Polluted Wood Preservation Sites Lisbeth M. Ottosen1, Alexandra B. Ribeiro2, Eckhard Melcher3
1
Department of Civil Engineering, building 204,Technical University of Denmark, 2800 Lyngby, Denmark e-mail:
[email protected] 2 Departamento de Ciências e Engenharia do Ambiente, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Quinta da Torre, P-2825-114 Caparica, Portugal. e-mail:
[email protected] 3 Federal Research Centre for Forestry and Forest Products, Institute of Wood Biology and Wood Protection, Leuschnerstr. 91, D-21031 Hamburg, Germany, e-mail:
[email protected]
Introduction
The major chemicals used in the wood-preserving industry are creosote, copper chromium arsenate (CCA) and pentachlorophenol (PCP). Soil contamination problems with one or more of these chemicals can often be expected at wood preservation sites, particularly at old sites. The chemicals used for wood preservation are chosen from several reasons, one being that they are toxic to microorganisms and fungus, which can degredade wood. Meanwhile the toxicicity is not specific for these organisms and thus these chemicals can cause serious problems, when spread to the environment.
During risk assessment, three different routes of exposure are typically considered: direct human exposure (ingestion, inhalation, and dermal absorption), leaching to groundwater, and ecological risk /Jang et al. 2002/. All three exposure routes can be relevant in relation to wood preservation sites and the present paper summarizes some pollution problems from wood preservation sites described in literature. For some European countries numbers of polluted sites are given, too.
Preservation methods and soil pollution
The different methods for wood preservation that has been used especially in the past did not include any protection of the surroundings against contamination with wood preservatives.
At sites where the sap displacement methods were used, as the Boucherie method or the modified Boucherie procedure, preservatives were often spilled directly on the soil. At e.g. a Danish site (Stenholtvang) where the Boucherie method was extensively used the vegetation at the area is still highly effected of the CCA that was spilled to the soil before 1954. Still in 2004 long narrow areas are without any vegetation. From old photographs from the site it is known that during the use of the Boucherie method the ends of the poles from where CCA dripped of was situated exactly in these areas. Other areas at this site are without vegetation, too. These areas are more squareshaped and they are placed exactly where the newly preserved wood were placed in stacks to drip-of. A map of growth categories for the vegetation of this site is shown in Figure 1. The map clearly indicates how damaged the vegetation at the site is still is these many years after the spill of CCA on the soil. This site is not a single case in Denmark. Lund and Fobian, 1991 reports of at least two other sites where about one third and three quarters of the sites, respectively, were without vegetation.
Figure 1: Map of growth categories of a Danish CCA polluted site (Stenholtsvang). The areas shown in black show no vegetation, the white areas are mainly grass and moss whereas the vegetation in the grey areas is normal or almost normal with trees /Samfundsteknik, 1989/
At sites where only pressure methods took place (Bethell, vacuum/pressure or full-cell process), the expected soil contamination is reduced, comparing with the former methods. However, there are still major areas of concern in a plant, such as: around solution storage tanks, near autoclave treatment site, in storage areas for freshly treated wood, in storage areas for final wood and in storage areas for sludge associated with dissolved salts of Cu, Cr and As.
Today, there are detailed manufacturers and statutory Codes of Practice regarding safe handling procedures with recommendations for enclosing drip-dry areas and having dedicated forklift trucks. There are also accelerated fixation technologies available to prevent the occurrence of handling wood wet with preservative and to speed up the production process. However, some plants still works with a high level environmental impact. Problems of soil contamination of today’s wood preservation sites is according to OECD mostly connected to the use of uncovered storing areas /OECD 2000/
Creosote polluted soils
Creosote contaminated soil and groundwater is a widespread problem in industrialized countries. Creosote has been used intensively for wood preservation and at asphalt factories. In addition, creosote is a waste product from the production of gas from coal /Broholm et al., 1999/. Soils from old gas works, asphalt factories and wood preservation (where creosote was used) are often highly contaminated.
Creosote consists of hundreds of different organic compounds of which only a small number have been identified. The composition of the organic compounds in creosote varies considerably depending upon the actual production technique. Usually polycyclic aromatic hydrocarbones (PAHs) constitute between 70 and 85% of the organic compounds in creosote, phenolic compounds about 10%, monoaromatic hydrocarbones (BTEXs) less than 3%, and heterocyclic compounds containing oxygen, nitrogen or sulphur in the ring structure (NSO-compounds) between 3 and 15% /Broholm et al. 1999/. Creosote is considered to be an environmental hazard and can e.g. cause skin and lung cancer in humans. A number of PAHs with four or more rings are able to induce cancer in mammals: these compounds are not themselves the active toxicants but are transformed in biological tissues to the active inducers, epoxydihydridiols or cation radicals /Cavalieri and Rovan, 1998/
Biological activity will occur at soils polluted with organic compounds. The natural organisms from the soil degrade some organic pollutants without any human activity. When soils are polluted with creosote the composition of the creosote will undergo changes as time passes and the low molecular PAH will be degraded. Meanwhile, the PAH with higher molecular weight are extremely slowly degrading in the soil. By Allard et al. 2000 it was reported that the critical factor for
degradation is the length of time during which the contaminant has been in the soil, the longer the less degradation.
From the soil the creosote compounds will spread to the surrounding environment and the compounds can be found in both groundwater and surface waters see e.g. /Ceroici, 1993/. With groundwater contamination originating from creosote contaminated sites, the phenolic-and NSOcompounds represent a larger problem than PAHs, although they constitute a small fraction of the creosote. This is due to their high aqueous solubilities relative to those of most of the PAHs /Johansen et al. 1997/. PAHs that are adsorbed to soil colloids further have potential for colloid-facilitated transport /Villholt, 1999/ and this must be taken into account when assessing a risk of groundwater contamination with compounds that are expected to adsorb in the upper layer of the soil. Clay-rich tills have often been considered to be efficient protection layers for underlying aquifers, due to their low hydraulic conductivity. However, within the last decade the presence of fractures in clayey tills has been documented and these fractures may increase the bulk hydraulic conductivity with several orders of magnitude. Broholm et al 1999 showed that transport of creosote low-molecularweight organic compounds in a large undisturbed column of clayey till was comparable to bromide transport, but the high-molcular-weight organic compounds were retarded significantly. Thus the groundwater is not protected against the low-molecular-weight compounds even under a clayey layer.
Soil pollution with polychlorinated phenols (PCP)
Chlorophenoles was widely used as fungicide against blue stained wood in e.g. Sweden from 1930s to the 1970s (Persson et al. 2003) and in Finnish sawmill industry from 1940s to the 1970s (Kitunen, 1990). The preserving facilities were built without adequate dripping facility and preserving solution dripped or was spilled directly to the soil (Kitunen, 1990). During the production of polychlorinated phenols (PCP) for wood preservation a number of byproducts are formed among
which polychlorinated dibenzo-p-dioxines (PCDDs) and
dibenzofurans (PCDFs) have received most attention (Persson et al. 2003) and these compounds are also found in the soil at the sites (Kitunen et al. 1990).
Spread in the environment pentachlorophenol causes great concern. It is toxic to a wide range of organisms and the compound is listed as a priority pollutant by e.g. the U.S Environmental Protection Agency because of its carcinigenocity and toxicity /Wall and Stratton, 1995/
Eventhough the use of chlorophenoles for wood preservation was banned in the late 1980s in both Sweden and Finland the actual treatment sites remain heavily polluted with both PCP and the byproducts PCDDs and PCDFs (Kitunen et al, 1990), (Persson et al. 2003). The soil form these areas may release PCP during tens of years, leading to contamination of surface and ground waters (Kitunen et al, 1990). PCPs, PCPPs and PCDF are all considerable persistent in soil /Kitunen et al. 1987/. Water solubility influences the behaviour of organic pollutants in soils. The more soluble a compound is the more mobile. Phenols are water-soluble organics, but the water solubility decreases with an increasing molucelar weight. Highly chlorinated phenols are poorly soluble and PCPPs are even less water-soluble. PCDDs/PCDFs are practically insoluble in water. Kitunen et al (1990) studied migration of different chlorophenols in the soil under a former sawmill and found that PCP was mobile and had leached through the soil profile (more than 1 meter that was the maximum depth of the investigation), while PCPPs and PCDFs accumulated in the top soil. This can be related to the water solubility of the compounds. In an other investigation by Kitunen et al. 1987 it was shown that this accumulation of PCPPs and PCDFs was irrespective of the soil texture, which was very different at the four sites included in the investigation.
An examples of a PCP polluted river close to a saw mill as a consequence of spillage is given by McNeill (1990). At the site severe soil pollution was found, too. The main source of pollution at this site originated from storing of newly preserved timber.
Soil pollution with CCA
Arsenic, copper and chromium occur naturally in both soil and water but at certain concentrations all three compounds are toxic. Arsenic is the element of the three that causes greatest concern due to the toxicity even at very low levels. Background levels depend on the local conditions and independent of this there is a geographical variation. On a global scale the natural background for arsenic is less than 10 mg/kg soil /J.O.Nriagu/. Because of the high concentrations of metals in the CCA-solution, relatively small leaks or spills may result in high concentrations of the metals in soil that are potentially harmful to human health and the environment /Jang et al. 2002/. At wood
preservation sites extremely high soil concentrations can be found. Concentrations as high as 20.000 mg/kg soil are reported from impregnation plants /Nordic Council of Ministers, 2000/.
To assess the environmental impact of contaminated soils, knowledge of the total concentration of a specific metal without considering its speciation is not sufficient. The physicochemical properties of soil can widely influence metal speciation and, consequently, its mobility, bioavailability and toxicity. Especially knowledge of As and Cr speciation is very important, since their toxicity is associated with changes in oxidation state. Dependent on pH and redox potential of the soil environment, As can occur in two stable oxidation states that form oxyanions: As(V) as arsenate species (HxAsO4x-3), and As(III) as arsenite species (HxAsO3x-3). In general, trivalent arsenic is of more environmental concern, because it is more mobile and more toxic. Cr mainly exists in two stable oxidation states (hexavalent and trivalent) in soils, the hexavalent form being far more toxic than the trivalent.
An investigation of a Norwegian wood preservation site showed that even 7 years after the preservation activities were ended at the site high concentrations of As was still found in the soil solution (as high as 50 mg/l). Meanwhile Cu was not detectable in the soil solution, not even at hot spots with a total soil concentration of 67000 mg/kg. As was also found to be far most mobile in the soil /Andersen et al. 1987/. Cu is normally regarded as strongly adsorbed to the soil and Kelsall et al. 1999 reported that Cu was immobile in an acid sandy soil of South West Victoria. Meanwhile there are examples of Cu being mobile e.g. migration into depths of up to 6 meters under a Danish wood preservation site in Hammel which was placed on glacial till /Boutrup and Clement, 1990/. In another Danish site Cu was found in very high concentrations in the depth of 2.3 m below surface. The high concentration was connected to a calcareous zone /Lund and Fobian, 1992/.
The immediate leaching potential of the initial ionic species was found to increase in the following order Cu2+ < HAsO42-/H2AsO4-
