Biodegradation and Transport of Crude Oil in Sand and Gravel Beaches of Arctic Alaska

Offshore oil production along Alaska’s arctic coast is expected to increase in coming years. While this will create large economic benefits for the state, crude oil spills may occur. Oil spills reaching the shoreline could create adverse ecological effects, so it is important to understand methods, such as bioremediation, that might expedite oil removal. Mass transfer processes play an important role in determining the fate of crude oil along shorelines. Biodegradation and mass transfer processes are strongly dependent on temperature and sediment material. It is, therefore, necessary to study how the beach matrix, temperature, and nutrient addition affect the fate of hydrocarbons. The effect of environmental conditions on the rate of crude oil removal was studied in the laboratory, simulating oil spills at arctic seashores. Laboratory microcosms were set up containing beach sediments collected from Barrow, spiked with North Slope Crude. The microcosms were spiked with a standard concentration of nutrients and incubated at varying temperatures (3°C vs. 20°C), salinities (30 vs. 35 g/L), and crude oil concentrations (1 vs. 5 mL/kg). Respiration rates (breakdown of hydrocarbons to CO2), hydrocarbons remaining in the sediment (GC/FID), and hydrocarbons volatilized and sorbed to activated carbon (GC/MS) were measured. Mini-column experiments to simulate the transport of oil through the sediment profile were conducted for two different sediment types (sandy-gravel versus pebble) obtained from Barrow and two different temperatures (20° and 3° Celsius). In all microcosms, higher respiration rates by naturally occurring microorganisms were observed at 20oC compared to 3°C. Surprisingly, the release of volatile organic compounds (VOC) was similar at both temperatures, for different crude oil concentration and salinities. Regardless of temperature, increased salinity had a positive impact on the rate of crude oil removal. At higher crude oil dosages, a larger amount of volatiles was released; however, CO2 production did not significantly increase with the contaminant concentration. A mass balance was established that showed that only a small fraction of the crude oil was mineralized, approximately 40% was volatilized and most TPH remained in the sediment. Mini-column data showed that the amount of crude oil pooling and its location was dependent on sediment structure and fertilizer addition. In sandy gravel sediment, TPH pooled in the middle of the column, six inches below the surface. In pebble sediment, the highest TPH concentrations initially occurred at the top and then at the bottom of the column. Overall, less TPH remained in pebble sediment compared to sandy gravel. Apparently hydrocarbons had been washed out more easily from pebble sediment. Both sediments had higher CO2 production at higher temperatures, with the highest respiration found in sandy-gravel, i.e., more biodegradation occurs in sandygravel sediment. While CO2 releases were slightly higher in sediments with the addition of fertilizer; overall the application of fertilizer did not have a significant impact on CO2 release. The results of this study will assist decision-makers in choosing effective spill response strategies for future crude oil spills in Arctic shorelines.