Heavy Metals in Dhaka's Urban Environment — What Active Research Shows

When you open a sample vial from a site near the Buriganga river and the sediment is visibly dark — charcoal-coloured with a faint chemical smell — you are looking at something that has absorbed decades of industrial discharge. It is not unusual for Dhaka samples to produce readings that require re-calibration checks: not because the instrument is wrong, but because the concentrations are genuinely that far above typical background values.

I am working on heavy metal analysis as part of ongoing research at BCSIR. This post is not a presentation of final results — those will appear in peer-reviewed form in due course. It is a researcher's account of the context: what the existing literature documents about heavy metal contamination in Dhaka's urban environment, what the sources are, how contamination is measured, and why this particular corner of environmental science matters beyond the laboratory.

Why Dhaka, and Why Now

Dhaka is one of the most densely populated cities on earth, with an estimated 22 to 23 million people in the greater metropolitan area. Its industrial base expanded rapidly through the 1970s and 1980s — tanneries, textile dyeing and processing units, battery manufacturing, steel re-rolling mills, and small-scale foundries — often without the effluent treatment infrastructure that would later become standard in high-income countries. The Buriganga, Turag, Balu, and Shitalakhya rivers absorbed much of what the industrial sector produced.

English-language scientific literature on Dhaka's heavy metal contamination is sparse relative to the size of the problem. Most published work appears in Bangladeshi and regional journals with limited indexing, and the most frequently cited studies are now more than a decade old. This creates a gap that is genuinely worth filling: anyone searching "heavy metals Dhaka soil" or "Buriganga contamination research" will find relatively little that is current, accessible, and written from an active research perspective.

The Sources: Where the Metals Come From

Heavy metal contamination in urban Dhaka comes from several distinct industrial and infrastructural sources. Understanding which metals to expect, and where, requires knowing the chemistry of each source.

Tanneries — Chromium and Lead

The Hazaribagh tannery district operated for decades as the centre of Bangladesh's leather processing industry, discharging liquid effluent containing hexavalent chromium (Cr[VI]), a compound used in the chrome-tanning process, directly into the Buriganga. The tanneries were officially relocated to Savar beginning in 2017, but the sediment legacy in Hazaribagh-adjacent waterways is long-lived. Chromium and lead are consistently the metals reported at highest concentrations in studies sampling sediments from the Buriganga between Hazaribagh and downstream sites. Studies including Hasan and colleagues (2019) and Islam and colleagues (2015) found total chromium concentrations in Buriganga sediments ranging from several hundred to over 2000 mg/kg in impacted zones — values that place the sediments in hazardous waste territory by most international reference standards.

Textile and Dyeing Units — Cadmium, Zinc, Copper

Textile processing uses a range of metal-based dyes and mordants. Cadmium, copper, and zinc compounds appear in effluent from dyeing operations, particularly around the Turag and Balu rivers where garment-sector industrial zones concentrated. Zinc, while an essential micronutrient, becomes toxic in excess and is persistent in sediment. Cadmium is particularly concerning: it bioaccumulates in kidney tissue, is classified as a human carcinogen, and has no known biological function. Published values for cadmium in Turag sediments from several studies have exceeded WHO sediment quality guideline values by factors of five to ten.

Battery Recycling and Vehicle Emissions — Lead

Unregulated lead-acid battery recycling, concentrated in several areas of old Dhaka and the urban periphery, is a major source of atmospheric and soil lead. The smelting of battery plates releases lead fumes that deposit within a radius of several kilometres from the facility. Vehicle emissions from the pre-2000 era of leaded gasoline also left a legacy of lead in roadside soils that is documented in studies across South Asian cities. Dhaka was among the last major cities to complete the transition to unleaded fuel, and roadside soil lead concentrations remain elevated compared to background in many parts of the city.

E-waste and Informal Electronics Recycling

The informal dismantling of electronic waste introduces a cocktail of metals — lead, cadmium, mercury, nickel, and copper — into local soils and drainage channels. This source is growing as electronic consumption increases faster than formal collection and recycling capacity.

The Metals: Key Properties and Why They Matter

A note on arsenic: the well-documented arsenic crisis in Bangladesh's groundwater is largely a geogenic phenomenon — naturally occurring arsenic in alluvial aquifer sediments that entered tube-well water when deep wells were drilled to avoid surface water contamination. Urban arsenic contamination from industrial sources is a distinct (if related) issue and requires separate treatment. Both matter; they have different spatial distributions and require different remediation approaches.

How Researchers Measure Contamination

The analytical workflow for heavy metal research in soil and sediment follows a standard sequence, but there is more method-dependence in the results than is sometimes appreciated in discussions of published data.

Sample Digestion

Soil and sediment samples are dried, sieved to remove coarse material, and digested in concentrated acid to release metals from the soil matrix into solution. The most common procedure uses aqua regia, a mixture of concentrated nitric acid and hydrochloric acid, which dissolves most metal fractions. Some laboratories use microwave-assisted digestion for more complete extraction and better reproducibility. The digestion method matters because "total metal content" in the soil is not the same as "plant-available" or "leachable" fractions — but aqua regia digestion is the standard for contamination assessment comparisons.

Analytical Instrumentation

The resulting solution is analysed using atomic absorption spectrometry (AAS) or inductively coupled plasma optical emission spectrometry (ICP-OES). ICP-OES allows simultaneous multi-element analysis, which is more efficient when screening for many metals at once. BCSIR laboratories are equipped with ICP-OES, which is part of why multi-metal screening studies are feasible. Detection limits for most heavy metals of concern in this work are in the range of 0.01 to 0.1 mg/kg in the digested extract, well below the concentrations typically found in impacted Dhaka samples.

Pollution Indices

Raw concentration data are contextualised using indices that compare measured values against reference background concentrations — either local pre-industrial background values or globally accepted shale values. Commonly used indices include the geo-accumulation index (I-geo), the contamination factor (CF), and the pollution load index (PLI) calculated across multiple metals. These indices convert raw numbers into comparative categories (uncontaminated, moderately contaminated, heavily contaminated, extremely contaminated) that allow researchers to communicate the severity of contamination to non-specialist audiences and to compare results across studies.

What Existing Studies Document

A synthesis of published studies on Dhaka's urban environment produces a picture that is consistent in its broad outlines, even where individual values vary depending on sampling location, depth, season, and method. Three themes recur.

Hotspots near industrial discharge points. The highest concentrations are found at or near historical discharge points: Hazaribagh-adjacent Buriganga sites for chromium and lead, textile-zone Turag and Balu sites for cadmium and zinc. Concentrations decline with distance from these points but do not return to background values within the Dhaka metropolitan boundary in most studies. The spatial pattern is well-documented; what requires updating is whether the pattern has shifted since the tannery relocation and what remains in legacy sediments.

Floodplain agricultural soils carry the contamination inland. The rivers that receive industrial discharge also irrigate agricultural areas during drier months. Leafy vegetables grown on low-lying land adjacent to the Buriganga and Turag have been documented with heavy metal accumulations — particularly cadmium and lead — above both soil thresholds and food safety guidelines in multiple studies. This pathway from industrial effluent to the food chain via irrigation water and soil uptake is the most direct public health concern in the literature.

Roadside and peri-urban soils show a lead and zinc signature distinct from river sources. Studies sampling soils along major Dhaka thoroughfares consistently find elevated lead and zinc, reflecting vehicle emission legacies and tyre wear rather than direct industrial discharge. The spatial gradient is steepest within the first 5 to 10 metres from the road surface. Children playing on or near roadside soils face the highest exposure risk through ingestion and inhalation of resuspended particles.

The Research Gap This Work Tries to Fill

Most published heavy metal studies on Dhaka draw on samples collected before 2018 and analyse only one environmental matrix — river sediment, or urban soil, or vegetable tissue — in isolation. What is missing is integrated, multi-matrix, multi-site analysis that can track contamination from industrial source through the soil-water-plant pathway, with sufficient spatial coverage to identify which parts of the metropolitan area have changed since the tannery relocation and which have not.

That is the focus of the ongoing work at BCSIR: not simply confirming that contamination exists, which is well-established, but generating current data with enough site coverage and methodological consistency to detect change and to identify priority remediation areas. Some of what we are finding is surprising in terms of which sites show continued elevation despite the relocation. That will be documented properly in the published outputs.

What Remediation Looks Like

For heavily contaminated river sediments, practical remediation is constrained by scale. The volume of contaminated material in a major urban river system is enormous, and options like dredging and containment — while feasible for point sources — cannot address diffuse contamination across tens of kilometres of waterway. The more tractable interventions operate at a different scale: phytoremediation using metal-accumulating plant species in contaminated agricultural soils, immobilisation treatments that reduce the bioavailable fraction of metals in soil without removing them, and source control at the industrial and waste management level.

Source control is the most important and the least glamorous option. Every year that untreated industrial effluent continues to enter the river system adds to a contamination legacy that will persist for decades after the source is stopped. The tannery relocation is a step in that direction; whether effluent treatment at the new Savar site is operating at design capacity is a separate question that researchers are actively examining.

If you are working on research that intersects with this area — urban soil contamination, environmental chemistry, or related public health questions — I am reachable through the contact page. Collaborative work and data-sharing across institutions are how the English-language gap in this field gets closed.