Effects of forest fire on the level and distribution of PCDD/Fs and PAHs in soil

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Abstract

Forest fires are believed to produce polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs); however, there is no firm evidence supporting this conjecture. To address this issue, we investigated whether PCDD/Fs and polycyclic aromatic hydrocarbons (PAHs) are formed by forest fires. The present work takes the indirect approach of measuring the levels of PCDD/Fs and PAHs in soil and ash samples 1, 5 and 9 months after forest fires. To determine if PCDD/Fs or PAHs were formed during forest fires, the levels of PCDD/Fs and PAHs measured in the burnt soil samples were compared with those in the corresponding unburnt soil samples. One month after forest fires, the concentrations of PCDD/Fs and PAHs in burnt soils were higher than in the corresponding unburnt soils. In addition, the homologue profiles differed between the burnt and unburnt soils. Five months after forest fires, however, the concentrations of PCDD/Fs and PAHs in the burnt soils were similar to those in unburnt soils. The data presented here strongly suggest that PCDD/Fs and PAHs form during forest fires and are then introduced into the soil. The results further suggest that the ash resulting from the combustion of wood and other organic matter is the main agent influencing the concentration of PCDD/Fs and PAHs in the soil.

Introduction

Polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs) and polynuclear aromatic hydrocarbons (PAHs) have attracted considerable attention in recent decades due to concern over their potential adverse effects on humans and wildlife (Harvey, 1991, Hester and Harrison, 1996). These compounds are persistent organic pollutants (POPs) whose resistance to environmental degradation has led them to become ubiquitous in the environment (Wania and Mackay, 1996).

PCDD/Fs are the unwanted products of various combustion processes and a range of syntheses involving chlorinated compounds (Hester and Harrison, 1996). In addition, natural combustion processes such as forest fires, lightning and volcanic eruptions are thought to produce PCDD/Fs. The fact that PCDD/Fs have been detected in deep sediments, pre-industrial archived soils and remote locations indicates that PCDD/Fs are produced via natural processes (Gribble, 1994, Hashimoto et al., 1990). PAHs are produced by the incomplete combustion of organic matter. The principal sources of PAHs in the environment are the combustion of fossil fuels for heat and power generation, refuse burning, coke ovens and automobiles. They are also introduced into the environment via natural combustion processes such as volcanic eruption, forest and prairie fires (Harvey, 1991, Menzie et al., 1992).

In 1980, Bumb et al. formulated their ‘trace chemistry of fire’ hypothesis, in which they put forward the view that PCDD/Fs can form via natural combustion (Bumb et al., 1980). Since this hypothesis was presented, many studies have been conducted to evaluate its validity. A number of studies on wood combustion products have been conducted to test for the formation of PCDD/Fs (Bacher et al., 1992, Nestrick and Lamparski, 1982, Thoma, 1988, Wunderli et al., 1996). These studies revealed significant levels of PCDD/Fs in wood combustion products, although these levels were lower than those found in waste incinerator products, the principal source of PCDD/Fs. The observation that wood combustion produces significant levels of PCDD/Fs led to the suggestion that forest and brush fires might be the major sources of environmental PCDDs and PCDFs (Bacher et al., 1992, Nestrick and Lamparski, 1982, Thoma, 1988, Wunderli et al., 1996). Based on the wood combustion data of Nestrick and Lamparski (1982), Sheffield (1985) estimated that approximately 58.7 kg of PCDDs are produced by Canadian forest fires each year. In the USA, forest fires are estimated to produce 208 g I-TEQ/year of PCDD/Fs, fourth place in the US emission inventory (Cleverly et al., 1998). The UK emission inventory, however, estimates that 0.4∼12 g I-TEQ/year of PCDD/Fs are released from natural combustion processes (Alcock et al., 1999). In addition to wood combustion product surveys, studies have been carried out on real forest fires and controlled field burnings (Buckland et al., 1994, Mahnke and Krauß, 1996, Martinez et al., 2000, Tashiro et al., 1990, Walsh et al., 1994). Some surveys have reported a fire-induced change in congener distribution. However, these studies have provided no firm evidence that forest fires act as natural sources of PCDD/Fs, contrary to expectations based on the studies of wood combustion products. Hence, there is continuing debate over whether PCDD/Fs are formed through natural processes.

Forests cover approximately 65% of Korea, and each year forest fires cause a great deal of damage. In April 2000, particularly large-scale forest fires occurred in areas on the east coast damaging over 25 000 ha (http://www.foa.go.kr). Given that forest fire is believed to act as a natural source of PCDD/Fs and PAHs, such forest fires may contribute greatly to the background levels of PCDD/Fs and PAHs in Korea. Thus, the aims of the present study were to assess the potential of forest fires to present a significant source to the environmental PCDD/F burden.

The present study investigates whether PCDD/Fs and PAHs are formed by forest fires. Ideally, such an investigation would include direct air sampling during forest fires to test for the formation of PCDD/Fs and PAHs. However, air sampling during forest fires is difficult because it is not known in advance when, where and how forest fires will occur. Hence, the present work takes the indirect approach of measuring PCDD/Fs and PAHs in soil and ash samples after forest fires. Soil samples were collected after forest fires at five sites in the eastern coastal region of Korea. First, we investigated the effect of forest fires on the physical and chemical characteristics of soils. After establishing the physical and chemical changes wrought by the fires, we investigated the levels and patterns of PCDD/Fs and PAHs and compared this data with the characteristics of the corresponding unburnt soil samples. PCDD/F and PAH levels were analyzed 1, 5 and 9 months after the event of fires had occurred.

Section snippets

Sampling

Soil samples were collected after forest fires at five sites in the eastern coastal region of Korea: Kosung (site A), Kangneung (site B), Samchock (site C), Donghae (site D) and Pohang (site E) (see Fig. 1). Large-scale forest fires affected sites A, B and C in April 2000, with a total of over 20 000 ha being burnt over several days. Sites D and E suffered relatively small-scale forest fires which burnt 15 ha in December 2000 and 20 ha in March 2001, respectively. The vicinities of these study

Physical and chemical properties of the soils

The ash deposition and heat associated with forest fire have been shown to affect the physical and chemical properties of the soil (Cass et al., 1984, Raison, 1979). Table 1 shows the physical and chemical properties of the burnt and unburnt soils analyzed in this study. The physical properties of the soils are similar for the burnt and unburnt soils, in agreement with previous findings that there is very little immediate alteration of the physical properties of a soil unless heating is severe.

Conclusions

In the present study, the levels and distributions of PCDD/Fs and PAHs were analyzed in burnt soil samples collected 1, 5 and 9 months after forest fires. These results were compared with those of the corresponding unburnt soils. Soil samples were collected at five sites in the eastern coastal region of Korea: Kosung (site A), Kangneung (site B), Samchock (site C), Donghae (site D) and Pohang (site E). One month after the forest fires, soils from sites A, B and C showed increased concentrations

Acknowledgements

This work was supported by a fund from Ministry of Government, Korea, as the ‘The Eco-technopia 21 project’ and partly by the Korea Science and Engineering Foundation (KOSEF) through the Advanced Environmental Monitoring Research Center at Kwangju Institute of Science and Technology.

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