A case study of long-term CCA preservative leaching from treated hardwood poles in a humid tropical condition

Abstract

Chromated copper arsenate (CCA)-treated Malaysian hardwoods have long been used as utility poles, posts, construction piles and motorway fencing in soil contact exposed to the threats of decay fungi and termites. Despite global concerns citing predominantly temperate conditions of long-term leaching of CCA toxic heavy metals from wood into surrounding soils and groundwater since the 1990’s, the preservative leaching severity in the tropics has been far less appreciated due to dearth of work in this area. In 2013 (after 30 years exposure), levels of total copper, chromium and arsenic within 20 treated hardwood poles of Sarawak and in soils surrounding these poles, installed in 1980 and 1981 at a plot located in Timber Research and Technical Training Centre, Kuching, Sarawak, Malaysia, were sampled. The ground is waterlogged after heavy rainfall. It is shown that there is insignificant variations of CCA salt retention in wood between 1300 cm above ground and 0-20 cm below ground (P<0.05). Nevertheless levels of these elements are significantly (P<0.05) elevated in soils surrounding, especially up to 25 mm away from, the poles than at distant sampling points (150 – 300 mm) from poles as well as at sites well away from the poles containing very low levels (<6 – 13.4 ppm) of such heavy metals. Metal levels were also highest at the soil surface directly in contact with the poles (0 – 50 mm soil depth position) and decreased with remaining 2 soil depth positions 150 – 200 mm and 300 – 350 mm. Mean extractable arsenic levels ranged from 14.5 to 100.1 ppm, chromium levels from 23.3 to 148.3 ppm and copper from 21.8 to 104.7 ppm. Results, rather than indicating relatively higher CCA leaching, concurred with that reported temperate experience and showed that soil closest to the treated poles are most contaminated, albeit slightly, after 30 years of in-ground exposure.

Keywords: Tropical hardwoods, CCA preservative, long-term leaching, contaminated soil

Full text

Proceedings IRG Annual Meeting (ISSN 2000-8953)
© 2016 The International Research Group on Wood Protection
IRG/WP 16-50324
THE INTERNATIONAL RESEARCH GROUP ON WOOD PROTECTION
Section 5 Sustainability and Environment
A Case Study of Long-term CCA Preservative Leaching from Treated
Hardwood Poles in a Humid Tropical Condition
Andrew H.H. Wong1 and Willy S.M. Chin2
1Universiti Malaysia Sarawak (Unimas),
Faculty of Resource Science & Technology,
94300 Kota Samarahan, Sarawak, Malaysia
Email: ahhwong@unimas.my
2Sarawak Forestry Corporation (SFC)
Lot 218, KCLD, Jalan Tapang
93250 Kota Sentosa, Kuching, Sarawak, Malaysia
Email: chinwsm@gmail.com
Paper prepared for the 47th Annual Meeting
Lisbon, Portugal
15-19 May 2016
Disclaimer
The opinions expressed in this document are those of the author(s) and are not necessarily
the opinions or policy of the IRG Organization.
IRG Secretariat
Box 5609
SE-114 86 Stockholm
Sweden
www.irg-wp.com

2
A Case Study of Long-term CCA Preservative Leaching from Treated
Hardwood Poles in a Humid Tropical Condition
Andrew H.H. Wong1 and Willy S.M. Chin2
1Universiti Malaysia Sarawak (Unimas),
Faculty of Resource Science & Technology,
94300 Kota Samarahan, Sarawak, Malaysia
Email: ahhwong@unimas.my
2Sarawak Forestry Corporation (SFC),
Lot 218, KCLD, Jalan Tapang,
93250 Kota Sentosa, Kuching, Sarawak, Malaysia
Email: chinwsm@gmail.com
Abstract
Chromated copper arsenate (CCA)-treated Malaysian hardwoods have long been used as
utility poles, posts, construction piles and motorway fencing in soil contact exposed to the
threats of decay fungi and termites. Despite global concerns citing predominantly temperate
conditions of long-term leaching of CCA toxic heavy metals from wood into surrounding
soils and groundwater since the 1990’s, the preservative leaching severity in the tropics has
been far less appreciated due to dearth of work in this area. In 2013 (after 30 years exposure),
levels of total copper, chromium and arsenic within 20 treated hardwood poles of Sarawak
and in soils surrounding these poles, installed in 1980 and 1981 at a plot located in Timber
Research and Technical Training Centre, Kuching, Sarawak, Malaysia, were sampled. The
ground is waterlogged after heavy rainfall. It is shown that there is insignificant variations of
CCA salt retention in wood between 1300 cm above ground and 0-20 cm below ground
(P<0.05). Nevertheless levels of these elements are significantly (P<0.05) elevated in soils
surrounding, especially up to 25 mm away from, the poles than at distant sampling points
(150 – 300 mm) from poles as well as at sites well away from the poles containing very low
levels (<6 – 13.4 ppm) of such heavy metals. Metal levels were also highest at the soil surface
directly in contact with the poles (0 – 50 mm soil depth position) and decreased with
remaining 2 soil depth positions 150 – 200 mm and 300 – 350 mm. Mean extractable arsenic
levels ranged from 14.5 to 100.1 ppm, chromium levels from 23.3 to 148.3 ppm and copper
from 21.8 to 104.7 ppm. Results, rather than indicating relatively higher CCA leaching,
concurred with that reported temperate experience and showed that soil closest to the treated
poles are most contaminated, albeit slightly, after 30 years of in-ground exposure.
Keywords: tropical hardwoods, CCA preservative, long-term leaching, contaminated soil

3
Introduction
The unique characteristics, versatility and renewable nature of wood makes it an extensively
used structural material and other wood products worldwide. However the current severe
shortage of naturally durable woods in for in-ground structures is offset by the use of
preservative-treated wood (eg. treated with Chromated Copper Arsenate and new
generation Copper-organic preservatives) has been traditionally used in ground-contact as
powerline and telecommunication poles, posts, motorway fencing barriers, and
landscapping structures.
In Malaysia, with humid tropical climate, chromate copper arsenate (CCA) are commonly
used to treat wood for structural uses like timber roof trusses under Hazard class H2 (Wong
et al. 2000, Wong et al. 2006) and utility poles, although treated wood markets now face
severe competition from alternative materials (ie. metals and concrete). In ground contact,
CCA preservative treated wood with high retention values may last considerably longer than
untreated wood (Ling and Wong 2005) which is why CCA is regarded among the most cost-
effective preservative in protecting wood for many years from a wide spectrum of terrestrial
wood degrading organisms (Wong et al. 2006, Wong and Lai 2007). However, amid
considerable health and environmental safety concerns of CCA-treated wood expressed by
developed economy nations, studies on CCA-leaching from treated wood of especially
arsenic has led to CCA treated timbers to be subjected to stringent environmental controls in
these countries where uses of heavy metals in wood preservatives have been slowly phased
out in recent years in preference for organic preservatives instead (Cooper 1994, Lebow
1996, Hingston et al. 2001, Wong et al. 2006). Hence use of CCA-treated wood is now
restricted to non-residential uses mainly in temperate climes (Cooper 1994, Lebow 1996,
Hingston et al. 2001, Stilwell et al. 2006) or even replaced with alternative preservatives or
nonbiocidal treatments. While CCA is widely used in the humid tropics, perhaps not much
work on CCA leaching from exposed treated wood in the tropics (apart from sub-tropical
Florida, US) is done to ascertain relative leaching severities between both climates. Only
with more supporting field CCA-leaching data emerging from humid/wet tropical regions can
real concerns of environmental impacts of such treated wood in these regions be raised
(Wong and Chong 2014).
The objective of this study is to determine if there is long term CCA preservative leaching
from ground-contact exposed treated hardwoods of Sarawak, Malaysia, into the surrounding
soil, by assessing total levels of Chromium, Arsenic, and Copper in soil surrounding CCA
treated poles as well assess residual CCA retention at aboveground, groundline and below
ground of treated wooden poles sampled.
Materials and Methods
Between 1980 and 1981 several 2.44 m long indigenous hardwood poles were vertically
installed at spacings of 2.5 m, 3 m or 4 m between poles and held 0.6 m below ground at
Timber Research and Technical Training Centre (TRTTC), Kota Sentosa, Sarawak, Malaysia.
The poles were treated to refusal by the full-cell process with Chromated Copper Arsenate
(CCA) Tanalith-C® formulation (35% Cu as CuSO4.5H2O, 45% Cr as K2Cr2O7 and 20% As as
As2O5.2H2O). Wood was sampled 30 years after exposure to direct sunlight, weathering, high

4
humidity, high rainfall with periodically waterlogged acidic (pH 4.8 – 5.6) podzolic soils
(Andriesse 1975).
Soil sampling
Soil sampling method is adapted from Morrell and Huffman (2004). Soil auger was used to
collect soil samples next to each pole and also at fixed horizontal distances away from the
sample pole (ie. 0-25 mm, 150 mm and 300 mm distances) while vertical depths were
confined to each of these 3 selected horizontal position (ie. depths of 0-50 mm, 150-200 mm
and 300-350 mm). Background (control) pH and metal levels in soils were accessed by taking
soil samples at least 6 m from the poles and at similar vertical soil depth levels. All the
samples were oven dried at 105°C for five hours, then crushed with mortar and pestle and
then sieved through a 0.425 mm mesh to obtain homogeneous sample for extraction and
analysis.
Soil pH
Soil pH was determined only for soil 6 m away from treated poles (background or control
soils)to represent soil pH in the study plot of <1 ha. Soil at 3 depths were collected, oven
dried at 105oC for 5 hr before grinding and sieving through a 2 mm sieve. Five gram soil and
25 mL distiled water was placed in a beaker. Beaker agitated for 2 hr, teh soil allowed to
settle and the pH measured recorded with a pH meter.
Soil metal extraction (Aqua regia digestion).
Pre-oven dry-weighed about 2 g of soil was put into 50 mL beaker. Next, 20 mL of aqua regia
(AR) solution [95 mL of 69-70% m/m nitric acid (HNO3) mixed with 15 mL of 37% m/m
hydrochloric acid (HCl)] was added and heated to 130°C for 2 hours. Then, the contents were
cooled. After this 1 mL of 69-70% m/m nitric acid and 20 mL of distilled water were added to
the contents before reheating at 130°C for 1 hour. The contents were cooled at room
temperature and the solution filtered through Whatman No. 42 filter paper. The filtrate was
transferred into 100 mL volumetric flask and distilled water added to make up the 100 mL
mark.
Wood sampling
Twenty poles were randomly sampled, but only 13 poles (Table 1) had initial CCA retention
(by uptake) records made 30 years ago. Wood samples about 12 mm in lateral depth from
the surface were removed from the pole at 1.3 m above ground and at 0-20 cm below
ground level. After that, the wood samples were pulverized using a mill and sieved through a
0.425 mm mesh. Then, the woodmeal was mixed evenly and oven dried at 105 °C for 24
hours.
Metal extraction from treated wood (Copper, Chromium and Arsenic)
Extraction of wood preservative components is adapted from BS5666: Part 3 (British
Standard 1991). Pre-oven dry weighed (ca. 2 g) of woodmeal was transferred into a 250 mL
conical flask. Then, 40 mL of 2.5M sulphuric acid (H2SO4) solution and 8 mL of 30% hydrogen

5
peroxide (H2O2) solution were added. The mixture was heated at 75°C in a water bath for 30
min with occasional agitation of the mixture.
After that, 80 mL of distilled water was added and reheated until ceasation of evolution of
bubbles signaling the full degradation of hydrogen peroxide. The mixture was filtered
through Whatman No. 42 filter paper and transferred into a 250 mL volumetric flask. Next,
20 mL of 3% sodium sulphate (Na2SO4) solution was added and distilled water used to top up
to the 250 mL mark.
Basic density of wood
Methods for the measurement of wood basic density were adopted from the ASTM D 2395-
93 (ASTM, 2000). Briefly, wood sample was oven dried at 105 °C for 24 hours and weighed.
Next, a beaker was filled ¾ full with distilled water and the wood sample was immersed in
the water. The weight of water displaced by the immersed sample was obtained which is a
measure of the wood volume (cm3).
Analysis and measurement of metal levels (Copper, Chromium and Arsenic)
The prepared extracts were then analyzed using Flame Atomic Absorption Spectrometry
(Thermo Fisher’s iCE 3000 Series AA). Results were recalculated to reflect the concentration
of the metals in the samples and then subjected to statistical analysis using SPSS Statistical
software Ver. 17.0. Flame AAS detection limit of Arsenic is at 0.12 ppm, Copper at 0.0045
ppm and Chromium at 0.0054 ppm.
As per BS5666:Part 3 method, the weight of retained CCA salts in wood was calculated from
metal concentrations obtained from AAS by multiplying with the following factors:
Copper as copper sulphate (CuSO4.5H2O) = 3.93
Chromium as potassium dichromate (K2Cr2O7) = 2.83
Arsenic as arsenic pentoxide (As2O5.2H2O) = 1.77
The dry salt retention in treated wood was expressed as kg/m3 by the followign expression
based on basic density of wood:
 
 
3
/
000,000,1 /mkgD
kgmgCBA 

where,
A = Concentration of As2O5.2H2O
B = Concentration of K2Cr2O7
C = Concentration of CuSO4.5H2O
D = Initial basic density of wood

6
Results and Discussion
CCA preservative levels in poles
All the poles belonged to the non-Dipterocarp species and only Hydnocarpus sp. (Senumpul)
and Koompassia malaccensis (menggris, kempas) species exhibited significantly higher basic
density in the range of 650-750 kg/m3 while the other species Macaranga spp., and
Cratoxylum spp. (Geronggang) are of much lower density ranges of 200-380 kg/m3 (Table 1).
From findings on the first 13 poles (Table 1), there were significant (P<0.05) losses of CCA
salt when mean CCA retention (determined by chemical analysis in present study) after 30
years exposure was compared with initial retention of CCA salt (by uptake basis determined
30 years ago). Such retention reductions are as that of earlier work on leaching of CCA
treated timber (Lebow 1996, Solo-Gabriele et al. 2000, Townsend et al. 2001, Morrell and
Huffman 2004).
Table 1. Species and retention of CCA salt (kg/m3) in treated poles planted in 1980 and 1981
exposed for about 30 years
Pole
No.
Species
Basic
Density
(kg/m3)
Before installing
After 30 years in service
Initial retention
(kg/m3)
1300 cm above
ground level
Final retention
(kg/m3)
0-20 cm below
ground level
Final retention
(kg/m3)
1
Macaranga sp.
349.37
65.81
62.39
56.70
2
Macaranga sp.
283.69
78.00
68.58
46.24
3
Macaranga sp.
334.98
61.10
55.39
53.93
4
Macaranga sp.
335.08
64.55
53.24
54.26
5
Macaranga sp.
202.88
25.10
16.03
18.44
6
Macaranga sp.
306.47
70.80
52.95
48.89
7
Macaranga sp.
342.23
52.00
53.23
45.31
8
Macaranga sp.
308.09
79.60
56.45
49.78
9
Cratoxylum sp.
283.37
26.00
32.98
50.11
10
Macaranga sp.
361.53
71.51
51.07
52.26
11
Macaranga sp.
292.64
36.50
33.95
58.06
12
Macaranga sp.
328.56
56.60
37.19
47.97
13
Macaranga sp.
351.11
59.77
49.83
52.64
14
Macaranga sp.
385.18
n.d.
60.77
52.44
15
Hydnocarpus sp.
679.04
n.d.
42.38
41.98
16
Cratoxylum sp.
293.82
n.d.
36.20
50.35
17
Koompassia malaccensis
703.94
n.d.
28.67
24.32
18
Artocarpus integer
519.50
n.d.
59.23
56.82
19
Artocarpus integer
331.35
n.d.
36.56
40.66
20
Macaranga sp.
340.87
n.d.
51.96
51.45
*n.d. is “not determined”.
Mean of difference in the retention values of CCA treated timber between 1300 cm above
ground and at 0-20 cm below ground level is 0.68±10.17 kg/m3. It is expected that CCA
components leached out more from the wood surface in contact with soil, but no significant
difference (P<0.05) between retention of CCA salt above ground and below ground were
found, which differ from observations of Morrell and Huffman (2004) of difference in surface
retentions between the below and above ground samples after 45 yrs exposure which led

7
them to suggest groundline loss of CCA had occurred over the soil-contact exposure period.
Probably, relatively stronger tropical weathering effect may contribute to the present result
but another possible explanation is that CCA components are redistributed within the wood
as the loss of CCA at the surface of wood to the environment are replaced by leached CCA
within the wood. This redistribution is observed by Choi et al. (2004) when during the first
year of exposure provides strong evidence of mobile CCA components redistributed within
wood.
Metal Levels in soils surrounding treated poles
Background (control) level of copper (Cu) and chromium (Cr) detected in soil have mean
concentration of 8.04 and 12.56 mg/kg respectively while arsenic (As) is at levels
undetectable by AAS. Levels of mean soil pH varied little, ie. 4.96 (sampled 150 – 200 mm
depth), 5.24 (0 – 50 mm depth) and 5.58 (300 – 350 mm).
For soils surrounding poles sampled, Cr leached out the most, followed by Cu and As (Table
2). Relative leachability of Cu, Cr and As were in proportion to initial concentrations of the
preservative solution regardless of fixation periods (Wong et al. 2006). With metal ratios of
its respective treatment salt solution as 35% Cu, 45% Cr and 20% As, it is expected that Cr
yield the highest leaching rate followed by Cu and As. Cr element had the highest mean
leaching range of 39.94 – 148.32 mg/kg adjacent to the pole and declined to a range of 23.31
– 44.06 mg/kg 300 mm away from the pole. Comparatively, mean Cu levels were elevated
adjacent to the pole ranging 40.99 – 104.72 mg/kg and declined away from the poles with
mean range of 21.83 – 30.02 mg/kg. As levels leached and contaminated the soil adjacent to
the pole at high mean concentrations ranging from 62.90 to 100.06 mg/kg and declined to
mean concentration of 14.53 – 20.86 mg/kg. The leached As was found to be multiple times
higher than the findings of others who had mean As of 28 mg/kg (Townsend et al. 2001) or
with a range of 1.74 – 8.19 mg/kg (Morrell and Huffman 2004) for long-term exposed wood.
Table 2. Residual copper, chromium and arsenic levels in soils at selected depths and
horizontal distances from poles treated with chromated copper arsenate (CCA)
Residual metal levels (ppm) at each sampling depth (mm)
Depth of soil
(mm)
Metal
elements
Horizontal distance from pole (mm)
0-25 mm
150-200 mm
300-350 mm
0-50 mm
As
100.06
(74.41)
44.91
(25.63)
20.86
(13.47)
Cu
104.72
(44.78)
56.43
(26.97)
30.02
(10.04)
Cr
148.32
(64.31)
72.49
(36.57)
44.06
(18.25)
150-200 mm
As
79.90
(91.86)
30.06
(19.49)
17.87
(4.43)
Cu
71.01
(44.47)
44.94
(27.79)
24.39
(9.16)
Cr
81.58
(46.14)
48.98
(23.63)
33.26
(17.00)
300-350 mm
As
62.90
(45.96)
16.84
(10.92)
14.53
(8.29)
Cu
40.99
(24.79)
26.30
(12.61)
21.83
(9.26)
Cr
39.94
(26.71)
26.79
(12.16)
23.31
(10.43)
aNumbers in parentheses represent standard deviation, bSample size, n = 20

8
Overall, unlike findings of Morrell and Huffman (2004) where Cu was the leached the most,
Cr in the present study was found here to be leached at a highest concentration compared
to other CCA elements. Cr as a fixative to bind Cu and As salts to the ligno-cellulosic wood
substrate if severely leached may explain a somewhat high degree of leaching of As and Cu
in the present study, when compared to findings of others (Morrell and Huffman 2004). This
relative degree of leaching among CCA components also can be explained by various reports
(Wong et al. 2006, Wong and Lai 2007, Salim et al. 2012) that total Cr, As and Cu were not
completely fixed in Malaysian tropical hardwoods regardless of fixation time. Salim et al.
(2012) concluded that there is a strong positive correlation between unfixed and leached
amounts of Cu and Cr, and this may be a reason for some leaching of these elements seen in
those treated Malaysian tropical hardwoods. Also short-term leaching test results can be
unrelated to long-term trials as longer time-frames could yield different outcomes. Example,
short-term (laboratory or rapid field) leaching tests of treated wood in soil contact higher
leaching losses of As than Cr and Cu have been reported instead (Hua et al. 2010, Wong and
Chong 2014).
Figures 1 – 3, adapted from Table 2, shows the mean concentration of each individual metal
at soil adjacent to treated poles (mg/kg) compared to background (control) metal levels
recorded at soil control site. Mean concentration of all heavy metals (chromium, copper and
arsenic) are found to be higher than the mean concentration of respective metal levels at
control site and declined significantly (P<0.05) the further the distance away from the poles.
Metal levels declined almost proportionally with soil depth. Lateral movements of CCA
leachates depend not only on soil profile, but also precipitation and gravitational pull.
Horizontal movements of metal elements depend on the soil properties, disturbances and
diffusion effect. The metals immediately decreases significantly in the first 150 mm distance
from pole probably attributed to diffusion effect of metal ions in the soil, whereby the soils
with higher concentration of metal ions will migrate to soils of lower concentrations
provided that the metal ions do not form with organic matter especially in the top soil.
Figure 1. Mean concentration of arsenic (mg/kg) in soil at different depth (mm) and
distance from pole (mm) compared to arsenic concentration of soil control

9
Figure 2. Mean concentration of chromium (mg/kg) in soil at different depth
(mm) and distance from pole (mm) compared to chromium
concentration of soil control
Figure 3. Mean concentration of copper (mg/kg) in soil at different depth
(mm) and distance from pole (mm) compared to copper concentration
of soil control

10
Table 3. Copper, Chromium and Arsenic levels in soils at various depths and distances from
poles of different CCA retention classes
Residual metal levels (ppm)
Metal Elements
CCA Retention
(kg/m3)
Soil depth (mm)
Horizontal distance from pole (mm)
0-25 mm
150 mm
300 mm
Copper (Cu)
>50 kg/m3
0-50 mm
95.84
(17.04)
64.89
(31.63)
32.32
(11.36)
150-200 mm
65.35
(31.48)
51.16
(35.50)
29.59
(9.36)
300-350 mm
36.48
(15.42)
28.86
(14.61)
22.57
(8.98)
40 - 50 kg/m3
0-50 mm
126.93
(76.88)
51.55
(16.60)
28.60
(9.02)
150-200 mm
83.06
(71.59)
37.97
(9.60)
19.19
(5.39)
300-350 mm
46.87
(39.86)
25.45
(12.00)
22.78
(12.22)
<40 kg/m3
0-50 mm
92.91
(26.45)
35.16
(8.58)
24.44
(5.24)
150-200 mm
67.64
(19.69)
36.09
(18.13)
17.50
(0.73)
300-350 mm
45.79
(20.66)
19.47
(2.13)
17.46
(1.03)
Chromium (Cr)
>50 kg/m3
0-50 mm
170.49
(49.05)
92.68
(35.87)
53.24
(18.79)
150-200 mm
88.68
(44.72)
59.56
(21.12)
44.53
(14.50)
300-350 mm
40.94
(18.37)
32.80
(11.98)
28.18
(10.71)
40 - 50 kg/m3
0-50 mm
127.76
(81.75)
52.57
(15.80)
32.62
(11.33)
150-200 mm
71.37
(60.64)
35.40
(12.17)
19.54
(7.15)
300-350 mm
28.90
(19.16)
18.28
(6.87)
16.45
(3.51)
<40 kg/m3
0-50 mm
108.15
(64.50)
38.29
(21.42)
33.32
(6.93)
150-200 mm
75.95
(19.82)
37.40
(36.53)
23.14
(13.20)
300-350 mm
58.31
(57.39)
23.80
(12.46)
20.80
(12.78)
Arsenic (As)
>50 kg/m3
0-50 mm
66.59
(27.52)
37.23
(19.06)
16.63
(6.31)
150-200 mm
37.16
(24.65)
23.05
(17.22)
16.16
(4.40)
300-350 mm
34.65
(26.60)
7.51
(2.34)
9.19
(4.24)
40 - 50 kg/m3
0-50 mm
140.75
(97.64)
51.56
(34.84)
24.57
(19.83)
150-200 mm
127.31
(100.76)
37.35
(15.78)
21.30 (2.19)
300-350 mm
96.98
(49.42)
26.17 (1.90)
22.54 (5.03)
<40 kg/m3
0-50 mm
141.44
(106.21)
48.02 (3.35)
27.76
150-200 mm
143.39
(166.99)
36.53 (36.87)
< 6.00
300-350 mm
64.55
(47.12)
< 6.00
< 6.00
- Retention class >50 kg/m3 has n=11, retention class 40-50 kg/m3 has n=6, retention class <40 kg/m3 has n=3
- Values in parentheses denote standard deviation. Values without parentheses denote single values
Mean concentrations of Cr and Cu significantly (P<0.05) declined with increased soil depths
from the groundline but mean As concentration did not vary significantly with increasing soil
depths despite As having higher mobility than the other metal elements and gravity

11
influences mobility of metals in undisturbed soils (Bergholm 1990, Rahman et al. 2004). Also
despite the notion that metal elements are expected to increase with soil depth due to
gravity and runoff, metal elements were only found to concentrate at the level 0-50 mm
below the soil surface (this study) as was also found by Morrell and Huffman (2004). This can
be explained by top soil having higher organic matter that can capture more metal elements
than soil with lesser organic matter. Organic matter is known to form chelating reaction with
metal ions and binds them to organic molecules at the soil surface. However high soil
organic matter may not necessarily influence higher leachability of CCA from wood in soil
contact, based on a broad statistical correlation analysis (Wong and Chong 2014). At least
most likely the acidic soils of the test site (this study) could account for the long-term CCA
leaching observed in support of previous work demonstrating the role of acid soils on
enhanced CCA leaching (Warner and Solomon 1990, Hingston et al. 2001, Wong and Chong
2014). By contrast with the present findings (Table 2, Figures 1 – 3), Morrell and Huffman
(2004) reported lower long-term CCA leaching from ground-contact treated wood on a sub-
tropical case that: (i) mean extractable As adjacent to the pole ranged from 1.74 to 8.19
mg/kg and at distance at 300 mm from pole mean As level decline to levels 0.06 to 0.80
mg/kg, (ii) Cu leaching ranged 15.8 to 301.0 mg/kg adjacent to treated pole and declined to a
mean range of <0.1 to 5.9 mg/kg at 300 mm away, and (iii) Cr was at low levels of 1.02 to
0.27 mg/kg next to the pole and declined close to background levels of <0.2 mg/kg and
ranging from 0.02 to 0.06 mg/kg at 300 mm away.
Previous studies report different results of leaching rates of CCA to the soil environment.
Utility poles treated with CCA are partially buried into the ground and when leaching of CCA
components occur, soils surrounding CCA-treated timbers are found to be elevated in As, Cr
and Cu (Townsend et al. 2003, Morrell and Huffman 2004, Mercer et al. 2012). In fact, As, Cu
and Cr concentrations are elevated in soils especially within 300 mm from the pole
(Chirenjea et al. 2003). Townsend et al. (2000) determined the concentration of arsenic,
chromium and copper from CCA treated timber decks in Miami, Florida to be respectively
28, 34 and 40 mg/kg compared to respective background levels of 1.5, 10 and 10 mg/kg. The
work of Morrell and Huffman (2004) on CCA leaching of 45 yr-exposed treated wood in a
sub-tropics (Florida, US) inspired the present work in Sarawak due to lack of data from CCA
leaching in high humidity and rainfall tropical regions.
Table 3 shows the leaching of each metal among CCA retention groups of treated timber as
an attempt to visualise any effects of CCA retention value on leaching rates. Clearly no
significant difference in leaching rate among CCA retention class were detectable by this
approach where maximum CCA retention were achieved with treatment to refusal in this
study. However, this needs more investigation with more pole samples. Elsewhere CCA
retention reportedly plays a big role in determining leaching of CCA as classical retention
factors such as type of timber, species of timber, intended usage, and the service life
expectancy could determine leaching rate of As, Cr and Cu (Chirenjea et al. 2003). Retention
of CCA salts or oxide in wood is controlled via specific industrial treatment schedules related
to the purpose of usage and also wood properties (Chirenjea et al. 2003). Furthermore,
incomplete fixation may occur due to presence of extractives which can interfere in the
fixation process of CCA components (Dahlgren 1975, Kennedy and Palmer 1994, Stevanovic-
Janezic et al. 1997). Additionally, Salim et al. (2012) via laboratory leaching tests with
Malaysian hardwoods and found that high amounts of CCA remain as unfixed state resulting
in higher leaching of metals in the leachate especially Ramin and Geronggang. Different
wood characteristics brought about different leaching rates and more permeable species

12
have a higher tendency to leach more (Cockroft and Laidlaw 1978). Nevertheless these and
other wood properties (eg. wood cell structure, secondary metabolites in wood) which may
govern preservative leaching much more than mere CCA retention differences, while
certainly some soil properties already have notable impacts on rate of leaching according to
the wealth of studies reported.
Overall against the background of long-term higher CCA leaching of treated wood in a humid
tropical environment than in sub-tropical or temperate climes, it is timely to re-affirm this
behaviour with more case studies in the humid tropics that will aid decisions about the
future developments of wood protection industries therein. Although treated timbers do not
play a significant part of the economy of timber industries in Malaysia, with the decline in
volumes of timbers produced and associated increased price per unit volume of timbers in
this country, there will be an increase in demand for treated timbers in the future as wood is
favoured over other materials as being environmentally friendly and fulfills governmental
strategies for climate change mitigation. More research should be done to prepare for the
adoption of environmentally acceptable wood protection technologies in the humid tropics
as alternatives to chromated and arsenate preservatives.
Acknowledgements
The authors thank the following for their cooperation: Mr JK Lai provided wood
identification and treatment records, Mr Willies Chin helped with field data collection, and
Mr Peter K.F. Chong, Ms Ting Woei and Mr Tomy Bakeh assisted with CCA analysis of soil and
wood.
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