GEOMORPHOLOGICAL CONTROL ON SPATIAL CORRELATION BETWEEN GROUNDWATER ARSENIC AND OTHER IONS IN ALLUVIAL AQUIFERS OF BANGLADESH

Elevated arsenic in alluvial aquifers of Bangladesh is considered as one of the worst cases of naturally occurring groundwater contamination in the world. It has been estimated that nearly 35 million people are now exposed to high arsenic in groundwater. Reductive dissolution of Fe- oxyhydroxides has so far been the widely accepted mechanism of widespread arsenic occurrences in Bangladesh. Spatial variability of arsenic and its correlation with other dissolved ions in groundwater are significantly controlled by geomorphic distribution of groundwater aquifers, surface elevation and subsidence due to rapid sedimentation and tectonism.

Multivariate statistics and geostatistical methods are applied on groundwater database from National Hydrochemical Survey of Bangladesh in 12 different geomorphic divisions. In most geomorphic divisions, dissolved Fe and P are found to be the most correlative ions with As. However, dissolved K, Mg, Ca, Ba, and also Mn show moderate to strong correlation with As. Maximum correlation coefficient between As and Fe is found about 0.784 in the Alluvial Sand. Maximum (0.780) correlation between As and P is also found in the same geologic unit, where mean As concentration is 66 µg/L. In Chandina Alluvium, where As is very high in groundwater, correlation between As and P is very strong, but correlation between As and Fe is relatively low. In contrast, correlations between As and Fe and As and P in the Barind and Madhupur Tracts are very weak, where groundwater is contained in Plio-Pleistocene aquifers. SO₄ does not show any strong correlation with any other ions, except for Mn, and indicates weak correlation occasionally with K, Ca and Mg. Factor and cluster analyses also illustrate similar results. Cross semivariograms show how As, Fe, P, and K can be correlated with each other in space, indicating a broad regional scale of spatial correlation. As concentrations and surface elevation show a negative correlation; i.e., where surface elevation is high groundwater As is low and vice-versa. This relationship indicates subsidence, tectonics, and low groundwater mobility play important roles in spatial variability of arsenic concentration within each geomorphic division of Bangladesh.