Revisiting the Contamination Status of Polycyclic Aromatic Hydrocarbons in the Sediments of the Cochin Estuary, Southwest Coast of India, After a Decade

Estuaries continuously accumulate contaminants in sediments due to increased anthropogenic activities and land drainage. Cochin estuary underwent unusual environmental control when exposed to the catastrophic Kerala floods in 2018, followed by a substantial reduction in industrial activities due to the countrywide COVID-19 lockdown (2020 and 2021). Large-scale flushing of the estuary during the flood and reduced pollutant input during the pandemic lockdowns would lead to significantly lower polycyclic aromatic hydrocarbons (PAHs) levels in estuarine sediments and biota. To test the above hypothesis, PAHs in sediments of the Cochin estuary were analysed on a seasonal basis (pre-monsoon, monsoon and post-monsoon, in 2023) and compared with corresponding levels reported a decade earlier by Ramzi et al., Marine Pollution Bulletin 114:1081–1087, 2017. The distribution of total PAHs (TPAHs) during 2023 revealed significant seasonal heterogeneity characterised by high concentrations during post-monsoon (428.19 to 2214.02 ng/g), monsoon (2.23 to 2254.89 ng/g) and pre-monsoon periods (48.467 to 1343.59 ng/g). PAH concentrations were five times lower than the baseline data (304–5874 ng/g pre-monsoon, 493–14,149 ng/g monsoon, and 194–10,691 ng/g post-monsoon). The substantial decrease in PAHs reflected significantly reduced PAH input into the estuary due to the large-scale flushing of the estuary caused by the unprecedented 2018 Kerala floods, limited anthropogenic activities during the 2020/2021 lockdowns, consequent to regulatory restrictions imposed by the local administration. However, low molecular weight (LMW) PAHs, during 2023, still exceeded the Effective Range Low (ERL) values, indicating an occasional adverse impact on the benthic fauna. PAH levels from the mussel species Mytella strigata (M. Strigata) were considerably low (371 ng/g), indicating minimum bioaccumulation. Risk assessments of dietary intake from mussels indicate no threats to regular consumption.

Graphical Abstract