RELATIONSHIP BETWEEN THE GEOMORPHIC FEATURES AND SPATIAL ARSENIC DISTRIBUTION PATTERNS IN PART OF MEGHNA FLOODPLAIN, BANGLADESH

This present study investigates a relationship between the identified geomorphic features and the spatial arsenic distribution patterns in lower stretch of the Meghna floodplain (Homna, Gazaria, and Daudkandi upazilas) in Bangladesh using Geographic Information Systems and remote sensing techniques.

Thirteen geomorphic features have been identified within the research area using an ASTER VNIR satellite image, which are: meander ridge, meander swale, older floodplain, new floodplain, new floodplain-1, major channel and rivulet, perennial water body and/or bog, oxbow lake, abandoned channel, chute, new char, point bar, and natural levee. The existing dataset of the study area collected from Bangladesh Arsenic Mitigation Water Supply Project and fieldwork suggested that 74% of the tested wells contain arsenic above the Bangladesh limit of >0.05 mg/L whereas arsenic content is within safe level (≤0.05 mg/L) in rest of the wells. Among the identified features, proportions of safe wells were highest at the ‘new char’ unit (55%) and lowest at the ‘point bar’, and ‘meander swale’ units (0%). The presence of clay-rich bottoms in the ‘meander swale’ aided in water logging, which attenuated percolation and thus the flushing of arsenic. Additionally, the relationship between the arsenic concentration and well depths has also been investigated for every identified unit. Most of the existing wells in the area are clustered at 50-100 feet depth ranges. Arsenic contents are found to be higher at these depth ranges in most of these identified geomorphic features. The safe deeper wells were found at the ‘older floodplain’, ‘natural levee’, and ‘new floodplain’ units. Shallow aquifers contain more arsenic at ‘point bar’, and ‘meander swale’ units. The proportion of safe wells declined with increasing well depths at ‘meander ridge’, and ‘new floodplain-1’ features. Shallow and deeper aquifers have been found safe only at ‘new char’ unit.

This study revealed that sand dominated areas in lower Meghna floodplains have more arsenic-safe wells that may be because of more flushing and dilution of arsenic.