GEOCHEMICAL ANALYSIS AND OIL DISPERSION SIMULATION IN URBAN BEACHES OF SALVADOR (BA): A COMBINED APPROACH

Oil spills in urbanized coastal environments are critical events with severe implications for ecological integrity and public health. This study aimed to identify the origin and dispersion trajectory of the oil that reached three beaches in Salvador (BA) in October 2023 by integrating numerical modeling and geochemical analyses. Oily material samples were collected from the beaches of Amaralina, Paciência, and Ondina in October 2023. These were subjected to physical-chemical cleaning and fractionation processes. The fractions of n-alkanes (saturated) and polycyclic aromatic hydrocarbons (PAHs) were analyzed by gas chromatography coupled with mass spectrometry (GC-MS). For isotopic analysis, the saturated fraction was treated with a urea adduct to isolate linear compounds, which were then analyzed by gas chromatography coupled with isotope ratio mass spectrometry (GC-IRMS). In parallel, oil trajectory simulation was performed using the OpenDrift model, fed with marine current data from the Copernicus Marine Service and wind data from the ERA5 reanalysis. The simulations, conducted on the Google Colab platform with Python scripts, considered physical-chemical properties of oil similar to the collected material (density: 831.50 kg/m3; viscosity: 6.32 kg·m-1·s-1) obtained from the NOAA OilLibrary. The release point was defined at -13° latitude and -38° longitude, corresponding to a maritime route approximately 90 km off the coast of Salvador. The simulations covered the period from September 23 to October 3, 2023, encompassing one week before and after sample collection. Regarding the geochemical results, chromatographic profiles revealed high similarity among the samples from the three beaches, with compatible Pristane/Phytane ratios and hopane/sterane profiles, suggesting a common origin. The n-C17/n-C27 ratios indicated algal contribution to the depositional organic matter, while the n-C18/Ph and n-C17/Pr ratios indicated biodegradation processes. These findings corroborate previous results pointing to oil originating from Kuwait (Middle East). The delta13C isotopic values ranged from -28.06 to -31.18 per mil . The isotopic dataset from the three beaches indicated similar behavior, with delta13C enrichment in the residual fraction, suggesting petroleum of the same origin. The observed 13C enrichment in the current samples, compared to previous records, evidenced that the oil underwent weathering and biodegradation processes. The simulations indicated a predominant southwest trajectory, reaching the entrance of Todos os Santos Bay, which is in good agreement with the location of the oily residues observed on the beaches. It is concluded that the analyzed oil is of foreign origin and has characteristics compatible with an anoxic depositional environment. The combined use of numerical modeling and geochemical analyses proved effective for tracing the origin and trajectory of oil in coastal environments.