Variability in Evaluating 

Environmental Impacts 

of Treated Wood 

Stan Lebow 

Paul Cooper 

Patricia K. Lebow 

United States

Department of 

Agriculture 

Forest Service 

Forest 

Products 

Laboratory 

Research 

Paper 

FPL−RP−620 
Abstract 

Preservative-treated wood contains components that may be 

toxic to non-target organisms if released into the environment in sufficient quantities. Numerous studies have been 

conducted to determine the rate of preservative release from 

treated wood and/or the extent of their subsequent accumulation in the environment. These studies have produced a wide 

range of results, with a corresponding range of interpretations and recommendations. This paper reviews research on 

wood preservative leaching and environmental accumulation 

and discusses sources of the variability in research findings. 

Variables such as wood properties, pressure treatment techniques, construction practices, exposure conditions, and site 

conditions are discussed. 

Keywords: Wood preservatives, treated wood, leaching, 

variability, environmental accumulation 

May 2004 

Previously published in the Proceedings of Environmental Impacts 

of Preservative-Treated Wood; February 8ñ11, 2004, Orlando, FL. Gainesville, FL: Florida Center for Environmental Solutions: 1ñ14. CD-ROM. 

Lebow, Stan T.; Cooper, Paul; Lebow, Patricia K. 2004. Variability in 

evaluating environmental impacts of treated wood. Res. Pap. FPL-RP-620. 

Madison, WI: U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory. 10 p. 

A limited number of free copies of this publication are available to the 

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Contents 

 Page 

Introduction........................................................................... 1

Laboratory Studies................................................................ 1

Selection of Test Specimens ............................................. 1

Preservative Treatment and Fixation ................................ 2

Controlled Leaching Exposures........................................ 3

Quantification of Leaching in Laboratory Exposures....... 4

Field Studies of In-Service Structures .................................. 4

Age of Structure................................................................ 4

Type of Exposure.............................................................. 5

Construction and Maintenance Practices .......................... 5

Application of Finish ........................................................ 6

Quantification of Leaching for In-Service Exposures ...... 6

Surface Area ..................................................................... 7

Number and Location of Samples..................................... 7

Site Characteristics............................................................ 7

Concluding Remarks............................................................. 8

Literature Cited ..................................................................... 8
Variability in Evaluating Environmental 

Impacts of Treated Wood 

Stan T. Lebow, Research Forest Products Technologist

Forest Products Laboratory, Madison, Wisconsin 

Paul Cooper, Professor 

Faculty of Forestry, Unversity of Toronto, Toronto, Canada 

Patricia K. Lebow, Mathematical Statistician 

Forest Products Laboratory, Madison, Wisconsin 

Introduction 

Concerns about the safety and environmental impact of 

preservatives used to protect wood from biodegradation have 

increased in recent years, as has research to quantify preservative leaching and environmental accumulation. 

Early studies of preservative leaching tended to focus on the 

ability of a preservative to provide long-term protection. 

Preservative permanence in the wood is critical to efficacy, 

and leaching studies remain an integral part of research to 

evaluate potential new preservative systems. These types of 

leaching trials emphasize comparative evaluations of preservative formulations, and they typically use methods that 

accelerate leaching. More recently, emphasis has shifted to 

studies that evaluate the environmental impact of wood 

preservatives. These later studies place greater emphasis on 

quantifying in-service leaching rates and measurement of 

environmental concentrations of leached preservative. Researchers who are relatively unfamiliar with preservative 

formulations, treatment practices, and wood properties often 

conduct these environmental impact evaluations. Not surprisingly, studies conducted by researchers with various 

fields of expertise and a range of research objectives have 

produced results that are often conflicting and may be difficult to compare and interpret. 

This paper discusses some approaches used to evaluate 

preservative leaching and/or environmental accumulation 

and the influence of various aspects of these methods on 

research results. Evaluations of preservative leaching and 

environmental accumulation are grouped into two general 

types: those in which study conditions are controlled and 

those that are more observational in nature. Controlled studies are often laboratory studies, and observational studies 

typically utilize existing in-service structures, although there 

is overlap between these groups. 

Laboratory Studies 

In controlled studies, researchers must consider selection 

methods for test specimens and preservative treatments, 

exposure of samples to a source of leaching, and determination of preservative loss. 

Selection of Test Specimens 

The size and dimensions of test specimens have a great 

effect on the percentage of preservative leached from the 

wood. Smaller specimens have a larger portion of surface 

area exposed to leaching and allow more rapid water penetration. The effect of grain orientation is also exaggerated in 

smaller samples. The rate of movement of liquids along the 

grain of the wood is several orders of magnitude greater than 

that across the grain, and samples with a high proportion of 

exposed end-grain will exhibit exaggerated rates of preservative leaching (Cooper 1991a, Haloui and Vergnaud 1997). 

The standard leaching method used by the American Wood 

Preserversí Association (AWPA) purposefully employs 

small blocks with a high proportion of exposed end-grain to 

accelerate leaching (AWPA Standard E11, AWPA 2002). 

Although a valuable comparative method, this method and 

others that utilize small specimens should not be used to 

predict the amount of leaching that will occur from productsized material in service. It may not be practical, however, to 

conduct a laboratory leaching study using full-length lumber, 

poles, or piles. To avoid the problem of end-grain effect, 

specimens may be cut from product-size material and endsealed with a waterproof sealer prior to leaching. 

Wood species can also greatly affect the rate of preservative 

loss from treated specimens. Permeability varies greatly 

among wood species, and those species that are more permeable tend to leach at a higher rate because of more rapid 

movement of water through the wood (Crockroft and Laidlaw 1978, Wilson 1971). In studying the leaching characteristics of small specimens cut from the surfaces of commercially treated poles, Cooper (1991b) found that rates of 

preservative leaching from red pine were approximately 

double those from lodgepole pine, Douglas-fir, and western 

redcedar. In subsequent studies, hardwoods such as maple, 

red oak, and beech were found to have a greater percentage 

of extractable arsenic than does red pine (Cooper 2002, 

StevanovicñJames and others 2000) (Fig. 1). Other studies 
2 

also indicate that preservative components may be more 

leachable from hardwoods than from softwoods (Cooper 

1990, Nicholson and Levi 1971, Yamamoto and Rokova 

1991). Wood species may also affect the distribution of 

preservative within the wood and the chemical reactions that 

fix water-based preservatives in the wood. Because of these 

species effects, it is important to use a species that is typical 

for the application under evaluation or to at least identify and 

report the wood species. 

The presence and amount of heartwood may also affect 

leaching of preservatives. In most wood species, the inner 

heartwood portion of a tree is much less permeable than the 

outer sapwood portion. Accordingly, heartwood portions of 

test specimens may contain much less preservative than does 

the sapwood and may also be more resistant to penetration of 

the leaching medium. These effects might be expected to 

result in lower leaching rates from heartwood, but this generalization may be confounded by differences in preservative 

fixation in heartwood or by the presence of a higher concentration of preservative at the heartwood surface. Because the 

presence of heartwood complicates the interpretation of 

leaching results, heartwood should either be avoided or 

quantified and reported. Heartwood represents a major proportion of the wood produced from some wood species, such 

as Douglas-fir, but a much smaller proportion of wood produced from Southern Pine species. 

Some studies characterize the rate of leaching from a particular species/preservative combination. In the design of 

such studies, the researcher must be aware that even within 

the sapwood or heartwood of a single tree species there can 

be variability in wood properties, including the rate of preservative leaching. Not surprisingly, wood properties typically vary much more between trees and boards than within 

a single board. Consequently, it is desirable to obtain specimens from as many different boards as possible. For example, if 10 replicates are to be used in a leaching evaluation, it 

is usually more appropriate to cut a single specimen from 

each of 10 boards than to cut 10 replicate specimens from a 

single board. Obtaining boards from a range of geographic 

locations can achieve an even greater sense of variability, as 

well as broaden the inference space. As mentioned previously, specimens cut from longer boards may be end-sealed 

to prevent exaggerated leaching rates attributable to exposed 

end-grain. 

Preservative Treatment and Fixation 

Obtaining preservative-treated specimens is a problematic 

step for many researchers. Many laboratories do not have 

ready access to stock solutions of commercial wood preservatives or the equipment needed to conduct pressure 

treatments. In these cases, researchers typically purchase 

commercially treated products for leaching trials. A 

disadvantage of this approach is that the researcher has no 

knowledge of the treatment process, treating solution concentration, and fixation conditions. Ideally, the treated products are purchased from several retailers over a period of 

time to make the sample more representative. In some cases, 

researchers have purchased commercially produced lumber 

and then cut specimens to smaller width or thickness than 

that of the original board. Because penetration of a preservative is often not uniform throughout the thickness of a board, 

specimens cut in this manner may have one or more faces 

that have a different (usually lower) preservative concentration than that of the original board face. 

When the researcher treats specimens, care should be taken 

to prepare or obtain a preservative solution that is nearly 

identical to the commercial formulation. Leaching of active 

ingredients can be sensitive to proportions and types of 

solvents used. For example, leaching of copper from copper 

amine preservatives can be increased if an excess of amine is 

used in preparation of the treatment solution. In addition, 

some types of preservatives may be or may have been produced in multiple formulations. Before chromated copper 

arsenate Type C (CCA-C) became the industry standard, 

wood was also treated with CCA-A and CCA-B. Arsenic 

release from wood treated with CCA-B was greater than that 

from wood treated with CCA-A or CCA-C (Lebow 1996). 

The treatment process should ensure adequate penetration of 

the specimens without development of surface deposits. 

With some preservative systems, extended soaking periods 

that allow evaporation of solvents may produce a precipitate 

surface residue on the wood. 

The fixation conditions that specimens are exposed to after 

treatment can also affect the outcome of a leaching study. In 

general terms, fixation refers to the series of chemical reactions that render water-based preservatives difficult to leach 

during service. Although the fixation reactions of preservatives differ, they all depend on solution concentration, time, 

temperature, and rate of drying. Complete fixation of CCA 

depends on the wood species; it requires 10 to 20 days at 

room temperature for pine species (Cooper 2002). Test 

specimens exposed to leaching within a few days after 

Figure 1óRelative leaching of CCA components from 

different species (AWPA E11 leaching test) (Stevanovicñ 

James and others 2000). 

0 

4 

8 

12 

16 

Maple Red 

oak 

Beech Yellowpoplar 

Aspen Basswood

Time (h) 

CCA-C extraction (%) 

Cr

Cu

As

Red 

pine 

White 

birch 
3

treatment may exhibit abnormally high leaching rates of 

chromium, copper, and arsenic. The fixation reactions also 

require moisture (Cooper 2002, Kaldas and Cooper 1996), 

and rapid drying of specimens after treatment may lead to 

inadequate fixation even after a lengthy fixation period 

(Fig. 2). This is particularly a concern for small specimens 

such as the 19-mm cubes specified by the AWPA leaching 

standard (AWPA 2002). For CCA, the rate of fixation and 

subsequent leaching of CCA components are dependent on 

wood species. In general, species in which fixation occurs 

very rapidly also tend to have a higher rate of arsenic leaching (Cooper 2002, StevanovicñJames and others 2000). 

Differences in the chemical composition of the wood, and 

especially the amount and type of lignin, can affect the rate 

of fixation and subsequent preservative leachability (Kartal 

and Lebow 2001). Again, it is important to identify wood 

species when reporting leaching results. The solution 

strength or retention of preservative in the wood can also 

affect the rate of fixation. For CCA, arsenic fixation is more 

rapid at higher solution concentrations, while fixation of 

chromium and copper is slowed. Higher retentions have also 

been reported to slow fixation of copper in amine copper 

based preservative systems (Pasek, in press). 

Controlled Leaching Exposures 

Most controlled leaching trials of preservative-treated wood 

expose samples to leaching through immersion. Immersion is 

perhaps the simplest type of leaching mechanism to control 

and replicate, and it provides a severe leaching environment. 

However, the immersion conditions can affect the results. In 

some situations, the leached preservative in the water may 

reach concentrations that inhibit further leaching (Brooks 

2002). This problem can be addressed by either frequently 

changing the leaching water, as specified in AWPA Standard 

E11 (AWPA 2002), or by constructing a flow-through leaching apparatus that circulates fresh leaching water (Brooks 

2002). In the latter case, care must be taken to accurately 

control or measure the flow rate so that the dilution factor 

can be calculated. 

The characteristics of the leaching water can also influence 

leaching of preservatives. Standardized methods, such as 

AWPA Standard E11, generally specify the use of deionized 

or distilled water to minimize these effects. Some types of 

inorganic ions in water have been reported to increase leaching from CCA-treated wood (Irvine and others 1972, Lebow 

and others 1999, Plackett 1984, Ruddick 1993), whereas 

with at least one type of preservative, inorganic ions in water 

have been reported to decrease leaching (Kartal and others 

2004). Water pH can also affect leaching of preservatives. 

Leaching of CCA is greatly increased when the pH of the 

leaching water is lowered to below 3, and the wood itself 

also begins to degrade (Cooper 1991a,b; Kim and Kim 

1993). Water pH ranges more typical of those found in the 

natural world are less likely to have a great effect on leaching (Murphy and Dickinson 1990), although the presence of 

organic acids may influence leaching at more moderate pH 

levels. Warner and Solomon (Warner and Solomon 1990) 

reported that adding citric acid to leaching water greatly 

increased leaching in laboratory tests. Although it is doubtful 

that high levels of citric acid will be a problem in service, 

surface waters containing high levels of humic or fulvic acid 

from peaty organic soils can have the potential for increasing 

CCA leaching (Cooper 1990, 1991b). In comparing CCA-C 

losses from jack pine blocks exposed in garden soil and 

organic-rich compost, Cooper and Ung (1992) found that 

leaching was more than doubled by compost exposure. 

Water temperature has also been reported to significantly 

affect leaching from wood treated with a CCA. In a study by 

van Eetvelde and others (1995), copper, chromium, and 

arsenic leaching were approximately 1.4, 1.6, and 1.5 times 

greater, respectively, from wood leached at 20°C than from 

wood leached at 8°C. Brooks (2002) also concluded that 

leaching of copper from CCA-treated wood could be substantially increased as water temperature increased from 8°C 

to 20°C. A similar temperature effect was noted in a study of 

release of creosote components from treated wood (Xiao and 

others 2002). 

The rate of water movement around the test specimens can 

also influence leaching, although this effect has not been 

well quantified. Xiao and others (2002) reported that release 

of creosote was greatest at the highest flow rate tested and 

that turbulent flow may have greatly increased leaching. Van 

Eetvelde and others (1995) also reported that leaching of 

CCA was greater when leaching water was stirred rather 

than static. The AWPA standard leaching test specifies the 

use of a slow stirring speed (e.g., a tip speed of 25 to 

50 cm/s) (AWPA 2002). However, care must be taken that 

Figure 2óEffect of wood moisture content on 

chromium fixation. 

0 

100 

200 

300 

400 

500 

600 

700 

800 

900 

1000 

20 40 60

Fixation time (days) 

Cr-VI (ppm) 

0% MC 

13% MC 

High MC (not dried)

High MC (resaturated)

25% MC 
4 

the method of stirring or agitation does not mechanically 

abrade the surface of the wood. 

Although an immersion leaching exposure may be relatively 

simple to simulate, most treated wood in-service is not 

placed directly in water. Terrestrial applications are more 

common, and in such cases the treated structure is above 

ground or water or in soil contact. Because studies have 

illustrated that soil composition may affect both leaching and 

subsequent mobility of CCA components (Bergholm 1992, 

Crawford and others 2002), efforts have been made within 

the AWPA to develop a standard method of evaluating 

preservative loss in soil exposures (Crawford and others 

2002). One challenge in this type of exposure is choosing a 

representative soil type; the authors recommend using at 

least three different soil types as well as characterizing and 

reporting soil properties. 

Both immersion and soil contact leaching tests are likely to 

greatly overestimate the amount of leaching that will occur 

from treated wood exposed above ground. However, laboratory evaluations of aboveground leaching are rare, in part 

because it is difficult to simulate natural rainfall. It appears 

that the rate of rainfall, not just volume, can affect the 

amount of leaching from wood exposed above ground. 

Studies in outdoor exposures have indicated this effect 

(Cockroft and Laidlaw 1978, Evans 1987), and recent laboratory evaluations (Lebow and others 2003) have attempted 

to quantify the effect of rate of rainfall on leaching (Fig. 3). 

Laboratory evaluations also indicate that exposure to UV 

light may increase leaching from CCA-treated wood exposed 

above ground (Lebow and others 2004). 

The orientation of the wood product (vertical or horizontal) 

can also affect the amount of water that enters the wood to 

facilitate leaching (Cooper 1991a). Although complex, 

further research is needed to determine principal factors that 

affect leaching from wood exposed above ground and to 

develop laboratory methods to predict leaching in service. 

Quantification of Leaching in 

Laboratory Exposures 

Regardless of the leaching exposure, one must somehow 

quantify the amount of preservative that has been lost from 

the wood. This is usually accomplished by either assaying 

the wood before and after leaching, or by analyzing the 

leaching water and calculating the rate of leaching and cumulative amount leached. Although analysis of the treated 

wood before and after leaching is a convenient way to assess 

leaching, this approach may not provide meaningful data 

unless substantial leaching has occurred. With well-fixed 

preservative systems, only a small percentage of preservative 

is typically lost during a laboratory leaching trial and error in 

measurement of preservative content in the wood can easily 

obscure or over- or underestimate leaching. Lower levels of 

leaching can be detected by analysis of leaching water, 

although care must be taken in calculating the dilution factor, and complex error structures may arise if repeated measurements are made over time. Analysis of leaching water 

also allows evaluation of changes in the rate of leaching over 

the course of the exposure period. 

Field Studies of In-Service 

Structures 

Evaluations of preservative release from in-service structures 

have recently increased. These are generally observational 

(not controlled) studies. Evaluations of in-service structures 

provide valuable information on leaching and environmental 

accumulation in actual applications. The disadvantage of 

these types of studies is that they are specific to the conditions at that specific site and are difficult to relate to other 

exposures. The original treatment may be unknown, and 

there may be little historical data to indicate whether the site 

was previously exposed to contamination from construction 

debris or other non-leaching sources. In-service leaching 

results are affected by a range of site-specific conditions in 

addition to the treatment, fixation, and species effects discussed in the previous section. These include the age of the 

structure, type of exposure, climate, and construction and 

maintenance practices. 

Age of Structure 

In general, the greatest rate of leaching occurs when the 

treated wood is initially exposed to the leaching medium. An 

initial wave of readily available and unfixed or poorly fixed 

components moves out of the wood, followed by a rapid 

decline to a more stable leaching rate (Bergholm 1992, 

Cooper 1991a, Evans 1987, Fahlstrom and others 1967, 

Lebow and others 1999, 2000) (Fig. 4). This time-dependent 

leaching pattern is a function of the size of the treated 

Figure 3óEffect of rate of rainfall on leaching of 

chromium, copper, and arsenic from CCA-treated 

decking. 

Rate of rainfall (mm/h) 

0.5 

1.5 

2.5 

3.5 

4.5 

0 5 10 15 20 25 30

Amount leached (mg) 

during 750 mm rainfall 

Cr Cu As
5

product, the amount and type of surface area exposed, and 

the extent to which preservative components are fixed. It 

also appears to depend on the severity of leaching exposure, 

with a steeper gradient occurring under a more severe leaching condition such as water immersion, and a flatter gradient 

occurring for wood exposed above ground. However, 

regardless of specific conditions, it is likely that rate of 

leaching occurring during the first year of exposure will be 

greater than that during subsequent years. Extrapolating 

early rates of leaching to longer periods may overestimate 

long-term leaching. 

Type of Exposure 

The type of exposure or application also greatly influences 

in-service leaching. Regardless of whether the treated wood 

is exposed to precipitation, freshwater, seawater, sediments, 

or soil, the movement and composition of water is the key to 

the leaching of preservative components from the wood. 

Structures that are only intermittently exposed to precipitation will have much lower leaching rates than those continually immersed in water, especially in water containing solubilizing organic or inorganic components. Cooper (1991a) 

proposed a hierarchy of leaching exposures based on application and site conditions (Table 1). Within each of these 

types of exposures is a range of conditions that may potentially affect leaching. These include temperature and composition of soil and water. Because most treated wood is exposed above ground, climate plays an important role in 

leaching. The amount and rate of rainfall affect leaching 

(Lebow and others 2000), and it is likely that temperature 

and the presence or absence of freezing temperatures do as 

well. Although these conditions cannot be controlled, they 

should be noted and factored into the interpretation of 

leaching results. 

Construction and Maintenance 

Practices 

Construction and maintenance practices for a structure can 

also affect the rate of preservative leaching or the amount of 

preservative detected in the environment. If treated wood 

sawdust or shavings generated during construction are allowed to enter soil or water below a treated structure, they 

make a disproportionately large contribution to environmental contamination. As shown in Figure 5, leaching of 

CCA from construction debris immersed in water is vastly 

greater than that from solid wood. Environmental samples 

removed from areas where construction debris was deposited 

are likely to have much higher elevations of preservative 

components than might be expected from leaching alone. 

This effect may be responsible for some of the higher soil 

arsenic levels reported in recent studies of soil adjacent to 

CCA-treated decks (Stilwell and Gorny 1007, Townsend and 

others 2001), while other studies reported much lower concentrations (Lebow and others 2000, Chirenje and others 

2003). Although associated with the treated structure, environmental contamination caused by construction debris is 

attributable to construction practices and is not an inherent 

characteristic of the treated wood (Lebow and Tippie 2001). 

Cleaning and maintenance practices such as aggressive 

scrubbing, power-washing, or sanding can also remove 

particles of treated wood and deposit them in soil or water 

beneath a treated structure. In addition, some ingredients 

used in deck cleaners have been shown to react with and 

potentially increase the solubility of preservative components (Taylor and others 2001). 

Figure 4óChange in rate of arsenic release from 

CCA-treated 2 by 4 specimens immersed in seawater. 

Table 1óHierarchy of severity of leaching exposures 

in order of increasing severity (Cooper 1991a) 

Exposure condition Typical or example application 

Partially protected 

from rainfall 

Covered patios, gazebos, siding, 

substructure of decks and bridges 

Occasional or partial 

exposure to rainfall 

Fence boards 

Complete exposure 

to rainfall 

Shakes and shingles, decking, railings, stairs, steps 

Exposure to soil Fence posts, poles, land piles, retaining walls, treated wood foundations 

Exposure to fresh 

surface water 

Cribs, lock gates, fresh water piles 

Exposure to seawater, acidified water, 

or warm water 

Marine piles, piers, cribs, cooling 

towers, acid lakes 

Exposure to metal 

complexing 

compounds 

Silos, bog water (hypothesized), 

wood stave pipes and tanks, citric 

acid 

0.001 

0.01 

0.1 

1 

0 5 10 15 20

Immersion time (months) 

High salinity

Low salinity

Release rate (μg/cm2/day) 
6 

Application of Finish 

While construction debris and cleaning activities may increase environmental release from a treated structure, the 

application of finish appears to have the opposite effect. One 

report indicated that a clear water-repellent finish greatly 

decreased CCA release from fencing (Cooper and Ung 

1997a). Even after 2 years, arsenic concentration in rainwater collected from the finished specimens was approximately five times lower than that from the unfinished specimens. An observational study of the concentrations of 

arsenic, copper, and chromium in soil under residential 

decks noted that levels appeared to be lower under a deck 

that had been painted, although the design of that study did 

not allow a controlled comparison (Stilwell and Gorny 

1997). A laboratory study has also indicated that latex paint, 

oil-based paint, and semitransparent penetrating stains are all 

effective in decreasing leaching from horizontal surfaces 

(Lebow and others 2002). Again, although construction and 

maintenance activities generally cannot be controlled in an 

in-service leaching evaluation, they should be considered in 

the interpretation of leaching results. 

Quantification of Leaching for 

In-Service Exposures 

Leaching is generally evaluated in service by either assaying 

the treated wood or by collecting and analyzing environmental samples adjacent to the treated wood. Determining 

preservative loss by assaying wood after exposure requires 

knowledge of the original preservative retention in the wood. 

The original retention is often assumed, based on the specified target or standard retention for treated wood used in that 

application. This assumption can be problematic, as preservative retention in a treated product can be substantially 

higher or lower than the target retention. This is particularly 

true for some oil-type treatments where retention is controlled by adjusting the treatment process, and not by adjusting the treatment solution concentration. Even with waterbased preservatives, retention can vary greatly between 

material in a single charge and even more greatly between 

treating plants. Figure 6 shows the distribution of CCA 

retention in nominal 2 by 6 (standard 38- by 140-mm) 

Southern Pine lumber purchased from several retailers over 

the course of 1 year. All the boards were treated to a target 

retention of 6.4 kg/m3

. It is evident that retention varies 

greatly between boards, and that leaching would be either 

overestimated or underestimated for most boards based on 

an assumed original retention of 6.4 kg/m3

. Variability in 

retention can be even greater in more difficult to treat wood 

species. 

Another technique used to quantify leaching in service is 

comparison of the aboveground or above-water retention to 

the below-ground or below-water retention, with the assumption that leaching is minimal for samples exposed 

above ground (Freeman and others 1994, Gjovik 1977, 

Nicholson and Levi 1971). This method can indicate significant losses in the lower portions of treated wood. However, 

it is vulnerable to underestimation of leaching because some 

leaching does occur from above ground and the preservative 

may redistribute within the wood during service (Gjovik 

1977, Hegarty and Curran 1986, Shelver and others 1991). 

Because of challenges associated with assaying the treated 

wood to quantify leaching from in-service structures, researchers may instead collect environmental samples adjacent to a treated structure. This approach has the advantage 

of providing information on environmental accumulation of 

leached preservatives, but it gives limited information on the 

amount of preservative released from the wood. Environmental sampling also introduces a range of sources of variability into a leaching study. In addition to leaching rate, 

environmental concentrations of preservative components 

will be a function of background concentrations, sampling 

location, and soil or water characteristics. 

Figure 5óComparison of amount of preservative 

released from solid wood or construction debris. 

Figure 6óRange of CCA retentions measured in 2 by 6 

Southern Pine lumber treated to target retention of 

6.4 kg/m3

. 

0 

50 

100 

150 

200 

250 

Chromium Copper Arsenic

CCA element 

Release after 28 days 

(μg/g of wood)

Solid wood 

Chain sawdust 

Spade bit shavings 

Circular sawdust 

0 1 2 3 4 5 6 7 8 9 10 11 12

2

4

6

8

10

CCA retention (kg/m3)

Number of boards 

12
7

Determining background or preconstruction environmental 

concentrations of preservative components is a key, but 

sometimes difficult, step in evaluating environmental accumulation. Many wood preservative components, including 

copper, chromium, and arsenic, have been widely used for 

other applications in the past, and soil and sediments may 

contain unpredictable concentrations of these components. 

This problem has generally been addressed by removing 

environmental samples at varying distances from the treated 

structure (Chirenje and others 2003, Stilwell and Gorny 

1997, Townsend and others 2001) and considering those at 

an extended distance from the structure as representing the 

background concentration. While generally a valid approach, 

there is the concern that human activities probably are, or 

have been, greater in close proximity to the treated structure, 

and thus the risk of other sources of contamination is greater 

in that area than may be in a nearby but less used area. 

Surface Area 

The surface area of a structure contributing to soil levels in a 

particular area is an important consideration in environmental sampling. In complicated structures such as decks, it 

may be difficult to determine the surface area of the structure 

that is contributing to soil accumulations in any specific 

sampling location. Other structures, such as utility poles, 

have a large aboveground surface that drains into a small 

volume of soil at the base of the pole, and it is not surprising 

that relatively high levels of preservative components have 

been detected in soil adjacent to poles (Cooper and Ung 

1997b). 

Number and Location of Samples 

For a field study, the specific parameters and/or hypotheses 

of interest relevant to the inference population or populations, such as a 95% confidence interval for the median 

amount of copper within 152 mm of a structure, need to be 

identified before the study starts. The best sampling strategy 

and analysis methodologies to address these information 

needs can then be selected. Selection and number of sampling locations for removal of environmental samples can 

also influence the levels of preservative components detected. Common preservative components such as copper, 

chromium, and arsenic are reactive with soil constituents 

(Lebow 1996) and are not freely mobile in soil. Thus, environmental concentrations tend to be concentrated in areas 

immediately adjacent to treated wood or where water drips 

off treated wood into soil. Even when soil samples are removed from directly under the drip line of a deck, environmental concentrations of leached preservative components 

can vary greatly (Lebow and others 2000). Because of this 

wide variation, a statistically designed sampling plan is 

needed to characterize preservative concentration in the 

environment adjacent to treated wood. Practical general 

advice on environmental studies can be found in van Belle 

(2002), while more specific statistical methodology is given 

in Gilbert (1987), Gibbons and Coleman (2001), and Manly 

(2000). 

Environmental sampling typically yields many samples with 

relatively low levels of preservative components and a few 

samples with much higher levels (Lebow and others 2000, 

Stilwell and Gorny 1997, Townsend and others 2001). Because of this skewness, traditional normality-based statistical 

methods directly applied to samples from an underlying 

skewed distribution may be overly sensitive to the ìoutlyingî observations and lack power in comparing parts of the 

distribution where there is less information. Lognormal 

distributions are commonly assumed in environmental sampling. Ott (1995) discusses in detail the physical and stochastic reasons why lognormal populations naturally arise in 

environmental settings. Gibbons and Coleman (2001) provide statistical methods for testing distributional assumptions 

as well as for testing for outliers. If the lognormal distribution can be assumed, the normality-based methods can be 

applied to log transformed data, and the results reverse 

transformed to the original scale, to estimate various population parameters as well as confidence limits. For example, 

the sample geometric mean provides a simple estimate of the 

median, which can be a better estimator than the sample 

median of the median preservative concentration within that 

area. However, for small sample sizes with high skewness, 

this estimator has higher levels of associated positive bias 

(Gilbert 1987). Parametric approaches can offer more sophisticated modeling approaches than do nonparametric 

procedures, but depending on the particular questions that 

are to be answered in a particular study, nonparametric 

methods may also be appropriate (van Belle 2002). 

Besides potential sampling, temporal, and spatial variability, 

analytical uncertainty is a consideration, as discussed by 

Gibbons and Coleman (2001). Care needs to be taken that 

analytical uncertainty is not used to characterize other types 

of variability. Also common to field studies are values that 

are censored below the quantitation limit or limits of a measurement device, necessitating appropriate statistical analysis 

methods to accommodate the censored data. Although there 

is agreement about using an appropriate statistical procedure, 

the particular choice depends on several things, including the 

objectives, degree of censoring, and ease of use (Gibbons 

and Coleman 2001). 

Site Characteristics 

Independent of leaching rates, site characteristics strongly 

influence environmental accumulation of leached preservative components. Leached preservative components are 

reactive with naturally occurring ligands in soil, sediments, 

and water, which limits their mobility. Movement in soil is 

generally limited but is greater in soils with high permeability and low organic content (Bergholm 1983, 1990; Bergholm and Dryler 1989; DeGroot and others 1979; Dowdy 

and Volk 1983; Holland and Orsler 1995; Lund and Fobian 
8 

1991; Murphy and Dickinson 1990). Mass flow with a water 

front is probably most responsible for moving metals appreciable distances in soil, especially in permeable, porous soils 

(Dowdy and Volk 1983). It is apparent that preservatives 

leached into water have the potential for greater migration 

compared with that of preservatives leached into soil, with 

much of the mobility occurring in the form of suspended 

sediment (Lebow and others 2000, Neary and others 1989). 

These environmental factors interact with leaching rate to 

create a pattern of environmental accumulation specific to 

a particular site. 

Concluding Remarks 

Evaluation of the leaching and environmental accumulation 

of preservatives from treated wood is a complex process, and 

many factors can influence the results of such studies. In 

laboratory studies, the effects of specimen dimensions, wood 

species, treatment practices, fixation, and leaching exposure 

must be considered. Evaluation of in-service structures 

introduces additional variability, with factors such as age of 

the structure, type of exposure, construction and maintenance practices, and site characteristics. There is no perfect 

study design to account for all of these factors, and in many 

cases they are out of the control of the researcher. However, 

the researcher should be aware of these factors and the relative importance of these sources of variability to a particular 

study should be considered when interpreting and reporting 

the study results. 

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