Volume II Chapter 3.0 Pages 5 of 10 page next page 6

3.5.3. The de novo Synthesis of CDDs/CDFs During Combustion of Organic Materials 3-75

3.5.4 Theory on the Emission of Polychlorinated Biphenyls 3-91

3.5.5. Evaluation of Naturally Occurring CDD/CDFs by Examination of Sediment Core Data 3-92

3.5.3. The de novo Synthesis of CDDs/CDFs During Combustion of Organic Materials

The third and last theory states that CDDs/CDFs are formed in combustion processes from materials and/or compounds that are not structurally related to CDDs/CDFs on a molecular level. As in Theory 2, synthesis is believed to occur in regions outside of the furnace zone of the combustion process where the combustion plasma has cooled to a range of temperatures considered favorable to formation kinetics. A key component to de novo synthesis is the production of intermediate compounds (either halogenated or non-halogenated) that are precursors to dioxin formation.

However, research in this area has produced CDDs/CDFs directly from the heating of carbonaceous fly ash in the presence of an inorganic ion without the apparent generation of reactive intermediates. Thus, the specific steps involved in the de novo process have not been fully and succinctly delineated. Laboratory experimentation has proven that MSWI fly ash, itself, is not an inert substrate, and the matrix can actually participate in the formation kinetics.

table Table 3-22 CDDs/CDFs Formed From the Thermolytic Reaction of 690 mg Benzene + FeCl3Silica Complex
Typically the fly ash is composed of an alumina-silicate construct with 5-10 percent concentrations of silicon, chlorine (as inorganic chlorides), sulfur, and potassium (NATO, 1988).

Twenty percent of the weight of fly ash particles are carbon, and the particles have specific surface areas in the range of 2-4 m2 (NATO, 1988).

The distinguishing feature of the de novo synthesis over the precursor synthesis is the thermolytic breakdown and molecular rearrangement of chemical species unrelated to CDDs/CDFs at the start of the process to yield precursor compounds. Theory 2 starts with the precursor compounds already adsorbed onto the surface of fly ash or present in the gas phase
...
expand table Table V2 3-22
... (Dickson et al., 1992). By this distinction, however, one could argue that Theory 3 is really an augmentation to Theory 2 because the generation of CDDs/CDFs may still require the formation of a dioxin precursor. Nevertheless, a distinction is presented here for purposes of describing additional pathways that have been suggested for the thermal formation of these compounds.

To delineate the de novo synthesis of CDDs/CDFs from unrelated matter, Stieglitz and coworkers (1989a) have conducted experiments involving the heating of particulate carbon containing adsorbed mixtures of Mg and Al-silicate in the presence of copper chloride as a catalyst to the reaction. The authors described annealing mixtures of Mg-Al silicate with activated charcoal (4 percent by weight), chloride as potassium chloride (7 percent by weight), and 1 percent copper chloride (CuCl2) (in water) in a glass tube at 300° C. The retention time was varied at 15 minutes, 30 minutes, and 1, 2, and 4 hours to obtain differences in the amounts of CDDs/CDFs that could be formed. The results are summarized in Table 3-23.

In addition to the CDDs/CDFs formed as primary products of the de novo synthesis, the investigators observed the formation of precursors at the varying retention times of the experiment. In particular, similar yields of tri- though hexa-chlorobenzenes, tri- through hepta-chloronaphthalenes, and tetra- through hepta-chlorobiphenyls, were quantified which were seen as highly suggestive of the role these compounds may play as intermediates in the continued formation of CDDs/CDFs. Table 3-24 summarizes the experimental yields of chlorinated benzenes as a function of the annealing time at 300° C. Stieglitz et al. (1989a) made the following observations:

table Table 3-23 De Novo Formation of CDDs/CDFs After Annealing Mg-Al Silicate, 4% Charcoal, 7% Cl, 1% CuCl2.2H2O at 300° table Table 3-24 De Novo Formation of Chlorinated Benzenes (CBzs), Polychlorinated Naphthalenes (PCNs), and Polychlorinated Biphenyls (PCBs) after Annealing Mg-Al Silicate, 4% Charcoal, 7% Cl, 1% CuCl2.2H2O
expand table Table V2 3-23 expand table Table V2 3-24
1. The de novo synthesis of CDDs/CDFs via the reaction of carbonaceous particulate matter exposed to a temperature of 300° C was clearly demonstrated. Additionally, the experiment yielded ppb-ppm concentrations of chlorinated benzenes, chlorinated biphenyls, and chlorinated napthalenes through a similar mechanism. When potassium bromide was substituted for potassium chloride as a source of halogen for the organic compounds in the reaction, polybrominated dibenzo-p-dioxins and dibenzofurans were formed as reaction products.

2. Copper chloride catalyzed the de novo synthesis of CDDs/CDFs on the surface of particulate carbon in the presence of oxygen to yield carbon dioxide and chlorinated/brominated aromatic compounds

3. Particulate carbon, which is characteristic of combustion processes, may act as the source for the direct formation of CDDs/CDFs as well as other chlorinated organics.

More recently, Stieglitz and coworkers (1991) investigated the role that particulate carbon plays in the de novo formation of CDDs/CDFs from fly ash containing appreciable quantities of organic chlorine. Stieglitz et al. (1991) found that the fly ash contained 900 µg/g of bound organic chlorine. Only 1 percent of the organic chlorine was extractable. Annealing the fly ash at 300-400° C for several hours caused the carbon to oxidize leading to a reduction in the total organic chlorine in the matrix, and a corresponding increase in the total extractable organic chlorine (TOX) (e.g., 5 percent extractable TOX at 300° C and 25-30 percent extractable total organic chlorine at 400° C). From this, Stieglitz et al. (1991) concluded that the oxidation and degradation of carbon in the fly ash are the source for the formation of CDDs/CDFs, and, therefore, are essential in the de novo synthesis of these compounds.

Addink et al. (1991) conducted a series of experiments to observe the de novo synthesis of CDDs/CDFs in a carbon-fly ash system. In this experiment, 4 grams of carbon-free MSWI fly ash were combined with 0.1 gram of activated carbon and placed into a glass tube between two glass wool plugs. The glass tube was then placed into a furnace at a specific temperature in the range 200 to 400° C. This was repeated for a series of retention times and temperatures. The investigators observed that the formation of CDDs/CDFs was optimized at the temperature of 300° C and at the furnace retention times of 4-6 hours. Figure 3-3 displays the relationship between retention time, temperature and the production of CDDs/CDFs from the heating of carbon particulate.

figure Figure 3-3 The de novo Synthesis of CDD/CDFs from Heating Carbon Particulate at 300° C at Varying Retention Times
Addink et al. (1991) also investigated the relationship between temperature of the furnaceand the production of CDDs/CDFs from the annealing of carbonaceous fly ash. Figure 3-4 displays this relationship.

In general, the concentration began to increase at 250° C and crested at 350° C, with a sharp decrease in concentration above 350° C.

The authors also noted a relationship between temperature and the CDD/CDF congener profile; at 300° C to 350° C, the lower chlorinated tetra- and penta-CDD/CDF congeners increased in concentration, while hexa-, hepta-, and octa-CDD/CDF congeners either remained the same or decreased in concentration.
expand Figure V2 3-3
The congener profile of the original MSWI fly ash (not subject to de novo experimentation) was investigated with respect to changes caused by either temperature or residence time in the furnace. No significant changes occurred, leading the authors to propose an interesting hypothesis for further testing: after formation of CDDs/CDFs occurs on the surface of fly ash, the congener profile remains fixed and insensitive to changes in temperature or residence time indicating some form of equilibrium is reached in the formation kinetics.
figure Figure 3-4 Relationship Between Temperature and the de novo Formation of CDDsCDFs
Gullett et al. (1994) developed a pilot-scale combustor to study the effect on CDD/CDF formation of varying the combustion-gas composition, temperature, residence time, quench rate, and sorbent (Ca[OH]2) injection. The fly ash loading was simulated by the injection on fly ash collected from a full-scale MSWI. Sampling and analysis indicated CDD/CDF formation or the injected fly ash at levels representative of those observed at full-scale MSWIs.

A statistical analysis of the results showed that, although the effect of combustor operating parameters of CDD/CDF formation is interactive andvery complicated, substantial reduction in CDD/CDF formation can be realized with high temperature sorbent injection to reduce HCl or Cl2 ...
expand Figure V2 3-4
... concentrations, control of excess air (also affects ratio of CDDs to CDFs formed), and increased quench rate.

The de novo theory also considers the generation of CDDs/CDFs from the combustion of PVC resin. Key to the de novo synthesis of CDDs/CDFs is the initial formation of HCl from combustion. Paciorek and coworkers (1974) thermally degraded pure PVC resin at 400° C and produced 550 mg/g HCl vapor as a primary thermolysis product, which was observed as being 94 percent of the theoretical amount based on the percent weight chlorine on the molecule.

Ahling et. al. (1978) have concluded that HCl can act as a chlorine donor to ultimately yield chlorinated aromatic hydrocarbons from the thermolytic degradation of pure PVC, and that these yields are a function of transit time, percent oxygen, and temperature. The data they observed from 11 separate experiments conducted with a range of temperatures from 570-1130° C indicated that significant quantities of various isomers of dichloro-, trichloro-, tetrachloro-, and hexachlorobenzenes could be produced.

Choudhry and Hutzinger (1983) proposed that the radical species Cl× and H× generated in the incineration process may attack the chlorinated benzenes thus formed, and abstract hydrogen atoms to produce ortho-chlorine substituted chlorophenol radicals. These intermediate radical species then react with molecular oxygen to yield ortho-substituted chlorophenols. As a final step, the ortho-substituted chlorophenols act as ideal precursors to yield CDDs/CDFs with heat and oxygen.

Although most of the aforementioned experiments have involved the pyrolysis of anthropogenic substances, the de novo formation of CDDs/CDFs is theoretically proposed to include the combustion of autochthonous (naturally occurring) organic substances (Choudhry and Hutzinger,1983) in the presence of a chlorine donor. This possibility was first advanced by scientists at Dow Chemical Co. in 1978 in a proposed working hypothesis known as "the trace chemistries of fire" (Crummett, 1982). This proposed working hypothesis was based on the following observations:

1. Combustion processes are seldom more than 99.9 percent efficient in converting carbonaceous fuel into carbon dioxide.

2. The remaining 0.1 percent of the fuel is converted into traces of organic species including complex halogenated aromatic hydrocarbons. Most of these compounds have not been identified in combustion emissions.

3. Municipal solid waste and fossil fuels contain complex mixtures of diverse chemical species at variable concentrations.

4. Combustion fuels contain chlorine in a range of 1-5000 parts per million.

5. Particulate matter that is emitted from oil-fired heating and power plants contain vanadium and nickel. Particulates emitted from coal-fired power plants contain vanadium, nickel, iron, and manganese. In combination with silicon and unburned carbon, these species can act as catalysts in the combustion process to form halogenated aromatic hydrocarbons.

6. Chemical reactions that occur in flames include pyrolysis, oxidation, and reduction. Ions, electrons, free radicals, and free atoms interact in a continuously changing environment.

7. Dow scientists found traces of CDDs/CDFs in all particulate matter samples taken from areas that were in close proximity to combustion sources.

8. Precursors for the formation of CDDs have been experimentally proven, and have been identified to be primarily chlorinated phenols and chlorinated benzenes. Because the pyrolysis of polyvinyl chloride (PVC) produces chlorobenzenes, the combustion of PVC may cause the formation of CDDs/CDFs.

Dow Chemical Co. invited the scientific community at large to give advice on ways in which the trace chemistries of fire hypothesis could be tested. The following studies were proposed as a means of testing the hypothesis (Crummett, 1982):

1. Determine if CDDs/CDFs are present in soils (having a relatively high carbon content) taken from drill cores beneath ancient lake beds at depths corresponding to 5, 12, and 35,000 years of sedimentation and deposition.

2. Determine if CDDs/CDFs are present in ice core samples taken from the center of an ancient glacier.

3. Determine if CDDs/CDFs are present in volcanic ash.

4. Determine if CDDs/CDFs are present in sea breezes from remote islands in the South Pacific.

5. Determine if CDDs/CDFs can be formed by the combustion of fossil fuels in the presence of chlorine or inorganic chloride.

6. Determine if CDDs/CDFs can be detected in fish species taken from rivers remote from chemical manufacturing but close to incinerators and fossil-fueled power plants.

Although these studies were proposed in 1978, only items (3), (5) and (6) have even been partially addressed. Thus the "trace chemistries of fire" remains largely a working hypothesis that is in need of further testing and proving through well designed and conducted field sampling and laboratory research programs. Nevertheless, there exists some empirical evidence in this area.

Liberti et al. (1983) showed that CDDs/CDFs could be produced from the combustion of pure vegetable extracts in the presence of chlorine gas and oxygen. Pyrolytic degradation of extracts of chestnut, mimosa, and tannic acid was accomplished in a bench-scale thermal reactor. When combustion proceeded without chlorine gas, phenolic compounds and cresol were formed as primary thermolysis products.

When the vegetable extracts were burned in association with chlorine gas or PVC plastic, chlorophenols and CDDs/CDFs were formed. Liberti et al. (1983) postulated that the PVC was acting as a chlorine donor in the formation of CDDs/CDFs from phenolic compounds, and that the chlorine gas directly formed the chlorinated precursor from a phenolic (pre-dioxin) ring structure. Table 3-25 summarizes these experiments.

There is some empirical evidence that the burning of wood, in the presence of chlorine or inorganic chlorides, may form CDDs/CDFs, although the evidence is not conclusive. Few of these experiments had ruled out contamination of the wood fiber by known chlorinated precursors through extraction and chemical analysis. None of the cited experiments attempted to determine if the wood fiber was contaminated by CDDs/CDFs prior to the conduct of the experiment.
table Table 3-25 CDDs/CDFs Formed from the Combustion of Vegetable Extracts in the Presence of Chlorine Gas
If the atmosphere serves to widely distribute CDDs/CDFs, and if CDDs/CDFs can exist in the vapor and particle phases in the ambient air, then trees and other biomass can become reservoirs of CDD/CDF contamination by means of particle deposition onto and vapor diffusion into the biomass.

Until these possibilities have been addressed and their impacts, if any, are quantified, experiments in which CDDs/CDFs are generated from the combustion of wood must be interpreted with a certain degree of caution, especially with regard to proving that CDDs/CDFs can be formed in nature without human intervention. Ahling and Lindskog (1982) demonstrated that the combustion of untreated wood in an open fire can generate relatively high concentrations of chlorinated aromatic ...
expand table Table V2 3-25

... hydrocarbons in the emissions. These compounds include established dioxin precursors such as di- through hexa-chlorobenzene and tetra- and penta- chlorinated phenols in ppbv-ppmv concentrations.

In addition, ppmv levels of benzene were produced. The presence of chlorinated precursors indicates that inorganic chlorides in the plant may be capable of chlorinating unsubstituted aromatic structures. Reaction kinetic experiments involving the formation of HCl vapor (Olie et al., 1983; Choudhry and Hutzinger, 1983) have shown that HCl can be formed from inorganic chlorides as a result of a reaction between sulfur dioxide and sodium chloride during combustion, as follows:

Diagram V2 3-3

The general reaction is:

Diagram V2 3-4

Olie et al. (1983) conducted wood burning experiments in a bench-scale combustion unit. Wood treated with pentachlorophenol was incinerated to generate CDDs/CDFs in one experiment, and 60-year-old wood from the demolition of a residence was burned in a separate experiment. The authors alleged that the 60-year-old wood predated the manufacture and use of phenoxy wood preservatives and, therefore, probably was absent any dioxin precursors; however, this was not analytically confirmed.

They did not directly monitor the smoke emissions for the presence of CDDs/CDFs, only the collected fly ash. CDDs/CDFs were detected in the fly ash in ppbw concentrations. However, the authors noted that the quantified CDDs/CDFs could have occurred as a consequence of the previous tests of burning wood treated with pentachlorophenol.

Nestrick and Lamparski (1983) conducted studies on residential wood combustion to evaluate the possibility that CDDs may form. This was accomplished through the evaluation of soot scrapings from the chimneys of wood burning stoves. Samples were taken at random from the eastern, central, and western regions of the United States. Average total CDD levels in the chimney flue scrappings were: 8.3 ppb in the eastern region, 42.5 ppb in the central region, and 9.9 ppb in the west.

EPA tested a freestanding noncatalytic residential wood stove for chimney flue gas emissions of CDDs/CDFs during the combustion of pine and oak (U.S. EPA, 1987d). Through a series of tests, it was determined that the wood fiber was free of known chlorinated precursors (e.g., PCBs, chlorinated benzenes, and chlorinated phenols). The total chloride concentration was found to be 125 ppm for the oak and 49 ppm for the pine wood prior to burning in the wood stove. The combustion of the wood generated ppm levels of aromatic hydrocarbon compounds.

This relatively high loading of emissions on the sampling device interfered with any speciation of CDD/CDF compounds in the emissions. However, combustion ash samples provided an alternative matrix for evaluation. The analysis of ash samples from the unit showed that only OCDD was present as a dioxin contaminant at a maximum concentration of 0.09 ppb (by weight). Wipe samples were also taken from inside the chimney flue. OCDD and hepta-CDD were detected in the chimney soot at a maximum concentration of 0.6 ppb and 0.04 ppb, respectively. No lower chlorinated CDDs nor any CDFs were found in the ash or soot wipes.

Choudhry and Hutzinger (1983) have postulated that the complex structure of lignin in wood fiber can pyrolyze to generate CDDs/CDFs if a chlorine donor is present. This theory is based on the experiments of Kirsbaum et al. (1972) in which two lignin preparations (spruce and asp) were thermally degraded in glass tubes at 475° C to yield an array of hydrocarbons, including phenol. If phenol could be formed, and if gaseous forms of inorganic or organic chlorine are available, then the phenol could be chlorinated to form a chlorophenol compound. The latter is then a precursor to the ultimate formation of CDDs/CDFs.

In addition, Choudhry and Hutzinger (1983) indicated that continued pyrolysis of other hydrocarbons identified as thermolysis products in the combustion of lignin could ultimately yield benzene. Nestrick et al. (1987) demonstrated that benzene can react with an inorganic chloride in the presence of heat to produce a variety of chlorinated aromatic compounds including CDDs/CDFs. If these thermolytic pathways are operational in lignin pyrolysis, then, in theory, it is possible that forest fires can generate CDDs/CDFs in the smoke, which has been proposed by Clement and Tashiro (1991).

Because of the potential importance of lignin pyrolysis as a potential, yet unverified, combustion source of CDDs/CDFs in the environment, additional research should be directed in this area. In the conduct of combustion experiments involving the pyrolysis of lignin, attention should be given to the identification of any CDDs/CDFs or precursor compounds that may exist as contaminants. Only after sample contamination has been completely ruled out can the researcher draw convincing conclusions from the experiment.

Coal is a naturally occurring substance having the potential to form CDDs when combusted. Mahle and Whiting (1980) first reported on the results of high temperature combustion of bituminous coal in a bench scale furnace with the addition of HCl, NaCl, or Cland air to yield CDDs. Table 3-26 summarizes these experiments.
table Table 3-26 CDDs/CDFs Formed from the Combustion of Coal in the Presence of NaCl, Cl2, or Hydrochloric Acid
In experiment III, tetra through octa-CDDs were formed from the oxidation of coal by air which had been bubbled through a solution of hydrochloric acid. In a review of this experiment, Choudhry and Hutzinger (1983) postulated that the hydrochloric acid aided in the chlorination of aromatic hydrocarbons produced as combustion byproducts.

This was also the case in experiment IV in which chlorine gas was introduced into the oxidation of coal. Experiment IV produced the highest yield of tetra- through octa-CDDs. When coal was combusted only with air, the exothermic reaction did not generate detectable quantities of tetra- or hexa- CDDs. Experiment I yielded only hepta- and octa-CDD in quantities close to the detection limit.
expand table Table V2 3-26

3.5.4 Theory on the Emission of Polychlorinated Biphenyls

The air emission of polychlorinated biphenyls (PCBs) from MSW incinerators is less understood. There are virtually no theories explaining the detection of these compounds in incinerator emissions nor other combustion sources, the exception being the intentional destruction of PCBs in hazardous waste incinerators in which case 99.9999 percent destruction rated efficiency (DRE) must be achieved.

When this occurs, 0.0001 percent of the initial amount of PCBs fed into the hazardous waste incinerator may be emitted out the stack. This may indicate that some small fraction of the PCBs present in the fuel fed into an incineration process may result in emissions of PCBs from the stack of the process.

PCBs have been measured as contaminants in the raw refuse prior to incineration in an MSWI (Choudhry and Hutzinger, 1983; Federal Register, 1991). It is possible to use this information to test Theory 1 involved in CDD/CDF emissions: that the PCB contamination present in the fuel is responsible for emissions from the stack.

The mass balance of total PCB beginning with measurement in the raw refuse and ending with measurement at the stack to an RDF MSW incinerator (Federal Register, 1991) can be used to calculate the destruction rated efficiency (DRE) of incineration of the PCB contaminated MSW. Using results from test number 11 at the RDF facility (Federal Register, 1991), a computation of DRE can be made with the following equation (Brunner, 1984):

Formula V2 3-1

In test 11, 811 nanograms of total PCBs/gram of refuse (ng/g) were measured in the MSW fed into the incineration system, and 9.52 ng/g of total PCB were measured at the inlet to the pollution control device (i.e., outside the furnace region, but preceding emission control). From these measurements, a DRE of 98.8 percent can be calculated.

Therefore, it appears that PCB contamination in the raw MSW that was fed into this particular incinerator may have accounted for the emission of PCBs from the stack of the MSW incinerator.
PCBs can be thermolytically converted into CDFs (Choudhry and Hutzinger, 1983; U.S. EPA, 1984). This process occurs at temperatures somewhat lower than typically measured inside the firebox of an MSWI.

Laboratory experiments conducted by EPA (U.S. EPA, 1984) indicate that the optimum conditions for CDF formation from PCBs are near a temperature of 675° C in the presence of 8 percent oxygen and a residence time of 0.8 seconds.

This resulted in a 3 to 4 percent efficiency of conversion of PCBs into CDFs. Because 1 to 2 percent of the PCBs present in the raw refuse may survive the thermal stress imposed in the combustion zone to the incinerator (Federal Register, 1991), then it is reasonable to presume that PCBs in the MSW may contribute to the total mass of CDF emissions released from the stack of the incinerator. This is speculative, and more definitive research is needed in this area before strong conclusions can be made regarding the causes of PCB emissions during combustion.

3.5.5. Evaluation of Naturally Occurring CDD/CDFs by Examination of Sediment Core Data

In the review of these theories, a question arises as to the contribution made by combustion of synthetic organic substances produced by humans versus the contribution made by natural sources to the overall thermolytic synthesis of CDDs and CDFs.

This question can be partially addressed using the results from analyses of the temporal distribution of CDDs/CDFs in sediment core samples taken from lakes located near the cradle of the U.S. industrial revolution (Czuczwa et al., 1984; Czuczwa and Hites, 1985; Czuczwa and Hites, 1986; Smith et al., 1992). Czuczwa and Hites (1985) analyzed sediment core samples taken in Lake Huron by the Great Lakes Research Station of the University of Michigan. Sedimentation rates within the core samples were determined by using Cs-137 and Pb-210 techniques of Robbins and Edgington (1973).

These rates were used as a basis of relating depth of core sample to era. CDDs/CDFs were detected in the core samples, and the results showed no appreciable degradation of CDDs/CDFs in the sediments over time. The most abundant CDDs/CDFs were OCDDs, and HpCDDs/CDFs. Analysis of depth of core sample versus era showed that the CDDs/CDFs increased steadily in concentration beginning at about 1940 and leveled off at about 1960.

Comparisons were made between this trend and the total production of synthetic chlorinated organic chemicals, as well as the total volume of coal combusted for energy production. If it is theoretically possible that the combustion of coal produces air emissions of CDDs/CDFs, then this should be reflected in the sediments.

The pattern of levels of CDDs and CDFs in the sediment cores seemed to track the total volume production of synthetic chloro-aromatics by the petrochemical industry in the United States, whereas the consumption of coal did not show a good correlation. Czuczwa and Hites (1985) concluded that the history of sedimentation rates of CDDs/CDFs in core samples from Lake Huron were reflective of atmospheric deposition from the combustion of synthetic chloro-aromatics, and, therefore, could only have come from the combustion of anthropogenic substances.

In a separate study, Czuczwa et al. (1984) and Czuczwa and Hites (1986) reported on the temporal variability of CDDs/CDFs in sediment core samples taken from a wilderness lake located in an uninhabited/undeveloped island (Siskiwit Lake, Isle Royale) in Lake Superior. Comparisons were made between the congener profiles found in the lake sediments to congener profiles found in urban air particulates.

A near perfect correlation was found (correlation coefficient = 0.998), leading to the observation that CDDs/CDFs entered the lake system from aerial transport and deposition. The historical record of CDD/CDF concentration in the core samples showed that CDDs/CDFs were virtually absent from the sediments until around 1940, therefore ruling out any significant contribution to background from natural sources such as forest fires.

Using a similar study design, Smith et al. (1992) investigated the temporal distribution by era in sediment core samples taken from Green Lake, New York, near Niagara Falls. Green Lake is a State Park, and removed from direct discharge of CDDs/CDFs into the system.

The investigators found a similar congener profile as Czuczwa et al. (1984) and Czuczwa and Hites (1986) in the sediments, and found excellent agreement with measured deposition flux of OCDD into the lake and the concentration of OCDD in the most recent sediment layer. This supports the observation of Hites (1991) on the importance of atmospheric transport and deposition as a major pathway of entry of CDDs/CDFs into the aquatic environment.

Smith et al. (1992) found that CDDs/CDFs could be detected in sediments dating back to 1860 - 1865, although concentrations were found to be low (e.g., CDDs = 7.0 ppt with 98 percent being OCDD; CDFs = 2.1 ppt with 75 percent being OCDF). These low concentrations remained essentially steady until about 1920 when concentrations significantly increased.

Between 1920 and 1940, CDDs increased from about 1-10 ppt to about 250 ppt, and CDFs increased from 5 to about 100 ppt. Between 1940 and 1960, CDDs increased to about 680 ppt, and CDFs increased to about 300 ppt. From 1960 to 1980, CDDs continued to increase to approximately 950 ppt, whereas CDFs significantly declined to about 150 ppt.

From the work of Czuczwa et al. (1984), Czuczwa and Hites (1986), and Smith et al. (1992), it appears that anthropogenic combustion sources, taken in their entirety, probably represent the largest mass flux of CDDs/CDFs into the environment, and that natural combustion activity (i.e., forest fires) probably is insignificant by comparison.

For example, if it is assumed that the 7 ppt CDDs in sediments dating back to the 1860s is reflective more of natural sources, and if the CDDs that were found to be 950 ppt in 1980 are more reflective of human sources, then a simple comparison would indicate that anthropogenic sources may exceed natural sources by a factor of 100:1.