The explosion of the Deepwater Horizon (DWH) oil platform in April 2010 on the Gulf of Mexico (GOM) released more than 700 million liters of crude oil. This catastrophe is the largest oil spill in US history and one of the largest in the world that resulted in over 1000 km of oiled shoreline across four different states (FL, AL, MS, and LA). When the floating emulsified oil approached sandy beaches, a portion of the emulsified oil interacted with suspended sediments and sank, forming submerged oil mats (tarmats). Since their formation, hydrodynamic processes fragmented the tarmats to form tarballs. The tarballs are highly mobile in the nearshore environment and can be found on GOM beaches to date. The objective of this proposal is to complete a comprehensive long-term chemical characterization study to investigate the fate of petroleum biomarker compounds used for chemical-fingerprinting of the tarballs collected at different times over the past 10 years along the Alabama shoreline. Petroleum biomarkers are geochemical organic compounds present in crude oils that can be related to their unique biological precursors. Currently, the most common method used for identifying and fingerprinting the oil spill source is characterizing these biomarkers. Hence, the long-term fate of three groups of biomarker compounds (terpanes, steranes, and triaromatic steranes) present in the tarballs collected from the Alabama shoreline is investigated for using them for fingerprinting purposes. There have been contradicting literature data on the degradation levels of these biomarkers, and this field-scale characterization study demonstrates that most of the investigated biomarkers remain recalcitrant after 10 years of natural weathering. We also quantified the changes in the concentration levels of toxic polycyclic aromatic hydrocarbons (PAHs) present in these tarballs, and the results show that many of these PAHs are recalcitrant. These results are consistent with previous studies that indicated that certain heavy toxic and carcinogenic PAHs, such as chrysene, can be recalcitrant and remain in the tarballs for a long time. Therefore, it is important to track the levels of these recalcitrant PAHs in the tarballs to determine their potential toxicity effects.