Wood preservation facilities, pentachlorophenol thermal: chapter F-3

3. Environmental Effects

PCP is an anthropogenic chemical that is ubiquitous in the Canadian environment as a result of extensive historical use in the wood preservation industry.

Impurities in technical-grade PCP, which may include tetrachlorophenol, trichlorophenols, hexachlorobenzene, polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) and chlorinated phenoxyphenols, are contributors to the compound’s toxicity. Chronic toxicity studies indicate that technical-grade PCP can be up to 10 times more potent than purified PCP due to the presence of these impurities (9).

Results from previous studies demonstrate that PCP has low bioaccumulation tendency in terrestrial invertebrates. PCP is metabolized rapidly in plants, so that while PCP products may be detected in plants, little intact PCP is found in plant tissues (9).

3.1 Aquatic Toxicity

On an acute basis, PCP is very highly toxic to aquatic invertebrates and highly toxic to fish. Acute LC₅₀ values for fish ranged from 20 µg/L to 600 µg/L.

Pentachlorophenol is hydrolytically stable in water at pH 4 to pH 9, precluding hydrolysis as a major degradation process in the environment. Chemical degradation of PCP in water will occur mainly through photodegradation. In surface water, PCP will rapidly photodegrade when exposed to direct sunlight, with more rapid degradation occurring with increased pH (when the compound is dissociated) (10).

The exposure of aquatic organisms to PCP could result in both short-term (acute) and long-term (chronic) toxic effects. At low concentrations, PCP is not considered a persistent contaminant in the environment because of documented photochemical degradation and microbial breakdown in surface waters, soil media and sewage effluents (11). However, PCP is widespread at low concentrations in the environment (11). The environmental effects are dependent upon a complex array of parameters including concentration, pH, adsorption to suspended solids, temperature, biodegradation rate and photodecomposition rate.

Based on extensive reviews of the literature and unpublished information, regulatory agencies have derived upper limits for PCP in the environment. As of July 1987, upper limits for Canadian waters have been defined under the auspices of the following regulatory agencies or commissions: the International Joint Commission (IJC), for Great Lakes waters (11); Health Canada, for maximum acceptable concentrations in drinking water (12); and the Canadian Council of Ministers of the Environment (CCME), for protection of aquatic life (13). The upper limits are summarized in Table 3.

Provincial guidelines are applicable and should be consulted. Provincial guidelines may differ from or be more specific than national guidelines. Provincial regulations may require additional measures that may enhance, but not reduce, protection.

3.2 Air Pollution

Pentachlorophenol is a relatively volatile compound, while its sodium salt is non-volatile. In the atmosphere, volatilized PCP may undergo photolytic degradation or may react with photochemically produced hydroxyl radicals. Atmospheric PCP that is associated with particulate matter or moisture will be lost from the atmosphere through wet deposition. Based on PCP’s low Henry’s Law constant, volatilization from aqueous systems will not be a significant mode of transport in the environment (10).

Pentachlorophenol contains chlorinated dibenzodioxins and chlorinated dibenzofurans (CDDs and CDFs) as contaminants formed during the manufacturing process. The CDDs and CDFs in the product (utility poles) may be released into the environment via volatilization and leaching. In addition, CDDs and CDFs may enter the environment during the thermal treatment of the utility poles, as well as when the utility poles are removed from service and are disposed of in landfills. These compounds are inherently toxic, as well as environmentally persistent, and their presence may increase the ecological risk associated with the use of PCP (10).

According to the Proposed Re-evaluation Decision, PRVD2010-03 Heavy Duty Wood Preservatives, “The wood preservation industry continues to be a source of dioxins and furans into the Canadian environment, however, a reduction in the amount of pentachlorophenol used in wood preservation due to the availability of alternatives for some uses and the measures taken by the technical grade active ingredient registrant to reduce levels of Track 1 contaminants in its technical product.” (6)

Section 4 addresses the potential health effects of exposure to air pollution from wood preservatives. Air pollution should be considered when evaluations of potential chemical discharges are made in Section 5.

3.3 Soil Contamination

Soil contamination can be an issue at wood preservative facilities if no effective measures are in place. PCP may be photodegraded, making degradation products available for being mobile in water. Contaminated soil can be spread by vehicles and wind, but it will mostly migrate into runoff water and can potentially contaminate drinking water. The design and operational recommendations presented in Sections 7 and 8 contain measures to minimize soil contamination.

Adsorption of PCP to soil is influenced by soil pH and organic carbon content (14). In general, adsorption was found to increase as soil pH decreased. As adsorption increases, PCP is less bioavailable and the rate of biodegradation tends to be reduced (15).

Leaching of PCP tends to increase with high PCP input, high soil moisture, alkaline soil conditions and low organic matter content in the soil (16). Over a range of environmentally significant temperatures and pH, the solubility of PCP was found to vary from 5 to 8000 mg/L.

Biodegradation is an important process particularly under aerobic conditions. Biodegradation processes reported include reduction, dechlorination, methylation, demethylation, acetylation and hydroxylation. By-products include lower chlorinated phenols, methyl ethers and pentachloroanisole. The rate of biodegradation in soil is affected by temperature, pH, moisture, adsorption and cation exchange capacity. Microbial species known to biodegrade PCP include Pseudomonas, Flavobacterium and Arthrobacter species. Several species of fungi are also known to be capable of PCP degradation (9).

The CCME has developed the Canadian Soil Quality Guidelines for the Protection of Environmental and Human Health. Limit concentrations for industrial sites are set as follows:

• SQG_HH = soil quality guideline for human health 7.6 mg/kg dry weight
• SQG_E = soil quality guideline for environmental health 28 mg/kg dry weight

The CCME recommends the application of various check mechanisms, when relevant, in order to provide a broader scope of protection. This additional information on check values can be found in the CCME Canadian Soil Quality Guidelines.