Seven years after the spill of an estimated 140,000 gallons of a mixture of No. 6 and No. 2 fuel oils into the Patuxent River, a study was conducted to assess recovery at 24 oiled sites compared with 24 unoiled sites. Metrics included stem density, stem height, belowground biomass and polynuclear aromatic hydrocarbons (PAH) at depths of 0–10 cm and 10–20 cm, and soil toxicity of the 0–10 cm interval.
Half of the soil samples contained the spilled oil; total PAHs varied by 1–2 orders of magnitude between the top and bottom intervals, reflecting high oil heterogeneity. The oil had lost 22–76% of its initial PAH content, although there had been little to no additional PAH weathering since Fall 2000 and Summer 2001. Stem density and height were significantly lower in oiled versus unoiled sites for Spartina alterniflora but not S. cynosuroides.
In contrast, belowground biomass was significantly lower in S. cynosuroides but not S. alterniflora. Based on toxicity tests and sediment quality benchmarks, 25% of the soils were expected to be toxic to many organisms (ESB-TUFCV values > 3.0; PMax > 0.65).
There are likely two factors limiting natural weathering processes in the marsh soils: slow physical removal processes and low oxygen availability.
The interior marsh habitat is flooded by daily tides through many small channels. The marsh surface has a lot of micro-topography with low areas between dense clumps of stems that hold pools of water during low tide. The sediments in these low areas are very soft and water saturated.
Obviously, during spring low tides, the marsh soils do drain as low as 30 cm, because the oil penetrated to these depths in some areas. The falling tide drains through dense vegetation. Tidal flushing may have been a mechanism for removal of bulk oil stranded on the surface initially; however, it would not be effective at mobilizing oil from below the marsh surface. There are few bioturbating benthic biota in these marshes. Photo-oxidation does not occur below ground. Therefore, the only other removal mechanism would be microbial degradation.