Geology Reference
In-Depth Information
North Dakota lignite (Figure 11.3.2A), contains low concentrations of PAHs (16 EPAPAHs = 1680 ng/g and 45
TPAHs = 11 500 ng/g) dominated by petrogenic three- and four-ring PAHs (e.g., alkylated phenanthrenes and
pyrenes), lower proportions of petrogenic two-ring PAHs and dibenzothiophenes, and a strong perylene peak.
The low concentrations of pyrogenic five- and six-ring PAHs are likely attributed to soot derived from prehistoric
forest/marsh fires. The Wyoming subbituminous coal (Figure 11.3.2B), also contains low concentrations of PAHs
(16 EPAPAHs = 1900 ng/g and 45 TPAHs = 15 600 ng/g) dominated by petrogenic three- and four-ring PAHs,
lower proportions of petrogenic two-ring PAHs and dibenzothiophenes, and a strong perylene peak. The low
concentrations of pyrogenic five- and six-ring PAHs are likely attributed to soot from prehistoric forest/marsh fires.
Some source signature differences between the lignite and subbituminous coals are evident in the different
proportions of specific pairs of PAHs, like fluoranthene relative to pyrene, perylene relative to benzo[
]pyrene,
dialkyl dibenzothiophenes relative to dialkyl phenanthrene, and others. The Wyoming subbituminous coal (Figure
11.3.2B), also contains low concentrations of PAHs (16 EPAPAHs = 1900 ng/g and 45 TPAHs = 15 600 ng/g)
dominated by petrogenic three- and four-ring PAHs, lower proportions of petrogenic two-ring PAHs and diben-
zothiophenes, and a strong perylene peak. The low concentrations of pyrogenic five- and six-ring PAHs are likely
attributed to prehistoric forest fires, coal fires, or volcanic activity. Some source signature differences between the
lignite and subbituminous coals are evident in the different proportions of specific pairs of PAHs, like fluoranthene
relative to pyrene, perylene relative to benzo[
a
a
] pyrene, dialkyl dibenzothiophenes relative to dialkyl phenanthrene,
and others. The Colorado high-volatile bituminous coal (Figure 11.3.2C), contains higher PAH concentrations
compared to the lignite and subbituminous coals (16 EPAPAHs = 10 000 ng/g and 45 TPAHs = 73 100 ng/g). The
greater aromatization corresponds with the appearance of high proportions of petrogenic two-ring PAHs and
progressively lower proportions of petrogenic three- and four-ring PAHs.
The PAH ratios are unique among the coals in this study (Table 11.3.3). The pyrogenic five- and six-ring PAHs are
still present at concentrations comparable with the lignite and subbituminous coals suggesting a preindustrial flux
of combustion by-products into prehistoric North American watersheds. The Colorado medium-volatile bitumi-
nous coal (Figure 11.3.2D), contains intermediate PAH concentrations compared to the subbituminous and high-
volatile bituminous coals (16 EPAPAHs = 2000 ng/g and 45 TPAHs = 32 800 ng/g). The medium- and high-volatile
bituminous coals share general features, such as the high proportions of petrogenic two-ring PAHs and progres-
sively lower proportions of petrogenic three- and four-ring PAHs. However, the source specific PAH ratios
demonstrate many unique features as well (Table 11.3.2). The pyrogenic five- and six-ring PAHs are present at
low concentrations compared to the lower rank coals suggesting the possible importance of watershed size
and proximity to combustion sources, or more likely the increasing role that condensation plays on reducing
the complexity and molecular size of the extractable hydrocarbons occurring at higher ranks. The Pennsylvania
anthracite coal (Figure 11.3.2E), contains trace PAH concentrations compared to lower ranked coals
(16 EPAPAHs = 38 ng/g and 45 TPAHs = 193 ng/g). The devolatilized and highly condensed nature of this coal
likely resulted from higher temperatures and pressures during formation as indicated by the enrichment of parent
relative to alkylated two- to four-ring PAHs. The higher proportion of sulfur in eastern coals is evident in the high
proportions of dialkyl dibenzothiophenes relative to dialkyl phenanthrenes, which likely reflects the greater marine
influences in the Carboniferous eastern coal basins.
The PAH results from the analyses of coals from different ranks highlight several trends in the semivolatile
hydrocarbon signatures. First, lignite and bituminous coals primarily contain petrogenic two- to three-ring PAHs.
The PAH concentrations increase by about one order of magnitude from lignite to high-volatile bituminous coal
with a commensurate enrichment of petrogenic two-ring PAHs. The PAH content declines by more than an order of
magnitude with increasing rank between high-volatile bituminous to anthracite coal. This loss in the amount of
extractable semivolatile hydrocarbons throughout the two- to six-ring PAH range and corresponds with an increase
in the abundance of nonalkylated, pyrogenic PAHs (e.g., the parent PAHs are higher than the alkylated PAHs in
lower rank coals). In short, the character of the PAHs extractable from coal reflects the nature of the initial organic
matter and the stage of geochemical maturation.
Saturated Hydrocarbons
T
he effect of coal type and rank are also reflected in the saturated hydrocarbon signature (Figure 11.3.3). The
plant wax patterns and relative abundances vary considerably in coal due to the variable organic matter from
which they formed and geochemical maturity. A considerable degree of transformation occurs during the
formation of bituminous coals. As rank increases,
the declining proportion of plant wax coincides with
