Oil spill pollution on/in sandy beachesmorphosedimentary forcings and natural degradation

Resumo

This thesis is focused on the study of oil spill pollution of both surface and subsurface in the intertidal area of sandy beaches. The central objective is to provide increased insights regarding the forcing mechanisms of oil arrival, subsequent burial, and natural degradation in order to improve the current oil spill response management protocols. This study entails a threefold approach: field surveys, numerical modelling, and experimentation. The surveys have been mainly conducted on Prestige oil polluted sandy beaches, Nemiña and O Rostro beaches (NW Spain), from 2004 to 2011. The beach monitoring program has provided evidence of recurrent Prestige oil pollution in both surface and subsurface (up to 1.8 m depth) even nine years after the accident. The study of sediment colour has been used as a tool to detect contamination when oil concentration is below the detection limits of conventional hydrocarbons analysis. Numerical modelling wave propagation has predicted which beach areas are more exposed to recurrent contamination. Hydrodynamic conditions and beach morphodynamics have revealed as the main factors controlling surface contamination distribution and oil burial and exhumation cycles both early and later stages of black tides. These two factors also have conditioned the maximum depth of oil burial. Theoretical estimation of the depth of oil burial along the intertidal area has been obtained from an equilibrium beach profile combined with a shoreline evolution model. Simulated test cases have revealed the existence of two main patterns of beach profiles behaviour under varying oceanographic and sedimentological conditions. Type A and Type B are characterized by intertidal slopes of time-constant and time-varying steepness, respectively. Type A underscores the existence of burial or exhumation along entire intertidal zone";" whereas Type B combines both burial and exhumation depending on the considered intertidal area. These outcomes have a direct effect in the own definition of the depth of oil burial. Laboratory experiments under controlled environmental conditions have been designed to study the behaviour of buried oil. The results have underlined the influence of sediment composition in the buried oil degradation process from tar balls to oil microparticles and/or oil coatings. They draw attention to carbonates which enhance the appearance and development of oil coatings. The findings of this research have opened new prospects for assessment, monitoring and response programs for oil polluted sandy beaches. These can be addressed in different stages: identification of the problem, dimensioning the affected zone and improving the clean-up operations. The identification of sensitive areas of beaches could be performed due to the knowledge of its exposure to oceanographic conditions. Based on the changes in the oceanographic and sedimentological parameters, the affected area mainly burial depth could be predicted at low computational cost. The current clean-up techniques could be improved, including the addition of bioclastic carbonate sands to retain the oil in a specific site, and hence, limiting its expansion toward more sensitive areas.