Groundbreaking Discovery: Carcinogenic PAHs Accumulate in Common Wetland Weed

A recent study from Jadavpur University has uncovered alarming levels of cancer-causing polycyclic aromatic hydrocarbons (PAHs) in the common weed Alternanthera ficoidea from the East Kolkata Wetland (EKW). This Ramsar-designated site, vital for natural wastewater treatment and supporting over 100,000 livelihoods through fisheries and agriculture, now faces heightened scrutiny due to pollutant bioaccumulation in its flora. Researchers Aparna Dhara and Ratna Dutta from the Department of Chemical Engineering analyzed plant parts and sediments, revealing concentrations up to 420 micrograms per gram in leaves from polluted sites—nearly ten times higher than control areas. This finding underscores the urgent need for enhanced monitoring in urban wetlands.

What Are Polycyclic Aromatic Hydrocarbons (PAHs)?

Polycyclic aromatic hydrocarbons (PAHs) are a group of organic compounds with two or more fused aromatic rings, formed primarily through incomplete combustion of organic materials. Common sources include vehicle exhaust, industrial emissions, coal combustion, and biomass burning. Of the 16 priority PAHs identified by the US Environmental Protection Agency (USEPA), several—such as benzo[a]pyrene (BaP), benzo[b]fluoranthene, and dibenz[a,h]anthracene—are classified as probable human carcinogens by the International Agency for Research on Cancer (IARC). In the environment, PAHs persist due to their hydrophobic nature, binding to sediments and bioaccumulating in organisms via food chains.

Step-by-step, PAH exposure occurs through inhalation of airborne particles, dermal contact, or ingestion of contaminated food and water. Once absorbed, they metabolize into reactive epoxides that damage DNA, leading to mutations and cancer risks, particularly lung, skin, and bladder cancers. In India, where air pollution contributes significantly, PAH-related health burdens are substantial, with studies linking them to elevated cancer incidences in urban areas.

The Ecological and Cultural Significance of East Kolkata Wetlands

Spanning 12,500 hectares, the East Kolkata Wetland (EKW) is the world's largest natural sewage treatment system, processing about 75% of Kolkata's municipal and industrial wastewater through constructed bheris (ponds). Designated a Ramsar site in 2002, it sustains pisciculture yielding 20,000 tons of fish annually for city markets and irrigated agriculture on 100 hectares. However, rapid urbanization, encroachment, and pollution from the adjacent Eastern Metropolitan Bypass threaten this fragile ecosystem.

Local communities rely on EKW for livelihoods, but bioaccumulation of toxins in fish and vegetables poses health risks. Recent studies highlight heavy metals and now PAHs in sediments, amplifying concerns for the 140,000 residents dependent on these resources.

Jadavpur University's Innovative Approach: Study Methodology

Chemical engineering researchers at Jadavpur University selected Captain Bheri (CB), a heavily polluted pond near the highway, as the study site, contrasting it with the relatively pristine Kansabati River Basin (KRB). Sediment and A. ficoidea samples—leaves, stems, roots, and rhizobium—were collected, extracted using optimized methods for 16 USEPA PAHs, and analyzed via gas chromatography-mass spectrometry (GC-MS).

• Sediment PAH quantification compared CB vs. KRB.
• Plant bioaccumulation assessed via bioconcentration factor (BCF) and translocation factor (TF).
• Air pollution tolerance index (APTI) calculated for plant resilience.
• Source apportionment using diagnostic ratios (e.g., Fla/Py, BaA/CHR).
• Carcinogenic risk via BaP-equivalent toxicity (TEQ).

This rigorous, multi-faceted methodology provides a baseline for future PAH monitoring in Indian wetlands.

Shocking Findings: PAH Levels and Bioaccumulation Patterns

Sediments at CB showed fourfold higher total PAHs compared to KRB, dominated by high molecular weight (HMW) compounds. In A. ficoidea, leaves accumulated the highest (420.23 ± 0.60 μg/g at CB), followed by stems (230.83 ± 4.87 μg/g), rhizobium (84.42 ± 7.42 μg/g), and roots (68.45 ± 7.63 μg/g)—confirming atmospheric deposition as a primary uptake route due to high BCF and TF values.

Lipid content strongly correlated with PAH uptake (r > 0.99), highlighting the weed's efficiency as a bioindicator.

Carcinogenic Risks Exceed Safe Limits

The BaP-equivalent toxicity in CB sediments surpassed Canadian guidelines, indicating significant lifetime cancer risk for exposed populations. HMW PAHs, more persistent and toxic, dominated, posing threats via food chain magnification to fish and humans consuming EKW produce. In India, PAH exposure contributes to 1-2% of lung cancers annually, exacerbated in polluted urban wetlands like EKW. While direct weed consumption is low, grazing livestock or indirect transfer amplifies concerns.

Learn more about the study in the full paper: Marine Pollution Bulletin.

Alternanthera ficoidea: A Tolerant Bioaccumulator with Remediation Potential

With APTI >17, A. ficoidea thrives amid pollution, making it ideal for passive biomonitoring. Its hyperaccumulation suggests phytoremediation viability—plants absorb, translocate, and stabilize PAHs, preventing leaching. Similar studies show Alternanthera spp. effective for heavy metals; extending to PAHs could restore EKW sustainably.

• High BCF: Air-to-leaf uptake dominant.
• TF >1: Efficient root-to-shoot transfer.
• Pros: Cost-effective, eco-friendly vs. chemical cleanup.
• Challenges: Harvesting/disposal to avoid re-release.

For aspiring researchers, explore research jobs in phytoremediation at Indian universities.

Pollution Sources Threatening EKW

Diagnostic ratios pinpoint mixed origins: pyrogenic (60-70%) from traffic exhaust along the bypass, biomass/coal combustion in nearby slums, and petrogenic from oil spills/leakages. Kolkata's traffic (over 2 million vehicles) and industrial hubs amplify deposition into bheris. Seasonal monsoons mobilize PAHs from roads to wetlands.

Health and Ecosystem Implications for Kolkata Communities

EKW fish and vegetables reach 80% of city markets; PAH transfer risks chronic exposure, especially to low-income fishers. Ecosystem-wide, bioaccumulation disrupts aquatic food webs, reducing biodiversity. Recent EKW studies note rising heavy metals alongside PAHs, compounding toxicity.

Nature's coverage highlights the urgency: Carcinogenic compounds in common weed.

Wetland Pollution Challenges Across India

India's 49 Ramsar sites face similar threats; e.g., high PAHs in Chilika Lake sediments from agriculture runoff. Urban expansion pollutes 70% of wetlands, per recent MoEFCC reports. Jadavpur's work sets a model for nationwide biomonitoring protocols.

More on Indian higher ed env research: STEM initiatives.

Photo by Nadia Amad on Unsplash

Pathways Forward: Policy, Remediation, and Research Opportunities

Recommendations include regular PAH screening using A. ficoidea, stricter bypass emissions controls, and phytoremediation pilots. EKW Management Authority could integrate biomonitoring into 2021-2026 Action Plan. For students, Jadavpur exemplifies careers in environmental engineering—check Kolkata higher ed jobs and career advice.

In conclusion, this study spotlights actionable science. Explore professor ratings at Rate My Professor, pursue higher ed jobs, or advance your path with career advice and university jobs.