Leaching Characteristics of CCA-Treated Wood Waste: A UK Study

Mercer, T.G.a,* and Frostick, L.E.b

a School of Physical and Geographical Sciences, Keele University, Staffordshire, England, ST5 5BG

bDepartment of Geography, University of Hull, Cottingham Road, HU6 7RX

*To whom correspondence should be addressed. Phone: + 44 (0)7587231305 E-mail: t.g.mercer@esci.keele.ac.uk

Abstract

CCA-treated wood is expected to increase in the UK waste stream over the next 20-50 years. The potential pollution from this waste has been evaluated through two leaching studies, one based upon batch leaching tests and another based upon a series of lysimeter tests. The aim of the studies was to characterise the behaviour of arsenic (As), chromium (Cr) and copper (Cu) from this wood when applied to soil as a mulch. Results demonstrate that all three elements leach from CCA waste wood, occasionally in concentrations exceeding regulatory thresholds by two to three orders of magnitude. In the lysimeter study, wood mulch monofills and wood mulch in combination with soil were used to monitor the leaching of As, Cr and Cu. Peak concentrations for As, Cr and Cu were 1885μg/l, 1243μg/l and 1261 μg/l respectively. Freshly treated wood leached 11, 23 and 33 more times more Cu, Cr and As, respectively than weathered wood. The toxic and mobile species of arsenic (As III, As V) were detected. Leaching in the CCA wood monofill was influenced by rainfall, with higher concentrations of metal(loid)s produced in lower intensity events. As and Cu were mobilised preferentially, with all metals exhibiting similar temporal trends. Retention of leached metal(loid)s was observed in lysimeters containing soil. Leaching processes appear to be favoured by the chipping process, diffusion and weathering. This study has shown that weathered waste wood mulch can cause significant pollution in soil water with potential impacts on both the environment and human health.

Keywords: CCA-treated wood waste; leaching; soil; mulch

1. Introduction

Chromated copper arsenate (CCA) is a water-based inorganic wood preservative that has been extensively used in the UK. The risks from arsenic (As), chromium (Cr) and copper (Cu) leaching from CCA-treated wood and potential impacts on the environment and human health have been widely reported in recent years. It has been established that CCA-treated wood leaches chemicals whilst in-service, particularly from the surface (Stilwell and Gorny, 1997, Solo-Gabriele et al., 2003, Zagury et al., 2003, Weis and Weis, 2004). However, limited leaching in-service can result in high levels of CCA remaining in the wood when it is taken out of service, particularly in the unexposed portions of the wood (Christensen et al., 2006).

The potential risks from CCA-treated wood have prompted the introduction of legislation worldwide. In the UK, the chemical components used in CCA treatment, the treatment process and end-uses are now highly regulated. The Marketing and Use of Arsenic Directive 2003/2/EC that was transposed into UK regulations (The Environmental Protection – Controls on Dangerous Substances Regulations, 2003), introduced a partial ban on the use of CCA as a preservative in the UK from the 30th of June, 2004 (HSE, 2009). The ban also extended to the use of CCA-treated timber. The recycling of treated timber is only allowed for ‘professional and industrial use where the structural integrity of the wood is required for human or livestock safety and skin contact by the general public is unlikely’ (OPSI, 2005). However, these restrictions are not applied to existing wood structures.

CCA wood waste is identified as hazardous under the Hazardous Waste England and Wales Regulations 2005 and the List of Waste Regulations (WRAP, 2004, EA, 2005, OPSI, 2005, DEFRA, 2002). At the end of its service life, CCA-treated wood waste must be disposed of in hazardous waste landfills unless the risk of the chemical components can be reduced (DEFRA, 2000).

Although the use of CCA as a wood preservative has now decreased as a result of EU and UK legislation, it is likely to remain a major component of the wood waste stream due to both its service life of 20-50 years (Christensen et al., 2006) and extensive use to date (WRAP, 2005). A study by Murphy et al. (2004) on future projections of the CCA wood waste streams for the UK, found that the amount of CCA-treated wood requiring disposal in 2004 was 62,000m3 and would increase to 870,000m3 by 2061. Furthermore, they predicted that the proportion of CCA-treated timber within the post-consumer wood stream would increase from 0.9% in 2001 to 12.3% in 2061. Therefore, managing this waste and ensuring safe disposal will be a major issue well into the future.

Several recent scientific studies have looked at the leaching properties of this waste and potential environmental and human health impacts. All have agreed that CCA-treated wood waste can leach significant As, Cr and Cu into the environment. Cooper et al. (2000) looked at leaching from CCA poles that have been removed from service and found leaching of copper in areas with ground contact and arsenic at all depths. Jambeck (2004), Jambeck et al. (2006) and Khan et al. (2006a) studied leaching of waste wood in simulated landfill scenarios using the wood in monofills, construction and demolition waste mixtures and MSW landfill mixtures. Arsenic was preferentially leached, with As (V) present in the monofill and As (III) in the Municipal Solid Waste (MSW) lysimeters. Copper and chromium were less mobile in the disposal environment.

Laboratory leaching methods have also been used to characterise leaching from CCA-treated wood waste. Townsend et al. (2003) and Shibata et al. (2006) reported that CCA-treated wood waste can inadvertently find its way onto the landscape mulch market as a result of construction and demolition processes. They studied leaching of CCA wood waste containing mulch using synthetic leaching procedures and reported significant leaching of all components, resulting in concentrations above regulatory levels. Further studies have looked at natural exposure of the wood. Shibata et al. (2006) set up six observation boxes and exposed the wood to rainfall. The mass of As leached over a year period was between 10-15% of the initial levels, demonstrating significant leaching. Hasan et al. (2010) also set-up outdoor leachate-collection systems and exposed weathered wood blocks to the elements. They found that As leached more rapidly than Cr or Cu and leaching was influenced by both rainfall and weathering.

Most past studies have reported on leaching of CCA chemicals added directly to soil or leaching from CCA lumber. The objective of the current study is to characterise leaching from CCA-treated wood waste mulch using batch leaching tests and field lysimeters. Similar methods are widely used in waste degradation experiments and allow exposure to natural conditions with some degree of control over the environmental variables. Only a limited amount of studies have focussed on leaching resulting from CCA mulching under field conditions. Field lysimeters have been employed in other research to look at the effects of CCA-treated wood mulch following a two month weathering period (Gifford et al., 1997). No existing studies examine the effects on top soil of applying commercial waste wood with a long service life (15-20 years) as soil mulch. This is the horizon where most of heavy elements accumulate and warrants investigation (Bergman, 1983, Bergholm, 1989, Dagan et al., 2006).This gap in current knowledge is addressed in this study where the leaching products were subjected to a range of laboratory analytical methods to determine metal partitioning within the solid and liquid media. The results are compared against UK Environment Agency Environmental Quality Standards for surface water and World Health Organisation Guidelines and used to infer the potential factors affecting leaching rates.

2. Experimental Methods

2.1. Laboratory Leaching Potential Study

A widely used laboratory based synthetic precipitation leaching procedure (SPLP) was undertaken to determine the potential for CCA metal(loid)s to leach from both the freshly treated and weathered CCA wood used in this study. This method is based on the United States Environmental Protection Agency Method 1312 (EPA, 1994b) which is used to assess the risk of leaching from contaminated soils to groundwater (Townsend et al., 2004). Ultrapure water (resistivity of 18Ωm cm-1, and weight equivalent to 20 times the weight of the woodchips) was acidified to pH 4.2 ± 0.05 using sulphuric and nitric acids (3:2). The woodchip samples were then immersed in a plastic container and shaken for 18 ± 2hr. The resultant mixture was measured for pH, filtered and microwave digested. The digestates were analysed using an inductively coupled plasma optical emission spectrometer (ICP-OES, Perkin Elmer Optima 5300DV, Conneticut, USA). In this study the technique was used to evaluate the maximum leaching potential and to facilitate comparisons with previous leaching studies.

2.2. Lysimeter Design

The main design criteria for lysimeters include size and shape, time period for the experiment, type and frequency of monitoring, environment and the number of replicates (Smith, 2005). Dimensions are of secondary importance in wood preservative leaching experiments (Melcher and Peek, 1996).

In this study, simple lysimeters were selected as there were fewer factors that need to be taken into account such as weighing mechanisms, intricate water application systems and leachate collection systems. Seventeen lysimeters were constructed and placed in a secure outdoor location (University of Hull Botanic and Experimental Garden, Cottingham, UK). Each lysimeter consisted of a plastic tray 65cm long, 35cm wide and 9.5cm deep supported by a raised and tilted wooden frame (Figure 1). The leaching trays and frames had two holes at the base with plastic washers and tubing attached to the base for leachate collection and sampling. Leachate was collected in sealed plastic containers. These were protected by dark plastic sheet flaps to prevent exposure to direct sunlight and to prevent contamination.

Figure 1: Experiment lysimeter Design

Samples of CCA-treated wood waste were collected from a local industrial source where it had previously been used in a cooling tower for at least 15 years. Other CCA leaching studies have used weathered wood posts, marine piling (Hasan et al., 2010), ramps, walkways, play structures and garden fences that had been exposed to natural conditions during service life (Townsend et al., 2005). The wood in the current study was selected

due to its availability as a waste and commercial application. Initial concentrations in the wood were more than three orders of magnitude higher than the highest retention weathered wood that had been used as marine piling in a field leaching study (Hasan et al., 2010). This is most likely due to its initial treatment. At the time of treatment, wood used in cooling towers were treated to hazard category 4 (highest hazard) requiring initial preservative solution concentrations of 40-50g/L and extended vacuum and pressure periods (BSI, 1989). Retention levels were typically 40kg/m3. Furthermore, the indoor containment of the wood indicated that it had undergone weathering from water temperature, not conventional weathering from exposure to UV radiation (Schmalzl et al., 2003). Therefore, this wood may have a greater potential to leach once it is exposed to natural conditions. However, it represents a potentially more hazardous portion of the existing CCA wood waste stream (indoor applications) that warrants study. Similar but untreated wood was obtained for comparison with weathered wood. Freshly pressure treated sapwood blocks of Scots Pine (Pinus sylvestris - mean retention levels of 5.86kg/m3) were also produced using a modified version of vacuum impregnation following BS 4072 (1999) and BS 4072:2 (1987).

Wood samples were chipped using a Retsch SM100TM cutting mill to a maximum particle size of 2mm and digested using CEM methods for wood samples (CEM, 2004). ICP-OES analysis was used to determine mean concentrations of As, Cr and Cu (Table 1). The soil used in the study was also analysed for metal(loid) concentrations. The ultrapure water used in the SPLP and acidified SPLP solution did not have detectable concentrations of arsenic, chromium or copper. The rain water in the lysimeter leaching study was sampled on two occasions and was also found to be free of detectable arsenic, chromium and copper.

Table 1: Initial concentrations of arsenic, chromium and copper in the experimental materials (± 1 standard deviation)

2.3. Experimental Setup

The experimental design consisted of fourteen combined and three separate (monofill) soil and mulch lysimeters. Seven lysimeters contained weathered CCA wood mulch and soil, a further seven contained untreated wood mulch and soil and a further three contained only weathered CCA woodchips, untreated wood chips or soil. All leachate collection containers, lysimeters and other equipment were acid washed with 5% nitric acid followed by a rinse with ultrapure water to remove any contamination. A protective ground sheet was laid beneath the lysimeters to prevent growth of weeds and limit pests. Sieved and sterilised top soil was added to a depth of approximately 7.5-8cm and an average weight of 20kg. The weathered and untreated wood were chipped separately using a Jensen Industrial Chipper with the untreated wood processed first to avoid contamination. In between chipping, the machine was brushed out and cleaned with compressed air. Lysimeters with the untreated wood were filled first to prevent contamination of the wood, soil and/or equipment with CCA components. The wood was well mixed and 4.5kg was added to each lysimeter.

2.4. Leachate Sampling and Analysis

The experiment was set-up on the 9th of May, 2007 and ran for 21 weeks. Rainfall data was monitored hourly using a Campbell ScientificTM Basic Mounted Weather Station. Leachate was sampled after periods of rainfall and total volume measured using a graduated cylinder. Redox was evaluated with a handheld ExStikTM ORP RE300 and pH with a Fisherbrand Hydrus 300 pH probe. Leachate volumes were inconsistent across all lysimeters due to loss of leachate from overspill caused by flooding, absorption of rainfall by the wood and/or

Freshly Treated Wood CCA-treated Waste Wood Untreated Wood Soil

Arsenic 2060 ± 220 mg/kg 5430 ± 1340 mg/kg 0 mg/kg 3 ± 1 mg/kgChromiu

m3310 ± 410 mg/kg 10650 ± 3350 mg/kg 0 mg/kg 11 ± 2 mg/kg

Copper 2110 ± 460 mg/kg 2920 ± 970 mg/kg 0 mg/kg 24 ± 1 mg/kg

soil and possible differences in the distribution and amount of rain reaching each lysimeter. On all occasions, leachates from lysimeters with the untreated wood were measured first. Upon emptying, the leachate collection dishes were washed out with ultrapure water and wiped clean before replacing. Leachate samples were transferred into cleaned 100ml nalgene containers. These were immediately returned to the laboratory and stored at 4°C prior to elemental analysis and frozen prior to speciation analysis.

Metal speciation analyses were carried out on a sample from each of the lysimeter treatment types. A novel rapid technique was used allowing for simultaneous determination of inorganic species of both arsenic and chromium. Previously frozen leachate samples were defrosted and 20μl of the liquid injected into a C18 monolithic silica column. The column used in this study has been shown to give rapid separation of the species with equivalent or superior peak efficiencies to traditional columns (Pearson et al., 2007). The mobile phase was set-up for ion-pair chromatographic separation of arsenic and chromium species (Pearson et al., 2007). Separation was achieved by ion-pair liquid chromatography where ions were paired with the additive to the mobile phase; Tetrabutylammonium Bromide (TBAB). The species were then separated in the column based on the polarity of the sample and therefore retention time in the column (Pearson, 2007). The column was coupled with an inductively coupled plasma mass spectrometer (ICP-MS) for identification of species.

Samples were analysed for total As, Cr and Cu by ICP-OES following microwave acid digestion. Leachate samples were diluted with 70% concentrated HNO3 to according to the methods of the EPA (EPA, 1994a). The detection limits of the ICP-OES were 8.8, 2.5 and 3.7 μg/l respectively. Standards, blanks and spikes were run throughout for quality control. During periods of high rainfall, not all leachate could be sampled due to limits on equipment availability. In addition, on some occasions there were inevitable losses of leachate due to broken and lost sample tubes so that a full range of samples was not available. Where data was missing or minimum measureable levels resulted in data gaps, statistical multiple imputations were applied to facilitate data analysis (Mercer et al., 2011). The fraction of missing leachate was 10% of the total measured. However, imputations were carried out successfully due to the sufficient replication of the treatments and were further tested using standard indices as described by Junninen et al. (2004).Representative batches were used from the 18 collection dates were selected covering the entire experimental period.

Statistical analyses were performed with SPSSTM (version 16) and Microsoft Excel (2007) with confidence levels of 95% or 99%. Significant differences between datasets were analysed using one way ANOVA with Tukey tests or t-tests. Where test assumptions were violated, the non-parametric equivalents were used.

3. Results

3.1. Initial CCA Concentrations in the Wood

Although the weathered wood had been in service for 15 years, concentrations levels of all metal(loid)s were higher than those in the freshly treated wood (1.4 times higher for copper, 2.6 times higher for arsenic and 3.2 times higher for chromium). This probably relates to higher initial loading values. The retention of metals in the weathered CCA wood used in this study suggests that most of the preservative has remained fixed in the wood whilst in-service. The proportions of the As, Cr and Cu in the wood reflect the original preservative formulation with chromium in highest concentrations followed by copper then arsenic. However, in the weathered wood, arsenic values were higher than copper, suggesting that the elements leached out differentially whilst in-service.

3.2. SPLP Results

In freshly treated wood, arsenic was leached preferentially followed by copper then chromium (Cr<Cu<As). For weathered wood, copper was most mobile followed by arsenic and chromium (Cr<As<Cu). Concentrations in

leachate from the freshly treated wood were orders of magnitude higher than those from their weathered equivalents. Untreated wood did not yield any detectable metals confirming that the clean wood used in this study was uncontaminated (Table 2).

Table 2: Average pH and Metal(loid) concentrations in the SPLP extract (± 1 standard deviation)

Freshly Treated CCA Wood

CCA-treated Waste Wood

Untreated Wood

Final pH 5.6 ± 1.2 7.0 ± 0.6 4.1 ± 0.3Arsenic 12.37 ± 0.38 mg/l 0.38 ± 0.07 mg/l 0 mg/l

Chromium 4.94 ± 0.13 mg/l 0.21 ± 0.01 mg/l 0 mg/lCopper 9.39 ± 0.4 mg/l 0.83± 0.04 mg/l 0 mg/l

The retention levels in the weathered wood suggest that most of the preservative has remained fixed in the wood whilst in-service. Freshly treated wood lost 24% arsenic, 6% chromium and 18%copper by weight, compared with weathered wood, which lost 0.28%, 0.08% and 1.14% by weight. The concentrations of all metal(loid)s leached from freshly treated wood and weathered wood exceeded the maximum Environmental Quality Standard (EQS) of the Water Framework Directive (EA, 2006) by two to three orders of magnitude (arsenic EQS: 50 μg/l, chromium EQS: 32 μg/l, copper EQS: 28 μg/l).

3.3. Rainfall and Leachate Physico-Chemical conditions

Rainfall intensity and duration are important factors in the leaching process as they control leaching as the amount of leachant and contact time with the leachant (Kartal et al., 2005, Taylor and Cooper, 2005). In this study there was 522.7mm of rainfall over the 147 days of the experiment, with very intense rain in a single two week period when there was a 1 in 150 year rainfall event in June 2007 (Coulthard et al., 2007, Coulthard and Frostick, 2010). Many of the lysimeters became waterlogged and leachate overflowed. These increased total quantities of metals leached but also diluted the leachate.

3.3.1. Leachate Redox and pH

The conditions in the lysimeter leachate remained moderately reducing over the entire experimental period (between +100 and +400 mV, Figure 2). Significant variation in redox readings (F(4,71) = 9.47, p < 0.01) suggested that lysimeter conditions differed . Conditions in the untreated wood monofill were more reducing than those in the CCA-treated and untreated wood lysimeters (p < 0.01 in both cases). Both wood monofills had more reducing redox conditions than the soil monofill (p < 0.01 in both cases). Under normal summer conditions and with the shallow soil depths in this study, conditions should have remained fully oxidising (> 400mV). However, anaerobic conditions developed in the soil as a result of unusual rainfall events (Coulthard and Frostick, 2010).

The pH of the CCA-treated wood monofill and untreated wood monofill leachates were significantly lower than those of lysimeters containing soil (Tukey post hoc comparisons: CCA wood monofill (M = 6.4) against CCA-treated wood + soil (M = 7.58) and untreated wood + soil (M = 7.5) lysimeters p < 0.01; untreated wood monofill (M = 5.5) against CCA wood monofill, CCA-treated wood + soil and untreated wood + soil lysimeters p < 0.01; Figure 2). Values were fairly consistent with leachates from the soil-containing lysimeters between pH 6.8 and 8.7, those from CCA wood monofills between pH 6 and 7 and those from untreated wood significantly lower between pH 5 to 6.7. Fluctuations within these ranges (particularly between days 35 and 63) coincide with periods of heavy rainfall.

Figure 2: 2a. Average leachate redox (± 1 standard deviation). Lines at 100mV and 400mV denote redox class boundaries. 2b. Average leachate pH (± 1 standard deviation)

3.3.2. Arsenic, Chromium and Copper Concentrations in Lysimeter Leachate

Over the course of the lysimeter study, the temporal leaching trends were similar for all three elements (Figure3). The highest concentrations came from the CCA wood monofill with As, Cr and Cu at 1.89 mg/l, 1.26 mg/l and 1.24 mg/l respectively. Furthermore, there was considerable variability between sampling dates, although the overall trend was for a significant increase over time (arsenic r2 = 0.66, p < 0.01; chromium r2 = 0.83, p < 0.01; copper r2 = 0.63, p < 0.05). Comparisons with the results from the SPLP method show differences throughout the experimental period. Leaching from the CCA-treated wood and soil lysimeter showed no significant trend over time (arsenic r2 = 0.18, p = 0.44; chromium r2 = 0.14, p = 0.12; copper r2 = 0.13, p = 0.12).

Both the CCA-treated wood monofill and the CCA-treated wood and soil lysimeters leached in a similar way with a dip in concentrations on days 51 and 101 and peaks on days 43, 64, 127 and 148. Low concentrations occurred after rainfall events diluted the leachate. Day 127 samples included drainage from waterlogged soils during a long dry spell. Comparison of concentration with rainfall intensity confirms that higher concentrations of metal(loid)s in leachate are generally associated with low rain intensities in the CCA wood monofill (Figure 4) and that the decrease with increasing rain intensity follows a power law (p < 0.05 ). The inverse is true in the CCA-treated wood and soil lysimeters (p<0.05 for Cr) due to the complexing of metals in the soil and subsequent flushing out following higher intensity events. CCA wood monofills on average leached more

050

100150200250300350400450

Mean Untreated Wood + SoilMean CCA-Treated Wood + SoilCCA Wood MonofillUntreated Wood MonofillSoil Monofill

Number of Days into Experiment

Redo

x (m

V)

4

5

6

7

8

9

10

Number of Days into Experiment

pH

2a

2b

Reducing

Oxidising

Moderately Reduc

ing

metal(oids) than those containing soil and wood mulch together(As: U = 3.00, p < 0.001, Cr: U = 0.00, p < 0.001, Cu: U = 0.00, p < 0.001). Leachate data for the untreated wood monofill, untreated wood and soil lysimeters and soil monofill are not displayed as the concentrations are very low and fall below the EQS.

Figure 3: Arsenic chromium and copper leachate concentrations from the CCA monofill and CCA-treated wood and soil lysimeters over time ± 1 standard deviation (As EQS: 50 μg/l; Cr EQS: 32 μg/l, Cu EQS: μg/l)

0 7 14 21 28 35 42 49 56 63 70 77 84 91 98 105 112 119 126 133 140 147 154 1610

200400600800

100012001400160018002000

f(x) = 5.99079392677493 x + 369.945436682997R² = 0.631493277262405f(x) = 7.4997742729746 x + 40.1998886948402R² = 0.832151229489663

f(x) = 11.0145303174166 x + 270.453796351524R² = 0.662291448150777

Mean CCA-Treated Wood and Soil Lysimeters (As) CCA wood monofill (As)Linear (CCA wood monofill (As)) Mean CCA-Treated Wood and Soil Lysimeters (Cr)CCA wood monofill (Cr) Linear (CCA wood monofill (Cr))Mean CCA- Treated Wood and Soil Lysimeters (Cu) CCA wood monofill (Cu)

Number of Days Into ExperimentConc

entra

tion

(μg/

l)0

20406080

100120

Rain

fall

(mm

)

0

1

2

3

Num

ber o

f Po

re W

ater

Vo

lum

es

Figure 4: The influence of rain intensity on metal(loid) concentration in the leachate from CCA-treated wood lysimeters. Nb log/log plots are used

0.1 1 10 1003

30

300

3000

f(x) = 37.4130965786171 x^0.312769867980639R² = 0.378941606928154

f(x) = 867.89957303587 x^-0.405487454571347R² = 0.726347527928128

Chromium

CCA Wood Monofill

Power (CCA Wood Monofill)

Rainfall Intensity Before Sampling(Rainfall Volume/Number of Days)

Conc

entra

tion

(μg/

l)

0.1 1 10 1003

30

300

3000

f(x) = 41.2929148610001 x^0.196227150680565R² = 0.298900558307403

f(x) = 1118.60045592799 x^-0.293746285320398R² = 0.716287594279305

Copper

CCA Wood MonofillPower (CCA Wood Monofill)

Rainfall Intensity Before Sampling(Rainfall Volume/Number of Days)

Conc

entra

tion

(μg/

l)

0.1 1 10 10010

100

1000

10000

f(x) = 69.3226083094279 x^0.259945943570723R² = 0.251035423640166

f(x) = 1441.2853615018 x^-0.354888647019097R² = 0.514303459423591

Arsenic

CCA Wood MonofillPower (CCA Wood Monofill)

Rainfall Intensity Before Sampling(Rainfall Volume/Number of Days)

Conc

entra

tion

(μg/

l)

3.3.3. Proportions of Metal(loid)s Leached

Data for the CCA-treated wood and soil lysimeter show that, the metal(loid)s leach out in significantly different concentrations (H(2) = 10.78, p < 0.01). Using Mann-Whitney tests with a Bonferroni correction (0.0167 level of significance) concentrations of arsenic were significantly higher than both chromium (U = 1365, p < 0.0167) and copper (U = 1453, p < 0.0167). For the CCA-treated wood waste monofill, metal(loid)s leached in the order Cr<Cu<As, though the differences between the medians of the three elements were not significant. This differs from the patterns derived using the SPLP method (Cr<As<Cu).

In terms of leaching patterns, significant positive correlations between the metal(loid)s were observed in both the CCA-treated wood and soil lysimeters and the CCA wood monofills (Treated lysimeters leachate As vs. Cr r = .872, p < 0.01; As vs. Cu r = .602, p < 0.05; Cr vs. Cu r = .734, p < 0.01) (CCA wood monofill leachate As vs. Cr r = .881, p < 0.01; As vs. Cu r = .905; p < 0.01; Cr vs. Cu r = .976, p < 0.01). This suggests that although metal(loid)s vary in solubility, they follow a similar leaching pattern.

3.3.4. Arsenic and Chromium Inorganic Species in Leachate

An understanding of metal species transformation is essential to determine toxicity, solubility, mobility and therefore potential impact. The speciation of arsenic and chromium is affected by both redox potential and pH amongst other things (Masscheleyn et al., 1991, Smedley and Kinniburgh, 2002). Eh/pH diagrams are widely used as a graphical representation of stability relationships (Stumm and Morgan, 1981). They can help predict which species are stable under certain Eh and pH ranges in aqueous solutions at 25°C and 1 atmosphere. However, they can only be used as a general guide as most natural waters are ‘in a highly dynamic state rather than in or near equilibrium’ (Stumm and Morgan, 1981, p. 441).

Eh/ pH diagrams were used to evaluate the stability of arsenic (Figure 5) and chromium (Figure 6) species in all leachate samples. For arsenic the data show the conditions favour leaching of arsenic (V) as H 2AsO4

- and HAsO4

2-. However, there is also the potential for arsenic to leach as As (III) (H 3AsO3). The chromium species favoured by the conditions was chromium (III) as CrOH+2, Cr(OH)2

+1 and Cr(OH)3. A few data points lie near the theoretical boundary of the conditions that favour Cr (VI) suggesting this could be present in the leachate.

In all the treatments containing soil, inorganic species were present as arsenic (III) and chromium (III). In the leachate from the CCA-treated wood monofill, arsenic is present as both As (III) and As (V), with no evidence of chromium species. The untreated wood monofill leachate did not contain any detectable inorganic arsenic or chromium species. This shows that this wood and leachate was not contaminated during experiment set-up, sample collection or analysis. These results are in agreement with the data points plotted on the Eh/pH diagrams.

Figure 5: Eh/pH stability diagram for arsenic in the leachate samples. Boundaries adapted from Solo-Gabriele et al. (2004)

Figure 6: Eh/pH stability diagram for chromium in the leachate samples. Boundaries adapted from Solo-Gabriele et al. (2004)

HAsO4

2-

As (V)

H2AsO4

-

As (V)

H3AsO3

As (III)

CrO4

-2

Cr (VI)

CrOH+2

Cr (III) Cr(OH)3

Cr (III)

Cr(O

H)2+1

Cr (I

II)

4. Discussion

4.1.1. Comparison of Leaching Characteristics between Freshly Treated and Weathered CCA Wood

The lysimeter and SPLP experiments have demonstrated that metal(loid)s in freshly treated chipped wood leach in greater concentrations than metal(loid)s in weathered chipped wood. This may be because freshly treated wood (not aged) loses CCA rapidly in the initial stages of exposure of the wood surface to a leaching medium compared to weathered wood, due to more freely available surface deposits (Lebow et al., 1999, Khan et al., 2004a, Waldron et al., 2005). In addition, the movement of preservatives to the surface of the wood is accelerated by the steep concentration gradient between the wood and leachant (Cooper, 1994).

The proportions of metals in the leachants do not reflect original proportions within the preservative (47% CrO3, 19% CuO and 34% As2O5 w/w) suggesting preferential leaching of arsenic and copper over chromium. Furthermore, there are differences in the ratios of arsenic and copper leaching concentrations as observed in both the SPLP and lysimeter experiments. These differences may be influenced by many factors including the original CCA formulation used and the age of the wood.

The proportion of leachable arsenic depends on the original retention levels of chromium and the ratio of As:Cu in the treatment formulation. Where formulations have been used with high As:Cu ratios, less hexavalent chromium is formed (Moghaddam and Mulligan, 2008). Hexavalent chromium is responsible for the rate of fixation of the arsenic and copper to the lignin of the wood. Therefore lower concentrations result in a slower fixation and potential release of arsenic. With a low As:Cu ratio, the production of hexavalent chromium increases, accelerating the fixation of arsenic and copper in the wood (Moghaddam and Mulligan, 2008). This results in slower arsenic release whilst in-service. The formulation of the treatment is therefore an important factor determining metal leachability. Furthermore, variability in retention can occur even in the same piece of lumber (Schultz et al., 2002, Townsend et al., 2004).Variability in the data is therefore to be expected irrespective of leaching processes.

In all experiments, chromium was the most stable element, a result which is reflected in other studies (Warner and Solomon, 1990, Cooper, 1991, Hingston et al., 2001, Lebow et al., 2003, Taylor and Cooper, 2003, Stefanovic et al., 2006, Shibata et al., 2007). Copper tends to bind weakly to labile wood cellulose whilst chromium binds more firmly to the slower degrading lignin (Warner and Solomon, 1990). Arsenic binds to all components of the wood, forming copper and chrome arsenates whose bonds can be disrupted by ion exchange reactions (Warner and Solomon, 1990). The smaller concentrations of chromium leached can also be explained by the reduction of hexavalent chromium to the less mobile trivalent chromium during the fixation process (Townsend et al., 2005). In aged wood, most of the Cr(VI) is reduced to Cr(III) which also results in higher retention in both wood and soil.

4.1.2. Environmental Quality and Drinking Water Standards

In this study, leachate from both the CCA wood monofill and CCA-treated wood and soil lysimeters exceeded the EQS. The exceedance of the maximum EQS has wide-ranging implications. The standards are put in place to ensure that the conditions of surface and/or groundwater can support ecosystems (EA, 2006). The leachate composition demonstrates that contamination of the soil could result in adverse impacts on soil flora and fauna. Furthermore, if the mulch is used in a landscape setting human health may be at risk. This is particularly the case if there is direct contact through ingestion, skin absorption or inhalation of CCA components from the wood or through indirect contact via contaminated crops or livestock. The potential mobility of the metal(loid)s in soil, as evident in the leachate taken from the base of the CCA-treated wood and soil lysimeters, suggests significant risk of contamination to both surface and groundwater. The World Health Organisation (WHO) set

drinking water standards for arsenic at 10µg/l and chromium at 50µg/l (WHO, 2004). However, the maximum concentrations found in the leachate exceeded these values by 42.6 times for arsenic and 4.6 times for chromium. Leachate would therefore have to be diluted more than 40 times to allow safe ingestion.

4.1.3. Temporal Leaching Trends

Concentrations of metal(loid)s in the CCA wood monofill leachate increased over time, confirming the results of Jambeck et al. (2006) who reported increasing concentrations for more than a year before they levelled off. The current experiments ran for 5 months sand it seems likely that the data may represent only the early stages of leaching. However, these results contrast with those of Gifford et al. (1997) who found that copper and chromium concentrations in leachate decreased steadily over an 18 month period and those of arsenic decreased for 9 months before increasing substantially. They attributed these trends to seasonal variations in rainfall and temperature. The experiments reported here suggest that temperature was not a significant influence, whilst rainfall intensity was.

The CCA wood monofill leachates contained significantly greater concentrations of metal(loid)s than the CCA wood and soil lysimeters over the experimental period. Furthermore, pH and redox were significantly lower, with trends suggestive of more extreme conditions. The temporal trends in leachate concentrations from lysimeters containing soil remained fairly constant over time, with increasingly oxidising conditions and neutral pH. These finding suggest that the soil had some buffering effects to changes in pH, redox and metal(loid) concentrations following the introduction of the wood. The leached metal results suggest that that there was some mechanism of attenuation in the soil that was exceeded following rainfall events. This is evident from Figure 4 where concentrations increase with rainfall intensity.

It has been established that metal(loid)s leach from fresh CCA-treated wood in the early stages of exposure to leachants and then decrease over time so that aged and weathered wood should not be readily leachable (Moghaddam and Mulligan, 2008). This was not the case in this study, probably as the wood used was deployed within a building where it was not exposed to rainfall and UV effects. The fact that the EQS were exceeded and that there is an increase in leaching over time does not fit in with assertions that CCA-treated wood waste is comparatively stable and that metal(loid) release only occurs in the early stages of exposure (Van Eetvelde et al., 1995, Lebow et al., 2003, Cockcroft and Laidlaw, 1978). Rather, the trend suggests that the degradation of CCA-treated wood waste is progressively releasing more metal(loid)s bound to the wood. This confirms the results of Hasan et al. (2008) who also found that leaching rates for weathered wood were almost double those of freshly treated wood. This raises concern over the longevity of the risks associated with the management of CCA-treated wood waste.

4.1.4. Factors Influencing Leaching Rates

A number of factors may influence temporal trends of metal(loid) leaching. The increased surface area caused by chipping exposes new surfaces which leach and weather rapidly (Hingston et al., 2002, Townsend et al., 2003, Townsend et al., 2004, Yoon et al., 2005, Cockcroft and Laidlaw, 1978, Lebow et al., 2004). In the early stages, rapid leaching is caused by dissolution of metal(loid)s bound on the surface of the wood in a similar way to leaching from freshly treated wood (Jambeck et al., 2006). Metal(loid) leachability increases with decreasing particle size (Townsend et al., 2004) and so small shavings and particles resulting from the chipping process will have contributed to metal(loid) concentrations in the leachate. In addition, the effect of grain orientation (which results in higher leaching rates due to the structure of wood cells) may be exaggerated in small wood samples (Lebow et al., 2004). This is because CCA is most mobile along the longitudinal axis of the wood due to the longitudinal orientation of the xylem vessels (Ko et al., 2007). In this study, the exposure of new surfaces from chipping and resultant leaching means that recycling this material as mulch is more hazardous than re-use.

A rapid leaching stage is followed by a slower process involving water penetration into the wood causing hydrolysis and/or dissolution of the fixed and complexed components (Hingston et al., 2001). The preservative will then diffuse towards the surface of the wood due to the steep concentration gradient between the wood and leachant. Therefore, the first 6 months of leaching should be characterised by a rapid release phase followed by a plateau (Lebow et al., 1999). This study focussed only on the rapid release phase where this phase was compounded by prolonged contact of the wood waste with water resulting from waterlogging. This increased leaching potential (Townsend et al., 2004). In addition, low redox and associated microbial activity favour soluble species, particularly of arsenic.

The exposure of the waste wood to UV radiation could have further contributed to the increasing leaching leaching rates observed in the CCA wood monofill. UV exposure results in the breakdown of lignin in the wood and therefore release of the bound metal(loid)s. In addition, cumulative weathering effects from moisture and evaporation conditions can cause warping, cracking and splitting, exposing more surfaces for leaching (Khan et al., 2006b, Cooper and Ung, 2009).

4.1.5. Speciation of Metal(loid)s in Leachate

The leachate Eh/pH data showed that conditions in this study favour arsenic as As (V) and chromium as Cr (III). However, some of the data points sit near the boundaries for inorganic As (III) and Cr (VI) species. The presence of As (III) in the analysed leachates of the soil-filled lysimeters is of concern as it is toxic, more soluble and more mobile than As (V) (Masscheleyn et al., 1991, Balasoiu et al., 2001, Hingston et al., 2001, Macur et al., 2001, Jambeck, 2004, Khan et al., 2004b, Solo-Gabriele et al., 2004, Dobran and Zagury, 2006). Furthermore, it causes contamination and health risks (Pantsar-Kallio and Manninen, 1997).

It is possible to find As (III) under conditions that theoretically favour As (V). In the soil lysimeters, any available As (V) may have adsorbed to the soil leaving the more mobile and soluble As (III) to pass into the leachate. Soil organic matter has also been shown to influence the formation of As (III) in aerobic environments by facilitating the reduction of As (V) to As (III). The organic matter of the soil was high (12-20%) in comparison to previous CCA leaching studies (Gifford et al., 1997, Crawford et al., 2002, Lebow et al., 2006)and as such could have contributed to the presence of As (III). Reduction of As (V) to As (III) by bacteria, fungi and algae has also been demonstrated (Cai et al., 2006).

The CCA wood monofill leachate contained both inorganic species of arsenic with As (V) being predominant. These results are in agreement with previous leaching studies using wood without soil. Arsenic is present as As

(V) in weathered CCA wood therefore the presence of As (III) in weathered wood leachate must be due to the reduction of As (V) biologically and/or chemically whilst the wood is in service (Khan et al., 2004b).

The presence of Cr (III) as opposed to Cr (VI) in this study was expected as it is the most commonly found species in wood, leachate and soil and occurs naturally in the environment (Nico et al., 2004). Chromium (VI) is usually only found under alkaline (pH>9.0) and highly oxidising conditions (James and Bartlett, 1982, Song et al., 2006). Cr (III) was only found in the soil-filled containers suggesting that the topsoil may be enhancing the mobilisation of Cr from the mulch. Chromium was not detected in the leachate from the CCA wood monofill, confirming that chromium is stable under most soil conditions.

5. Conclusion

The relatively short term leaching observed in these experiments resulted in significant pollution of the leachate. This raises issues of ensuring that there is effective management of this waste stream at the end-of- service life to prevent environmental contamination. As the partial ban on the use of CCA for wood preservation in the UK does not apply to wood currently in service, the waste stream for this wood is expected to increase over the foreseeable future. Current identification techniques used by the waste industry including visual observations and qualitative stains neither of which are completely effective in detecting the presence of the preservative. More robust analytical techniques such as ICP-OES or ICP/MS are not time or cost-effective and as such, misidentification of wood will inevitably persist. The probable presence of CCA wood in recycled waste streams, such as those used for mulch, will have to be considered when applying the material to soil in the future. In particular, this study has highlighted the very high levels of risk from waste CCA wood with high preservative retention levels. The environment on the surface of soil typically associated with the use of mulch will only serve to accelerate leaching rates and lead to elevated concentrations, particularly of arsenic, in both soil and leachate. The pollutants will then be available for uptake by plants and soil organisms causing contamination of the food chain.

6. Acknowledgement

The authors would like to thank the following for their assistance: the two anonymous reviewers whose comments greatly improved the manuscript; Gervais Sawyer for his help with wood species identification and laboratory preservation of the wood; Leeds university for the use of their cutting mill; Bob Knight for his assistance with metal(loid) analysis; Professor Gillian Greenway for assistance with reading drafts; Dr. David Sands for help with analysis, staff at the University of Hull Facilities for assistance with experimental setup; field assistants (Andrew Kythreotis and Edwina Mercer). Funding for this project was provided by a UK Engineering and Physical Sciences Research Council studentship.

REFERENCES

BALASOIU, C. F., ZAGURY, G. J. & DESCHÊNES, L. 2001. Partitioning and speciation of chromium, copper, and arsenic in CCA-contaminated soils: influence of soil composition. The Science of The Total Environment, 280, 239-255.

BERGHOLM, J. 1989. The Mobility of Arsenic, Copper and Chromium in CCA-contaminated soils. Stockholm: Swedish Wood Preservation Institute.

BERGMAN, G. 1983. Analysis of soil and ground-water at three wood preserving plants in Sweden. Stockholm: Swedish Wood Preservation Institute.

BSI 1987. Wood preservation by means of copper/chromium/arsenic compositions - Part 2: Method for timber treatment. London: British Standards Institution.

BSI 1989. BS 5589:1989 Code of Practice for Preservation of Timber. London: British Standards Institution.BSI 1999. Wood preservation by means of copper/chromium/arsenic compositions - Part 2: Method for

timber treatment. London: British Standards Institution.CAI, Y., SOLO-GABRIELE, H., TOWNSEND, T., KHAN, B., GEORGIADIS, M., DUBEY, B. & TOWNSEND, T. G.

2006. Chapter 7: Elemental Speciation and Environmental Importance Associated with Wood Treated with Chromated Copper Arsenate. In: TOWNSEND, T. G. & SOLO-GABRIELE, H. (eds.) Environmental Impacts of Treated Wood. Boca Raton: Taylor & Francis.

CEM 2004. Mars Xpress Microwave Sample Preparation Note: XprAG-4: Wood Chips. CEM.CHRISTENSEN, I. V., PEDERSEN, A. J., OTTOSEN, L. M. & RIBEIRO, A. B. 2006. Electrodialytic remediation of

CCA-treated waste wood in a 2m3 pilot plant. Science of the Total Environment, 364, 45-54.COCKCROFT, R. & LAIDLAW, R. A. 1978. Factors affecting leaching of preservatives in practice. IRGWP, 10th

Annual Meeting. Scotland: International Research Group on Wood Preservation.COOPER, P. A. 1991. Leaching of CCA from treated wood: pH effects. Forest Products Journal, 41, 30 - 32.COOPER, P. A. 1994. Leaching of CCA: Is it a Problem? Environmental Considerations in the Manufacture,

Use and Disposal of Preservative-treated Wood, 45-57.COOPER, P. A., JEREMIC, D. & TAYLOR, J. L. 2000. Residual CCA levels in CCA treated poles removed from

service. IRGWP 31st Annual Meeting. Hawaii: International Research Group on Wood Protection.COOPER, P. A. & UNG, Y. T. 2009. Component leaching from CCA, ACQ and a micronized copper quat

(MCQ) system as affected by leaching protocol. IRGWP 40th Annual Meeting. Beijing: International Research Group on Wood Protection.

COULTHARD, T., FROSTICK, L., HARDCASTLE, H., JONES, K., ROGERS, D. & SCOTT, M. 2007. The June 2007 floods in Hull Interim Report by the Independent Review Body 24th August 2007.

COULTHARD, T. J. & FROSTICK, L. E. 2010. The Hull Floods of 2007: implications for the governance andmanagementof urban drainage systems. Journal of Flood Risk Management, 3, 223-231.

CRAWFORD, D., FOX, R., KAMDEN, P., LEBOW, S., NICHOLAS, D., PETTRY, D., SCHULTZ, T., SITES, L. & ZIOBRO, R. Laboratory Studies of CCA-C Leaching: Influence of Wood and Soil Properties on Extent of Arsenic and Copper Depletion. International Research Group on Wood Preservation, 33rd Annual Meeting, 2002 2002.

DAGAN, R., BITTON, G. & TOWNSEND, T. G. 2006. Metal Transport and Bioavailability in Soil Contaminated with CCA-Treated Wood Leachates. Soil & Sediment Contamination, 15, 61 - 72.

DEFRA 2000. Chapter 6. Handling hazardous waste. Waste Strategy 2000, England and Wales: Part 2. London.

DEFRA 2002. Soil Guideline Values for Chromium Contamination. R&D Publications. Bristol: Environment Agency.

DOBRAN, S. & ZAGURY, G. R. J. 2006. Arsenic speciation and mobilization in CCA-contaminated soils: Influence of organic matter content. Science of The Total Environment, 364, 239-250.

EA 2005. Hazardous waste: Interpretation of the definition and classification of hazardous waste Technical Guidance WM2. Environment Agency. Bristol: Environment Agency.

EA 2006. Classification and Environmental Standards in the Water Framework Directive. Environment Agency.

EPA 1994a. Method 3015: Microwave Assisted Acid Digestion of Aqueous Sample and Extracts.EPA 1994b. Test methods for evaluating solid waste. Washington, DC.: Office of Solid Waste and

Emergency Response.GIFFORD, J. S., MARVIN, N. A. & DARE, P. H. 1997. Composition of Leachate from Field Lysimeters

Containing CCA Treated Wood. Proceedings of the 93rd Annual Meeting. Selma, AL.: American Wood Protection Association (AWPA).

HASAN, A. R., HU, L., SOLO-GABRIELE, H. M., CAI, Y. & FIEBER, L. A. 2008. Leachability of Arsenic, Chromium and Copper from Weathered Treated Wood. IRGWP 39th Annual Meeting. Istanbul: International Research Group on Wood Protection.

HASAN, A. R., HU, L., SOLO-GABRIELE, H. M., FIEBER, L., CAI, Y. & TOWNSEND, T. G. 2010. Field-scale leaching of arsenic, chromium and copper from weathered treated wood. Environmental Pollution, 158, 1479-1486.

HINGSTON, J. A., COLLINS, C. D., MURPHY, R. J. & LESTER, J. N. 2001. Leaching of chromated copper arsenate wood preservatives: a review. Environmental Pollution, 111, 53-66.

HINGSTON, J. A., MOORE, J., BACON, A., LESTER, J. N., MURPHY, R. J. & COLLINS, C. D. 2002. The importance of the short-term leaching dynamics of wood preservatives. Chemosphere, 47, 517-523.

HSE 2009. Copper, Chrome and Arsenic Treated Timber in Children's Playgrounds. Local Authority Circular. Health and Safety Executive.

JAMBECK, J. 2004. The Disposal of CCA-Treated Wood in simulated Landfills: Potential Impacts. Dissertation/Thesis, University of Florida.

JAMBECK, J. R., TOWNSEND, T. & SOLO-GABRIELE, H. 2006. Leaching of chromated copper arsenate (CCA)-treated wood in a simulated monofill and its potential impacts to landfill leachate. Journal of Hazardous Materials, 135, 21-31.

JAMES, B. R. & BARTLETT, R. J. 1982. Behaviour of Chromium in Soils: VII. Adsorption and Reduction of Hexavalent Forms. Journal of Environmental Quality, 12, pp 171-181.

JUNNINEN, H., NISKA, H., TUPPURAINEN, K., RUUSKANEN, J. & KOLEHMAINEN, M. 2004. Methods for imputation of missing values in air quality data sets. Atmospheric Environment, 38, 2895-2907.

KARTAL, S. N., HWANG, W. & IMAMURA, Y. 2005. Effects of hard water, sea water and humic acid on the release of CCA components from treated wood. IRGWP 36th Annual Meeting. Bangalore: International Research Group on Wood Protection.

KENNEDY, M. J. & COLLINS, P. A. 2001. Leaching of preservative components from pine decking treated with CCA and copper azole, and interactions of leachates with soils. IRGWP 32nd Annual Meeting. Japan: International Research Group on Wood Protection.

KHAN, B. I., JAMBECK, J., SOLO-GABRIELE, H. M., TOWNSEND, T. G. & CAI, Y. 2006a. Release of Arsenic to the Environment from CCA-Treated Wood. 2. Leaching and Speciation during Disposal. Environmental Science & Technology, 40, 994-999.

KHAN, B. I., SOLO-GABRIELE, H. M., DUBEY, B. K., TOWNSEND, T. G. & CAI, Y. 2004a. Arsenic Speciation of Solvent-Extracted Leachate from New and Weathered CCA-Treated Wood. Environmental Science and Technology, 38, 4527.

KHAN, B. I., SOLO-GABRIELE, H. M., DUBEY, B. K., TOWNSEND, T. G. & CAI, Y. 2004b. Arsenic Speciation of Solvent-Extracted Leachate from New and Weathered CCA-Treated Wood. Environmental Science & Technology, 38, 4527-4534.

KHAN, B. I., SOLO-GABRIELE, H. M., TOWNSEND, T. G. & CAI, Y. 2006b. Release of Arsenic to the Environment from CCA-Treated Wood. 1. Leaching and Speciation during Service. Environmental science & technology, 40, 988-993.

KO, B.-G., VOGELER, I., BOLAN, N. S., CLOTHIER, B., GREEN, S. & KENNEDY, J. 2007. Mobility of copper, chromium and arsenic from treated timber into grapevines. Science of The Total Environment, 388, 35-42.

LEBOW, S., COOPER, P. & LEBOW, P. 2004. Variability in Evaluating Environmental Impacts of Treated Wood. Environmental Impacts of Preservative-Treated Wood Conference. Orlando, Florida.

LEBOW, S., WILLIAMS, R. S. & LEBOW, P. 2003. Effect of Simulated Rainfall and Weathering on Release of Preservative Elements from CCA Treated Wood. Environmental science & technology, 37, 4077-4082.

LEBOW, S. T., COOPER, P. A., LEBOW, P. K., TOWNSED, T. & SOLO-GABRIELE, H. 2006. Chapter 5: Study Design Considerations in Evaluating Environmental Impacts. Environmental Impacts of Treated Wood. Boca Raton: Taylor & Francis.

LEBOW, S. T., FOSTER, D. O. & LEBOW, P. K. 1999. Release of Copper, Chromium and Arsenic from Treated Southern Pine Exposed in Seawater and Freshwater. Forest Products Journal, 49, 80-89.

MACUR, R. E., WHEELER, J. T., MCDERMOTT, T. R. & INSKEEP, W. P. 2001. Microbial Populations Associated with the Reduction and Enhanced Mobilization of Arsenic in Mine Tailings. Environmental science & technology, 35, 3676-3682.

MASSCHELEYN, P. H., DELAUNE, R. D. & PATRICK, W. H. 1991. Arsenic and Selenium chemistry as Affected by Sediment Redox Potential and pH. Journal of Environmental Quality, 20, 522-527.

MELCHER, E. & PEEK, R. D. A comparison of the migration behaviour in soil of different waterborne wood preservatives and their leachates. International Research Group on Wood Protection, 28th Annual Meeting, Canada. IRG/WP/97-50091., 1997.

MELCHER, E. & PEEK, R. D. Dimension of lysimeters and the evaluation of the migration of wood preservative components in soil. International Research Group on Wood Preservation, 27th Annual Meeting, France. IRG/WP/96-50065., 1996.

MELCHER, E. & PEEK, R. D. Dimension of lysimeters and the evaluation of the migration of wood preservative components in soil. International Research Group on Wood Preservation, 27th Annual Meeting, France. IRG/WP/96-50065., 1996 1996.

MERCER, T. G., FROSTICK, L. E. & WALMSLEY, A. D. 2011. Recovering incomplete data using Statistical Multiple Imputations (SMI): A case study in environmental chemistry. Talanta, 85, 2599-2604.

MOGHADDAM, A. H. & MULLIGAN, C. N. 2008. Leaching of heavy metals from chromated copper arsenate (CCA) treated wood after disposal. Waste Management, 28, 628-637.

MURPHY, R. J., MCQUILLAN, P., JERMER, J. & PEEK, R. D. 2004. Preservative-treated wood as a component in the recovered wood stream in Europe - A quantitative and qualitative review. IRGWP 35th Annual Meeting. Slovenia: International Research Group on Wood Protection.

NICO, P. S., FENDORF, S. E., LOWNEY, Y. W., HOLM, S. E. & RUBY, M. V. 2004. Chemical Structure of Arsenic and Chromium in CCA-Treated Wood: Implications of Environmental Weathering. Environmental Science & Technology, 38, 5253-5260.

OPSI 2005. Statutory Instrument 2005 No. 894: The Hazardous Waste (England and Wales) Regulations 2005. England and Wales.

PANTSAR-KALLIO, M. & MANNINEN, P. K. G. 1997. Simultaneous determination of toxic arsenic and chromium species in water samples by ion chromatography-inductively coupled plasma mass spectrometry. Journal of Chromatography A, 779, 139-146.

PEARSON, G. F. 2007. Elemental Speciation and Miniaturised Sample Introduction Studies for Inductively Coupled Plasma Mass Spectrometry. Dissertation/Thesis, University of Hull.

PEARSON, G. F., GREENWAY, G. M., BRIMA, E. I. & HARIS, P. I. 2007. Rapid Arsenic Speciation using Ion Pair LC-ICPMS with a Monolithic Silica Column Reveals Increased Urinary DMA Excretion After Ingestion of Rice. Journal of Analytical Atomic Spectrometry, 22, 361-369.

SCHMALZL, K. J., FORSYTH, C. M. & EVANS, P. D. 2003. Evidence for the formation of chromium (III) diphenoquinone complexes during oxidation of guaiacol and 2,6-dimethoxyphenol with chromic acid. Polymer Degradation and Stability, 82, 399-407.

SCHULTZ, T. P., NICHOLAS, D. D. & PETTRY, D. E. 2002. Depletion of CCA-C from Ground-Contact Wood: Results from Two Field Sites with Significantly Different Soils. Holzforschung, 56, 125-129.

SHIBATA, T., SOLO-GABRIELE, H. M., DUBEY, B., TOWNSEND, T. G. & JACOBI, G. A. 2006. Arsenic Leaching from Mulch Made from Recycled Construction and Demolition Wood and Impacts of Iron-Oxide Colorants. Environmental Science & Technology, 40, 5102-5107.

SHIBATA, T., SOLO-GABRIELE, H. M., FLEMING, L. E., CAI, Y. & TOWNSEND, T. G. 2007. A mass balance approach for evaluating leachable arsenic and chromium from an in-service CCA-treated wood structure. Science of The Total Environment, 372, 624-635.

SMEDLEY, P. L. & KINNIBURGH, D. G. 2002. A review of the source, behaviour and distribution of arsenic in natural waters. Applied Geochemistry, 17, 517-568.

SMITH, R. 2005. The preservation and degradation of wood in wetland archaeological and landfill sites. PhD Dissertation/Thesis, University of Hull.

SOLO-GABRIELE, H., KHAN, B., TOWNSEND, T., SONG, J., JAMBECK, J., DUBEY, B., JANG, Y. & CAI, Y. 2004. Arsenic and Chromium Speciation of Leachates from CCA-Treated Wood: Final Report. Gainesville, Florida: Florida Center for Solid and Hazardous Waste Management.

SOLO-GABRIELE, H. M., TOWNSEND, T. G. & SCHERT, J. 2003. Environmental Impact of CCA-Treated Wood: A Summary from Seven Years of Study Focusing on the U.S. Florida Environment. IRGWP 34th Annual Meeting. Brisbane: International Research Group on Wood Protection.

SONG, J., DUBEY, B., JANG, Y.-C., TOWNSEND, T. & SOLO-GABRIELE, H. 2006. Implication of chromium speciation on disposal of discarded CCA-treated wood. Journal of hazardous materials, 128, 280-288.

STEFANOVIC, S., COOPER, P., TOWNSEND, T. G. & SOLO-GABRIELE, H. 2006. Chapter 6: Leaching of Chromated Copper Arsenate, Alkaline Copper Quaternary and Copper Azole Components from Wood Exposed to Natural Weathering Aboveground and Reaction of Leachates with Soil. Environmental Impacts of Treated Wood. Boca Raton: Taylor & Francis.

STILWELL, D. E. & GORNY, K. D. 1997. Contamination of Soil with Copper, Chromium and Arsenic Under Decks Built from Pressure Treated Wood. Bulletin of Environmental Contamination and Toxicology, 58, 22-29.

STUMM, W. & MORGAN, J. J. 1981. Aquatic Chemistry: An Introduction Emphasizing Chemical Equilibria in Natural Waters, Canada, John Wiley & Sons.

TAYLOR, J. L. & COOPER, P. A. 2003. Leaching of CCA from lumber exposed to natural rain aboveground. Forest Products Journal, 53, 81-86.

TAYLOR, J. L. & COOPER, P. A. 2005. Effect of climatic variables on chromate copper arsenate (CCA) leaching during above-ground exposure. Holzforschung, 59, 467-472.

TOWNSEND, T., DUBEY, B., TOLAYMAT, T. & SOLO-GABRIELE, H. 2005. Preservative leaching from weathered CCA-treated wood. Journal of Environmental Management, 75, 105-113.

TOWNSEND, T., TOLAYMAT, T., SOLO-GABRIELE, H., DUBEY, B., STOOK, K. & WADANAMBI, L. 2004. Leaching of CCA-treated wood: implications for waste disposal. Journal of Hazardous Materials, 114, 75-91.

TOWNSEND, T. G., SOLO-GABRIELE, H., TOLAYMAT, T. & STOOK, K. 2003. Impact of chromated copper arsenate (CCA) in wood mulch. The Science of The Total Environment, 309, 173-185.

VAN EETVELDE, G., WALDEMAR, J. H., MILITZ, H. & STEVENS, M. 1995. Effect of leaching temperature and water acidity on the loss of metal elements from CCA treated timber in aquatic applications: Part 2: Semi-industrial investigation. IRGWP 26th Annual Meeting. Helsinger: International Research Group on Wood Protection.

WALDRON, L., COOPER, P. & UNG, T. 2005. Diffusion modelling of inorganic wood preservative leaching in service. 26th Annual Meeting. France: International Research Group on Wood Protection.

WARNER, J. E. & SOLOMON, K. R. 1990. Acidity as a Factor in Leaching of Copper, Chromium and Arsenic from CCA-treated Dimension Lumber. Environmental Toxicology and Chemistry, 9, 1331 - 1337.

WEIS, J. S. & WEIS, P. 2004. Effects of CCA Wood on Non-Target Aquatic Biota. Proceedings of the Environmental Impacts of Preservative-Treated Wood Conference. Gainsville, Florida.

WHO 2004. Guidelines for Drinking Water Quality, Vol. 1, 3rd Edition. Chapter 8: Chemical Aspects. Geneva: World Health Organisation.

WRAP 2004. Treated Wood Waste: assessment of the waste management challenge - Summary Report. Oxon.

WRAP 2005. Options and risk assessment for treated wood waste: Summary Report. [Online]. Buckinghamshire: TRADA.

YAMAMOTO, K., MOTEGI, S. & INAI, A. 1999. Comparative study on the leaching of wood preservatives between natural exposure and accelerating laboratory conditions. IRGWP 30th Annual Meeting. Rossenheim: International Research Group on Wood Protection.

YOON, H., KANG, H. & HANG, Y. 2005. Long-Term Release of Arsenic, Chromium and Copper from CCA-Treated Wood and Ash. IRGWP 36th Annual Meeting. Bangalore: International Research Group on Wood Protection.

ZAGURY, G. J., SAMSON, R. & DESCHENES, L. 2003. Occurrence of Metals in Soil and Ground Water Near Chromated Copper Arsenate-Treated Utility Poles. Journal of Environmental Quality, 32, 507-514.