Abdul Wahab Khan
Panaji
Casino Point, Goa’s busiest floating casino hub in the capital city, has emerged as one of the state’s most persistent estuarine pollution hotspots, alongside Betim, Old Goa, Campal, Panaji, Dona Paula, Miramar and Vainguinim, according to a study that has revealed how seasonal pollution repeatedly overwhelms the Mandovi-Zuari estuary’s natural carrying capacity.
While the southwest monsoon temporarily restores water quality by flushing contaminants into the Arabian Sea, researchers have found that pollution quickly returns during drier months, leaving several urban and tourism centres vulnerable to deteriorating water quality, microbial contamination and oxygen depletion.
The study has said “monsoonal flushing plays a critical role in improving water quality by enhancing oxygenation and diluting pollutants,” warning that “persistent pollution hotspots necessitate site-specific interventions”.
The research, titled ‘monsoon-driven variability in water quality index and microbial pollution influencing the carrying capacity of the Mandovi-Zuari estuarine system, Goa’, was carried out by Ashutosh Shankar Parab, Mayukhmita Ghose and Cathrine Sumathi Manohar of the biological oceanography division, CSIR-National Institute of Oceanography (NIO), Dona Paula, and S Neetu of the physical oceanography division, CSIR-NIO.
The study was published in the peer-reviewed journal Estuaries and Coasts on July 23, 2025. The scientists investigated how seasonal monsoon cycles regulate physicochemical properties, microbial pollution and the estuary’s overall carrying capacity across 14 locations: Sinquerim, Coco beach, Reis Magos, Campal, Casino Point, Panaji, Betim, Old Goa, Miramar, Caranzalem, Dona Paula, Dias beach, Vainguinim and Odxel.
Surface water samples were collected during intermonsoon (January–March), premonsoon (April–May), monsoon (June–September), and postmonsoon (October–December) seasons in 2018 following standard seawater protocols.
The seasonal variability in physicochemical parameters clearly demonstrated the influence of the southwest monsoon. Premonsoon recorded the highest salinity, exceeding 35 PSU at Sinquerim, Coco beach, Vainguinim and Caranzalem, whereas monsoon freshwater inflow reduced salinity dramatically to just 9 PSU at Old Goa and around 16-17 PSU at Panaji and Betim.
Surface water temperatures declined by 3-4°C during monsoon, while pH dropped to its lowest values at Panaji and Odxel due to freshwater influence. Dissolved oxygen showed the most dramatic seasonal improvement. During monsoon every station exceeded the healthy threshold of 5 mg/L, reaching as high as 12 mg/L, whereas Caranzalem and Vainguinim recorded critically low oxygen during premonsoon and Casino Point remained below the recommended limit even during postmonsoon. Temperature, salinity and pH all showed highly significant seasonal differences (Kruskal-Wallis, p<0.0001), while dissolved oxygen also varied significantly across seasons (ANOVA, p<0.0001).
Microbial pollution revealed an equally concerning pattern. During intermonsoon, Panaji recorded the highest total coliform concentration of 16,000 MPN/100 mL, followed by Old Goa, Betim and Coco beach at 9,200 MPN/100 mL, far exceeding the CPCB limit of 500 MPN/100 mL. Premonsoon saw persistent contamination at Coco beach, Betim, Casino Point and Old Goa despite some reduction.
During monsoon, Betim and Old Goa again experienced major microbial surges, while Campal, Miramar and Dias beach also crossed 1,000 MPN/100 mL. Postmonsoon brought substantial improvement, although Vainguinim and Miramar continued recording elevated bacterial loads. Faecal coliforms painted a similar picture. Intermonsoon contamination peaked at Panaji, Caranzalem and Casino Point, while premonsoon showed the highest counts at Betim, followed by Casino Point and Miramar. During monsoon, Sinquerim, Betim, Old Goa and Dona Paula became the most contaminated locations before bacterial levels declined after the rains. The seasonal differences for total coliforms (H=14.51; p=0.0023) and faecal coliforms (H=10.49; p=0.0148) were statistically significant, confirming rainfall-driven microbial transport.
The Water Quality Index analysis showed Casino Point and Coco beach slipping into the “poor” category during intermonsoon, while Betim, Campal and Old Goa recorded further deterioration during premonsoon. Monsoon transformed the estuary, with Panaji, Caranzalem and Dias beach achieving “excellent” water quality, although Old Goa and Dona Paula remained localised hotspots. Postmonsoon conditions improved but Vainguinim still recorded a WQI of 110.2, remaining within the poor category. Seasonal differences in WQI were statistically significant, with researchers identifying premonsoon as the period of maximum environmental stress.
