Toxic Metals Build Up in Bangladesh’s Andharmanik River Sanctuary, Crabs Pose Health Risk

Along a 40-kilometer stretch of coastal Bangladesh, where freshwater from the Ganges-Padma system mingles with tidal surges from the Bay of Bengal, lies one of the country’s most treasured fish sanctuaries. The Andharmanik River, declared a protected refuge by the Government of Bangladesh, has long been celebrated for its extraordinary blend of freshwater, brackish and marine ecosystems that support a dazzling diversity of aquatic life. But a new study published in Discover Oceans reveals a troubling underside to this ecological gem: heavy metals from industry, ports and agriculture are quietly accumulating in the river’s sediments and, more alarmingly, in the flesh of the mud crabs that local communities harvest and eat every day.

The research, led by Md. Bokthier Rahman of Patuakhali Science and Technology University together with colleagues from several Bangladeshi and Australian institutions, represents the first comprehensive attempt to quantify heavy metal contamination across the entire Andharmanik ecosystem. Using inductively coupled plasma mass spectrometry, one of the most sensitive analytical techniques available, the team measured eleven metals: arsenic, cadmium, cobalt, chromium, copper, iron, manganese, nickel, lead, selenium and zinc. Samples of sediment, water and mud crab, Scylla serrata, were collected from five stations along the river during two contrasting seasons, the dry winter months of 2023 and the wet pre-monsoon summer of 2024.

The choice of seasons was far from arbitrary. During winter, when rainfall is minimal and river discharge drops, the Andharmanik’s flushing capacity weakens dramatically, allowing contaminants to linger and concentrate. Temperatures of 18 to 22 degrees Celsius prevail, and reduced freshwater inflow means less dilution of dissolved metals. Summer tells the opposite story: higher temperatures of 28 to 34 degrees Celsius and pre-monsoon rains swell the river, diluting dissolved metals and stirring up bottom sediments into resuspension. This seasonal hydrology proved decisive in shaping the study’s central finding: concentrations of nearly every metal were significantly higher in winter than in summer across all three sample types, a pattern the researchers attribute to the dilution effect of greater water volumes during the wet season.

In the sediments, the picture was stark. Arsenic, cobalt, chromium, copper, manganese, nickel and zinc all exceeded recommended guideline values established by the Food and Agriculture Organization, the World Health Organization and the United States Environmental Protection Agency. Iron dominated the metal profile, reaching concentrations as high as 3,028.65 milligrams per kilogram of dry weight in winter, while cadmium registered the lowest values. The overall abundance followed the sequence iron greater than manganese greater than nickel greater than chromium greater than zinc greater than copper greater than cobalt greater than lead greater than selenium greater than arsenic greater than cadmium. Statistical analysis revealed significant differences between stations and seasons for most metals, with station 3 consistently emerging as the most contaminated site in both seasons.

That station sits in a stretch of river surrounded by a dense cluster of pollution sources. The construction of Payra seaport and a power generation plant has transformed Kalapara into a rapidly industrializing area, and the study documents a litany of discharges reaching the water: ash from rice processing industries dumped directly at the riverbank, tar, dye and steel from ship breaking activities, sewage from lavatories installed along the coast, garbage from major fish landing centers at Mohipur, Alipur BFDC and Kuakata, and pesticide-laden agricultural runoff. The researchers also noted that poisonous substances were reportedly being used to catch fish in the Tegachia canal, which connects directly to the river near station 2. Spearman’s correlation analysis showed significant positive correlations among many pairs of metals in both seasons, suggesting they share common origins in these anthropogenic activities rather than in the river’s natural geology.

The water column itself offered a rare note of reassurance. Every metal measured in water samples remained below recommended guideline values, and the calculated Risk Index ranged from just 3.77 to 10.45, indicating that the river water does not currently pose a significant ecological threat. The sediment Risk Index, spanning 32.27 to 85.77, likewise fell within the low ecological risk category under the classification framework developed by Hakanson. In other words, the river’s open waters and its bed sediments, while showing concerning exceedances of some guidelines, have not yet crossed the threshold of serious ecological harm. The story changes dramatically, however, when the researchers turned their attention to the crabs.

Mud crabs are ideal sentinels of contamination because they live in close contact with sediments, the repository where metals concentrate, and they bioaccumulate a wide range of chemical contaminants in their tissues. They are also a dietary staple in coastal Bangladesh and a valuable export commodity: in the 2023-24 fiscal year alone, the country shipped roughly 10,782 metric tons of crab abroad. When the team analyzed crab muscle tissue, they found that arsenic, cadmium, chromium, copper, iron, lead and zinc all exceeded recommended limits set by Bangladeshi, European Union, United States and international food safety authorities. Iron again led the accumulation profile, followed by zinc and copper, with concentrations of iron reaching nearly 288 milligrams per kilogram of dry weight.

The health risk calculations are where the findings turn genuinely alarming. Using standard formulas that incorporate consumption rates of 49.5 grams per day and body weights of 70 kilograms for adults and 35 kilograms for children, the team computed target hazard quotients for each metal. For adults, all individual metals fell below the safety threshold of 1 except arsenic. For children, both arsenic and iron exceeded the limit. More critically, the total target hazard quotient, which sums the risks across all eleven metals, surpassed the acceptable value of 1 for both age groups, indicating medium to high potential non-carcinogenic health hazards from regular crab consumption. The carcinogenic risk assessment delivered an equally sobering verdict: while adults remained within the acceptable range of 10 to the power of minus 6 to 10 to the power of minus 4, the values for chromium and lead in children exceeded the safe threshold, suggesting that prolonged consumption of contaminated crab could pose a significant cancer risk for young consumers.

These results place the Andharmanik within a wider and worrying pattern across South Asian waterways. Comparable studies on the Rupsa, Dhaleshwari, Halda and upper Meghna rivers in Bangladesh, and on mud crab populations in India, Malaysia and Indonesia, have repeatedly found that benthic species accumulate higher metal loads than fish swimming in mid or upper water layers, and that rapidly industrializing coastlines are the dominant drivers. What distinguishes the new study is its completeness: by sampling water, sediment and crab simultaneously across seasons, it traces the full pathway by which industrial effluents enter a river, settle into its bed, and climb the food chain into human diets. The paradox is painful. The very features that made the Andharmanik a sanctuary, its sheltered depositional zones and productive mixing of fresh and salt water, are the same features that trap and concentrate its pollutants.

The researchers argue that the window for action remains open but is narrowing. Because the water and sediments have not yet reached high ecological risk levels, aggressive monitoring and enforcement of environmental laws could still prevent the river from sliding past the point of no return. They call on the relevant authorities to identify and control effluent sources, and they urge future research into bioaccumulation in fish and into stricter quality assurance protocols for contamination monitoring. For the communities who depend on the Andharmanik’s fisheries, and for the children who eat its crabs, the study’s message is unambiguous: a sanctuary on paper cannot protect an ecosystem that the law fails to shield in practice, and every season of unchecked discharge makes the eventual cleanup harder, costlier and less certain to succeed.

Subject of Research: Seasonal heavy metal contamination in the water, sediment and mud crabs of the Andharmanik River fish sanctuary in coastal Bangladesh and its associated human health risks

Article Title: Seasonal variations of heavy metals contamination in sediment, water and crab at a vital fish sanctuary connected to the Bay of Bengal and health risk assessment

Article References: Rahman, M. B., Smriti, S., Hussain, M., Yasmin, F., Begum, M., Ullah, H., & Rashid, H. (2026). Seasonal variations of heavy metals contamination in sediment, water and crab at a vital fish sanctuary connected to the Bay of Bengal and health risk assessment. Discover Oceans, 3(1), Article 34. https://doi.org/10.1007/s44289-026-00145-1

Image Credits: AI Generated

DOI: 10.1007/s44289-026-00145-1

Keywords: heavy metals, Andharmanik River, Bay of Bengal, fish sanctuary, sediment contamination, mud crab, Scylla serrata, health risk assessment, carcinogenic risk, ICP-MS, seasonal variation, Bangladesh