The coastal lake Veli-Akkulam is crucial to Thiruvananthapuram's urban watershed, yet it is under threat from pollution caused by urbanization and industrialization. Conserving freshwater resources is a key part of Sustainable Development Goals, and the only way to protect the lake is by consistently monitoring its water quality. This study aims at evaluating the pollution level of the lake throughout the seasons (pre-monsoon, monsoon, and post-monsoon) of 2022–2023 using the Carlson Trophic State and Water Quality indices. The lake's pollution condition using the indexes Carlson Trophic State (CTSI) and Water Quality Index (WQI) throughout the pre-monsoon, monsoon, and post-monsoon seasons in 2022–2023. The statistical tools such as Principal Component Analysis (PCA) and Pearson Correlation Matrix were used for analysing how various factors influence water quality. The impact of several factors on water quality was examined using Principal Component Analysis and the Pearson Correlation Matrix. PCA analysis revealed that high nutrient levels, suspended particles, and oxygen depletion were identified as the main drivers of the decline in water quality, and these factors were all linked to anthropogenic activity. The study's findings clearly show the Veli-Akkulam lake is severely polluted. The CTSI consistently classified the lacustrine system as eutrophic or hypereutrophic across all seasons, with mean values of 66.74 (pre-monsoon), 67.20 (monsoon), and 63.23 (post-monsoon). Similarly, the WQI classified the water quality as "poor," "very poor," and "unsuitable," with particularly high "unsuitable" values (306.6 to 398.47) recorded at multiple stations. The study concludes that human activity is the clear cause of the lake's current polluted and deteriorating state.