The Transport and Fate of Hydrocarbons in Benthic Environments

• Main Author: Boehm, Paul David
• Format: Text
• Language: unknown
• Published: DigitalCommons@URI 1977
• Subjects: Arctica islandica; Ocean quahog
• Online Access: https://digitalcommons.uri.edu/oa_diss/1316; https://digitalcommons.uri.edu/cgi/viewcontent.cgi?article=2324&context=oa_diss

Dissolved organic matter (DOM) in sea water solubilizes certain classes of hydrocarbons (Boehm, 1973). Several chemical and biological consequences of this solubilization phenomenon were explored, Experiments on the solubility behavior of No. 2 fuel oil showed that hydrocarbons can be dispersed in water in a soluble, a solubilized (colloidal micelles), and a particulate (accommodated) state. A fractionation of hydrocarbon mixtures (e.g., fuel oil) in sea water occurs due to the differences in the solubility behavior of the various classes of hydrocarbons within the mixture. Just as DOM promotes saturated hydrocarbon solubility, (i.e., the passage of a filter), solubilization by DOM results in these hydrocarbons being taken up (retained) less by a filter-feeding bivalve (Mercenaria mercenaria). This complementary set of results shows that solubility and uptake (as mediated by DOM) are both influenced by the physical-chemical state and size of the hydrocarbon moiety in water. Aromatic hydrocarbons are taken up by a different mechanism than are the saturates, probably by direct equilibration from water, and to a greater extent, by virtue of their greater thermodynamic water solubility. Their uptake is not directly mediated by DOM. Filter feeders were seen to take up a different suite of hydrocarbons than that introduced into the laboratory uptake chamber, due to solubility considerations. The ability of Mercenaria to depurate hydrocarbons accumulated through chronic exposure in the Providence River, Rhode Island, was examined. Clams which had been subjected to these high chronic hydrocarbon levels showed little ability to rid themselves of this hydrocarbon burden after a 120 day depuration period in clean water. Previous findings by other workers, illustrating rapid depuration, were shown to apply only to short-term acute exposure situations. Chronic exposure allows the hydrocarbons to reach stable compartments of the animal's lipid pool. In order to examine hydrocarbon transport routes in the benthos and to examine the relation between bivalves and the sediments in which they live, a detailed study of Rhode Island Sound's benthic hydrocarbons was undertaken. Hydrocarbon contents of surface sediments vary by two orders of magnitude (1.0 to 301 μg/g), with the lower values found in coarse sand of low organic carbon content, and the higher values in a dredge spoil deposit within the study area. Ratios of (1) total hydrocarbon: organic carbon content, and (2) the concentration of a prominent sediment cycloalkene:organic carbon content, were used to normalize large spatial heterogeneities in the benthic environment of the region. This data treatment permitted an assessment of the effect of dredge spoil disposal (of high hydrocarbon content) on the regular, background hydrocarbon geochemistry. The spoil is seen to be an unimportant source of hydrocarbons to the benthos beyond 2 kilometers from the disposal site. Hydrocarbon contents of the ocean quahog (Arctica islandica) do not reflect the sediment distributions quantitatively or qualitatively. Throughout the study area the clams' hydrocarbon contents vary by only a factor of 2.5 (2.6 to 6.4 μg/g wet). The hydrocarbon assemblage in the clams exhibits a lower boiling chromatographic distribution than that in the surrounding sediments. Key components of the surface sediments are two cycloalkene hydrocarbons of molecular weights 344 (C25H44) and 348 (C25H48). The concentration of the 344 compound covaries very significantly with the sediments' organic carbon content. Mass, infrared, and proton nuclear magnetic resonance spectroscopic studies were performed on the most prominent compound (C25H44). A major cycloalkene component of Arctica (C25H42), is absent from the sediments. Two anthropogenic chemical markers, petroleum hydrocarbons and PCBs, were located in a 40 cm sediment core from the Sound. A knowledge of the approximate dates of introduction of these materials to the environment plus analytical information on the depth at which they appear within the core, allows for the calculation of the recent sedimentation rate (1.4 to 3.3 mm/year) of Rhode Island Sound sediments. Analysis of herbal pollen concentrations in this same core indicated the entire core was deposited since 1700. An average recent sedimentation rate of approximately 1.5 mm/year based on this pollen analysis is supportive of the rate determined from organic chemical markers.