Determination of polycyclic aromatic hydrocarbons (PAHs) in sediments from the Irish Sea.

The analysis of 26 marine sediment samples taken from the Irish Sea was accomplished using high performance liquid chromatography (HPLC) and fluorescence detection. The air-dried, powdered samples used were subsamples of cores obtained from offshore of the Cumbrian coast and from the intertidal zones of the Solway Firth. The analytical technique permitted the quantification of fifteen PAHs, i.e. all the commonly encountered suite of sixteen PAHs specified by the USEPA with the exception of one member that could not be detected because of its non-fluorescent nature. The method was found to be both sensitive and selective and the limit of detection ranged from 0.4 μg/kg for acenaphthene to 0.03 μg/kg for anthracene.
Using the chromatographic data the concentrations and distribution patterns of the PAHs in the sediments could be examined, permitting conclusions to be drawn on the likely sources of the PAH in the sediments. For the Solway Firth samples total PAH concentration was found to range from 12 μg/kg to 1512 μg/kg and for the Cumbrian offshore coastal samples the range was 274 μg/kg to 4856 μg/kg. Inspection of the literature disclosed that such ranges are fairly typical of marine sediment PAH concentration levels.
The PAH distribution patterns revealed that the relative proportions of PAH were similar in all the samples regardless of sampling location and total concentration. The pattern corresponded with one that is encountered globally and typifies PAH assemblages resulting from high temperature combustion of fossil fuels. Such PAH generation is referred to as being pyrolitic in origin, distinguishing it from the petrogenic PAHs that arise from fossil fuel spillages etc. Unlike petrogenic PAH that is dominated by low molecular weight PAHs, pyrolitic PAH is marked by a predominance of the high molecular weight PAHs fluoranthene, pyrene, benz(a)anthracene, chrysene, benzo(a)pyrene, benzo(b)fluoranthene, indeno(1,2,3-cd) pyrene and benzo(ghi)perylene, together with some prominence for the low molecular weight PAH phenanthrene, the most stable of the three-ringed members. It was observed for the offshore Cumbrian sediments that these nine pyrolitic PAHs were the dominant species, in all except one core taken close to Whitehaven and its harbour. It was concluded that the pyrolitic PAH output from local coal-burning industry was the main contributor to the relatively high levels of PAH in the offshore Cumbrian coastal samples. The typical pyrolitic PAH signature was modified by a small petrogenic input, assumed to have resulted from incomplete fuel combustion and/or
leakage attending shipping activities in Whitehaven harbour.
For two offshore Cumbrian coastal cores where depth profiles were available the surficial sediment had the lowest total PAH concentration in both cases. Assuming that sediment from the surface is more recent than underlying sediment this may betoken the decline of coal-burning industry in the Cumbrian coastal region. However, because the surficial sediment is more oxic than deeper sediments, aerobic biodegradation of PAH, a more favourable process than anerobic biodegradation, may be responsible for the diminished surface concentrations. The only other core for which a depth profile was available was SF 00/7 (Figure 5) and this exhibited low total PAHs concentration down to 100 cms, but a comparatively high level of total PAHs concentration between 175-190 cms. Once more perhaps, this reflects the history of PAH deposition, i.e. the reduced use by modern shipping of the Solway Firth because of its comparative shallowness.
Unlike the offshore Cumbrian coastal sediments, the Solway Firth intertidal sediments displayed an appreciable presence of low molecular weight PAHs in addition to the nine pyrolitic members (Table 4). Naphthalene, acenaphthene and fluorene were frequently present in significant quantity. Their presence suggested a proportionally greater petrogenic contribution to the total PAHs than for the Cumbrian samples and this may stem from a locally flourishing leisure and tourist industry and the consequent likelihood of leakage and incomplete combustion of the fuel employed in boats.
Some correlation was found between the total organic matter content (TOC) and the total PAH concentration in all the sediments. Given the primarily pyrolitic nature of the PAHs from both regions, it was surmised that the PAH was probably bound to the soot particles, simultaneously generated during fossil fuel combustion, and that these formed a somewhat variable proportion of the total organic phase of the sediments leading to a loose though perceptible correlation. The TOC in the offshore Cumbrian sediments had a higher PAH loading than did the intertidal Solway Firth sediments. This implied larger concentrations of the soot combustion particles, containing PAH, associated with the TOC of the former, presumably, because the Cumbrian coastal region was more industrialised compared with the more rural Solway. Coal-burning industry then was held principally responsible for the greater PAH input to the offshore Cumbrian sediments.
The findings were supported by an alternative form of PAH pattern analysis that employed ratios for certain isomeric pairs of PAHs called molecular indices. The molecular indices revealed the same sort of conclusions that the ranking of abundance had produced, i.e. that the offshore Cumbrian sediments contained greater proportions of pyrolitic PAHs than did the intertidal Solway Firth sediments which in turn had increased contributions of petrogenic PAHs.