Final Report Evaluation of Polycyclic Aromatic Hydrocarbon Migration From Railway Ties Into Ballast and Adjacent Wetlands

In 1996, an electric utility replaced weathered creosote treated railway ties with newly treated ties as part of a routine maintenance operation on a railway spur carrying coal through a wetland into a power generating facility. The wetland was considered important habitat to Somatochlora hineana an endangered dragonfly. The US Fish and Wildlife Service considered the creosote treated railway ties a threat to this endangered dragonfly and required the utility to replace the treated wood ties with steel ties and to conduct a study to determine the potential risks to wetland environments associated with the use of creosote treated railway ties. Because there are many sources of polycyclic aromatic hydrocarbons associated with all forms of transportation, particularly railway lines carrying coal, a mesocosm study was used to determine the rate at which creosote derived PAH moves from treated ties into railway ballast and then into adjacent wetlands. This study included three cells containing newly treated ties, weathered (old) ties, and untreated red oak ties as a control. The mesocosms replicated conditions, including the wetland’s hydrology, found along the actual right-of-way. Concentrations of PAH were monitored quarterly in mesocosm ballast at distances of 5, 20 and 30 cm from the faces of all three types of railway ties and in sediments at distances of 0.0, 0.5 and 0.75 meters from the toe of the ballast for 555 days. The mesocosms were constructed without significant PAH contamination. Creosote was observed migrating from the newly treated ties into adjacent ballast to a distance of at least 30 cm during the summer of the first year following construction. A similar pulse of PAH was not observed in the weathered tie mesocosm. It appeared that this pulse of PAH was associated with warm ambient air temperatures experienced during July and August at the site and with solar insolation that may have significantly increased surface temperatures on the black colored ties, which act as a black body. Concentrations of PAH adjacent to the newly treated ties declined rapidly to low levels during the fall of the first year. This decline was likely associated with photochemical degradation in the dry ballast environment. A second pulse of PAH was not observed in any of the mesocosms during the second summer of monitoring and ballast concentrations remained low throughout the remainder of the study. Sediment concentrations of PAH remained low until near the end of the study when a small increase in sediment PAH was observed in the new tie mesocosm. The increase in sediment PAH attributable to the creosote treated railway ties was approximately 0.3 μg/g, which was equal to the observed atmospheric PAH deposition observed in this area. However, the observed increases were not statistically significant as a function of Distance, Treatment or Day of the study. Polycyclic aromatic hydrocarbons appeared to have migrated vertically downward to a depth of approximately 60 cm in the ballast. At the end of the study, the observed ΣPAH concentration was less than 0.85 μg ΣPAH/g dry ballast at any depth. PAH were detected in one of 16 water samples. Those samples were collected on the final day of the study. However the PAH concentrations were very low and an assessment using the sum of toxic units approach indicated that none of the samples approached concentrations associated with biological stress. Likewise, the PAH concentrations observed in the two most contaminated wetland sediment samples were not predicted to be stressful using the newly developed consensus sediment benchmark methodology of Swartz (1999). No adverse biological effects to even the most sensitive organisms were predicted at the PAH concentrations observed in wetland sediments during this study.