The extent of burial of the Rena Oil Spill within Bay of Plenty coastal sediments

The grounding of the MV Rena on October 5th, 2011 caused 350-400 tons of heavy fuel oil to spill into the ocean. A significant portion of the oil reached the Bay of Plenty coastline six days later. This thesis investigated the effectiveness of oil clean-up operations that involved spill response crew and some eight thousand volunteers. Previous oil spill studies discussed in literature, have established that spilled oil can mix with sediment beneath the beach surface. 26 Sediment cores averaging ~80cm in length were retrieved ~one year after the Rena oil spill at 12 locations between Waihi and Maketu and were chemically analysed to a depth of 40 cm (0-20 cm and 20-40 cm). GC/MS quantitative results concluded that polycyclic aromatic hydrocarbons (PAHs) were present in the sediment. However, only 7 of the 52 samples contained 4 of the 5 fingerprinted PAHs; phenanthrene, pyrene, benzo(a)anthracene and chrysene, that characterise the Rena oil, with naphthalene undetected in all of the 52 samples. Naphthalene degrades quickly due to its low molecular weight and commonly evaporates from the marine environment within the first few weeks of exposure. Therefore, the absence of naphthalene was anticipated. Variations of phenanthrene and pyrene were detected 27 other sediment samples at low, mid and high tide locations, in both surface 0-20 cm samples and the deeper 20-40 cm samples. Due to the absence of the most resilient PAHs benzo(a)anthracene and chrysene in the 27 samples characterise Rena oil, is likely that phenanthrene and pyrene have come from other hydrocarbon sources such as stormwater outfalls. Laboratory settling flask experiments demonstrated that heavy fuel oil formed droplets and would bind to sediment of both siliciclastic and bioclastic origin. Oil droplets would sink, float or stay suspended in the water column due to oil droplet density variations. Oil that bound to sand grains tended to sink and join the bottom sediments, and oil droplets that contained bubbles of water or air tended to float to the surface of the water column. Storms that occurred in the weeks after the initial Rena spill hindered oil clean-up operations and increased wave activity which potentially enabled oil to mix further into coastal sediments. The maximum depth of disturbance was recorded at Pukehina Beach at 28 cm during storm conditions which established that chemical analysis to a depth of 40 cm encompassed the maximum depth that oil could be mixed into the beach sediment in the intertidal zone. This study has set the foundations on understanding the geotechnical effects of spilled oil and coastal sediment interactions in the Bay of Plenty. The region would benefit from further depth of disturbance studies to assist in more efficient remediation of possible oil spills in the future