Occurrence and Distribution of Low Molecular Weight Polycyclic Aromatic Hydrocarbons (LMW PAHs) in Caño La Malaria, Cucharillas Marsh (Cataño, Puerto Rico): Spatial and Seasonal Variability, Environmental Implications, and Ecological Risk Assessment

Polycyclic aromatic hydrocarbons (PAHs) are persistent organic pollutants with significant ecological and public health implications, particularly in urban wetlands exposed to chronic anthropogenic stress. This study evaluates the occurrence, spatial distribution, seasonal variability, and ecological risk of three low molecular weight PAHs (naphthalene, phenanthrene, and anthracene) in surface waters of Caño La Malaria, the main freshwater source of Cucharillas Marsh, Puerto Rico’s largest urban wetland. Samples were collected at four locations across multiple seasons. Liquid-liquid extraction, rotary evaporation, and GC-MS were used for quantification. naphthalene was the dominant compound, with total PAH concentrations ranging from 7.4 to 2,198.8 ng/L, and most wet season samples falling within the mild-to-moderate contamination range. PAH levels were significantly higher in the wet season (mean = 4,434.66 ng/L) than in the dry season (mean = 1,245.08 ng/L), confirmed by Friedman test (p < 0.000001). Compositional shifts indicated increased levels of phenanthrene and anthracene during the wet season. No significant spatial differences were found (p = 0.753), and high correlations between sites (r = 0.96) suggest uniform input sources. Diagnostic ratios, inter-species correlations, and principal component analysis (PCA), all support a predominant pyrogenic origin. High correlations between phenanthrene and anthracene (r = 0.824) further suggest shared combustion-related sources. PCA revealed a clear separation between dry- and wet-season samples, with the latter showing greater variability and stronger associations with naphthalene and anthracene. Ecological risk assessment using hazard quotients (HQ) indicated insignificant acute toxicity risk across all sites and seasons, with all HQ values <0.01. The highest HQ (0.0095) occurred upstream during the wet season, approaching the threshold for low ecological risk. These findings support integrated watershed management practices to control PAH pollution and ensure long-term ecological health in this tropical urban wetland.