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The peripheral rivers of Dhaka are highly polluted through untreated industrial effluents and municipal discharges with limited regulatory oversight. This pollution becomes a multi-cascade phenomenon through inundation of floodplains exposing soil and crops to persistent pollutants like heavy metals that spread from one system to another and accumulate in a cascading manner, putting the livelihood, health, and well-being of the people at risk. A systematic study on understanding multi-cascade pollution and exposure pathways is lacking overall. The current study addressed this issue by focusing on the floodplains of the Tongi Khal, a highly polluted peripheral river of Dhaka, in the Uttarkhan area in northern Dhaka.
Field sampling and laboratory analysis were employed for pollution assessment of water, soil, and plants. Samples included eleven water samples from rivers, floodplains and ponds, fifteen soil samples from the floodplains, and forty-nine plant samples. Analysis included the heavy metals As, Cd, Cr, Pb, Co, Fe, Mn, Mo, Ni, Cu, and Zn, along with general ambient water quality parameters. The pollution in different cascades was evaluated through indices and potential health risks from consuming cultivated crops estimated. Human exposure pathway was explored using PRA tools and health impact was assessed with secondary data from survey.
The water quality was poor, with near zero dissolved oxygen and elevated concentrations of Cd, As, and Pb observed in rivers, floodplains, and ponds, rendering them unsuitable for sustaining aquatic ecosystems and subsistence use. Cd concentration ranged as high as 120 times the standard value. Pollution indices showed a ‘moderately polluted’ (1<CPI<2) state in monsoon to a ‘severely polluted’ (CPI>2) state in the dry period, where the largest CPI value of 5.8 was observed in floodplains close to the river. High concentrations of free ions of Cd, Pb, and Ni pose carcinogenic risks, while increased organometallic complexes in the dry season promote plant uptake, causing accumulation in plant cells and compounding the health risk.
Pollution load index categorized all the soil samples as polluted from heavy metals and concentrations of metals were correlated with organic enrichment. The enrichment factor revealed a strong human influence on metal pollution in soil with a very high degree of Cd pollution falling into the ‘severe’ to ‘extremely severe’ category with EF for value from 6.45 to 28.55. Prolonged inundation promotes metals availability to and uptake by plants, especially the ones with short roots, like vegetables.
As, Cd, Cr and Pb pollution were most prominent in crops especially in edible roots. Bio-concentration factor (BCF) shows high uptake of As and Pb for cattle food and vegetables. Water hyacinth showing highest metal accumulation poses risks to cattle but can be used for phytoremediation of polluted soil. As in vegetables pose the highest non-carcinogenic health risk followed by Cr. Health risk index (HRI) values for leafy and non-leafy vegetables were 3.11 and 11.12, respectively showing much higher disease potential from non-leafy vegetables. As is the most dominant carcinogen in the study area, followed by Cr and Cd. The carcinogenic risk from all potent carcinogenic metals is in the ‘moderate’ category (>10-3).
The major exposure pathways include dermal contact during subsistence water use, direct ingestion through consumption of vegetables grown on the floodplains, and indirect ingestion through consuming cattle, fisheries and ducks exposed to the pollutants. Cattle feeding on highly contaminated hyacinth, straw and water lettuce poses a serious human health risk. Awareness on metals pollution risks is lacking. Higher disease incidences than control site was observed at the site for people exposed to polluted water. Headache, skin diseases, hypertension, etc. were the major illnesses where skin diseases were common among the people actively involved in cultivation, fishing, bathing or washing in the river. Awareness building together with targeted interventions for pollution abatement and clean-up are essential to safeguard the health and well-being of the local people. |
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