent variables shared by Juhasz, et al. (2007b)
and Yang, et al. (2002). The four models represent different data subsets.....................................................................111
Table 4.2.13. Cross-validation summary table showing the ability of four validation models (VM1-VM4) to predict the
observed in vitro extractable arsenic concentrations, and the respective relative arsenic bioaccessibility reported in
Juhasz, et al. (2007b) and Yang, et al. (2002) , for different ranges of soil As concentrations .....................................111
Table 4.3.1. Ecological endpoints measured for wheat plants (10 plants per pot) submitted to an early growth bioassay test.
Values represent means of duplicate pots ± one single standard deviation....................................................................113
Table 4.3.2. Pearson correlation matrix for associations between the wheat biomass measures (shoot and root dry weight),
the germination success rate and the different soil arsenic determinations. All soil As determinations were log
transformed....................................................................................................................................................................117
Table 4.3.3. Pearson correlation matrix between the wheat bioassay ecological endpoints (shoot and root biomass, and
germination success rate) and selected group of soil properties....................................................................................120
Table 4.3.4. Parameter data and statistics describing a multiple linear regression model for estimating the wheat
germination success rate................................................................................................................................................121

x