Hydrocarbons in intertidal sediments and mussels from Prince William Sound, Alaska

Abstract

We collected and analyzed samples of sediments and mussels (Mytilus trossulus) for alkane and aromatic hydrocarbons from eight sampling stations adjacent to the oil tanker vessel transportation corridor through Prince William Sound, Alaska, during the period from 1977 to 1980, to determine baselines prior to the start of oil tanker movement through the Sound. We evaluated interannual variability of these analytes using a two factor analysis of variance of logarithm-transformed hydrocarbon concentrations determined in duplicate samples collected in July 1977 and in June 1978 at six of the stations. Intra-annual variability was evaluated using analyses of duplicate samples collected in May, June, and August 1978 at seven of the stations. In addition, total organic carbon and grain size distribution was determined in the sediment samples, the lipid content was determined in the mussel samples, and the surface seawater temperature and salinity was determined for each sampling station.

The hydrocarbon analyses indicated chronic, low-level hydrocarbon contamination that probably originates from small fuel spills, ballast water discharges, and fuel-combustion exhaust emissions of occasional vessel activity adjacent to three of the sampling stations: Constantine Harbor, Rocky Bay, and Mineral Flats, in decreasing order of contamination, respectively. Sediments at these three stations were contaminated by aromatic hydrocarbons found at concentrations that were generally less than 10 ng/g dry sediment weight, but above detectable limits (< 1.0 ng/g). In contrast, the remaining five stations showed no indication of petroleum hydrocarbon contamination, primarily because detected aromatic hydrocarbons were present only sporadically and at concentrations that were generally near detection limits. Both perylene, which was found at concentrations well above detection limits at all stations outside Port Valdez, and phenanthrene, which was also found sporadically at all sampling stations may have natural sources. Concentrations of aromatic hydrocarbons were frequently too low at most of the sampling stations to allow evaluation of intra- and interannual variability.

Concentrations of individual n-alkanes varied substantially in sediments and in mussels. The most abundant n-alkanes in sediments included normal alkanes with an odd number of carbon atoms and a molecular weight greater than tetradecane (C-14). Concentrations of these n-alkanes were generally in the range of 10 to 100 ng/g dry sediment weight and exceeded 1,000 ng/g at Constantine Harbor. The most abundant n-alkanes in mussels included decane (C-10) through heptadecane (C-17), and pristane, at concentrations generally ranging from 10 to over 1,000 ng/g dry tissue weight.

Sources of alkanes in sediments included terrigenous plant waxes, marine plankton, and possibly marine macrophytic algae at all the stations; petroleum-derived alkanes were also found at Constantine Harbor. Terrigenous plant waxes in sediments were indicated by high abundances of odd-numbered carbon n-alkanes of molecular weight greater than nonadecane (C-19) compared with even-numbered carbon n-alkanes in these sediments, and by slight but significant intra-annual variability of these odd-numbered carbon alkanes in sediments, which probably arose from seasonal deposition of senescent leaves. Marine planktonic and algal sources of pristane and normal alkanes were indicated by the presence of these alkanes in sediments and in mussels, and by the relatively high abundances of pristane, pentadecane (C-15), and heptadecane (C-17) in sediments and in mussels.

The concentrations of pristane, pentadecane (C-15), and heptadecane (C-17) varied significantly in sediments, in mussels, or in both, intra-annually or interannually. Pristane variability in sediments and in mussels was significantly correlated and was probably due to variability of populations of calanoid copepods in Prince William Sound. Neither pentadecane variability nor heptadecane variability were correlated in sediments and mussels, suggesting multiple biological sources of these alkanes.

These results indicate that, except in areas affected by localized vessel traffic, intertidal sediments and mussels in Prince William Sound were remarkably free of petroleum-contaminant hydrocarbons during the period of this study. The hydrocarbons found in sediments and mussels unaffected by vessel traffic can be adequately explained by known, natural sources. As a result, sediments and mussels contaminated by crude oil from the Exxon Valdez oil spill should be particularly apparent due to the general absence of other confounding sources of petroleum hydrocarbons.


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