Environmental factors influencing oil penetration and persistence in fine sediment tidal flats

Physical and sedimentological parameters determining the retention and penetration of oil slicks into fine sediments were examined experimentally in natural and reconstituted cores, in a simulated tidal system. Fifty-three undisturbed sediment cores (1 m, 6·7cm diameter, i.d.) were collected from two intertidal sites — fine sand tidal flat (Patricia Bay, Vancouver Island; mean grain size 152 ± 29 μm (n = 15); permeability 0·124 cm min⁻¹) and a coarser sediment beach (Island View Beach, Vancouver Island; mean grain size 280 ± 142 μm (n = 3); permeability 0·435 cm min⁻¹). Thirty-three were used for oiling studies, 20 for granulometric determinations. A further 12 reconstituted cores were prepared from mixtures of Patricia Bay and Island View Beach sediments with fine clay added, to yield an overall range of mean sediment grain size (132 to 287 pm), mud content (0·35 to 4·7%), and permeability (0·0035 to 0·44 cm min⁻¹).

Known amounts of an experimentally weathered heavy oil (Alberta Sweet Mixed Blend) were layered onto water standing over the sediment cores during a simulated flood period, and during simulated ebb were brought into contact with the core surfaces, where the oil layer was left in contact for varying tidal periods. Quantitative and qualitative measurements were made on gross oil fate, penetration of oil into core sediments, and weathering, using a combination of infrared spectroscopy and gas chromatography. Also investigated were the influence on oil penetration of sediment grain size, percentage mud content, and duration of tidal submersion.

Oil loading experiments with surface slicks of different thicknesses (0·5–10 mm) showed that increasing thicknesses of surface-applied oil resulted directly in increasing concentrations of hydrocarbons in the sediments. However, 96 to 100% of the oil was found primarily in the top 2 cm, with little penetration below 2 cm in fine sand cores when exposed to simulated tidal incursions.

Both penetration and hydrocarbon concentrations within the contaminated sediments varied inversely with mud content as index offineness. Penetration, in these well-sorted fine sand sediments, became increasingly less at a mud concentration of >2%. For the sediment types used here, the relationship between hydrocarbon penetration and mud content was described by the function log hydrocarbon concentration (mg kg m⁻¹) = 5·04 − 0·426 (% mud) (r = −0·97). Penetration was also influenced by tidal emergence, as indicated in experiments with different tidal regimes. The relationship was consistent. Thus, both penetration and subsurface hydrocarbon concentrations were considerably higher in sediments that were tidally exposed for 57% or longer of the tidal cycle, while submergence for 33% or longer of the tidal cycle resulted in much lower oil penetration and contamination.

These observations indicate that slightly muddy tidal flat sediments (0·35–5·0% mud) may be less vulnerable to oiling than was previously thought, with greater than 95% of oiling restricted to the top 2 cm of homogeneous sediments. However, penetration was sensitive to small increases in sediment permeability and mud content, while persistence of stranded oil was significantly influenced by the location of intertidal oiled sites relative to the mean water level.