Fossil fuels are of utmost importance to the world we live in today. However, their use can cause major impacts on the environment, especially on water resources. In this regard, algae have been intensively used as a strategy for remediation and monitoring of environmental pollution due to its efficient absorption of contaminants. In this work, samples of seaweed collected in Niterói/RJ—contaminated with kerosene and diesel—were analyzed by radiocarbon (14C) accelerator mass spectrometry (AMS) and by n-alkane quantification with gas chromatography to evaluate bioaccumulation in function of the dosage of contaminants. The biogenic content measured by radiocarbon analysis resulted in 95.6% for algae contaminated with 10 mL of kerosene and 67.6% for algae contaminated with 10 mL of diesel. The maximum intensity of n-C17 n-alkane in algae with 5 mL, 10 mL, and 15 mL of diesel was 768.2, 1878.1, and 5699.2 ng.g-1, respectively. While the maximum concentration of n-C27 in algae with 5 mL, 10 mL and 15 mL of kerosene was 3.3, 35.9, and 150.3 ng.g-1. We concluded that, for both contaminants, their incorporation into algae increases as the contamination dosage increases, making this methodology an effective technique for monitoring and remediation of urban aquatic ecosystems.