MANGANESE IN GROUNDWATERS IN THE SEDIMENTARY AQUIFERS OF BENGAL BASIN DELTA, EAST OF THE RIVER BHAGIRATHI, WEST BENGAL, INDIA AND BANGLADESH

Research on drinking waters of the Bengal Delta has documented elevated concentrations of arsenic for more than a decade, but recent literature has also noted high concentrations of manganese in drinking waters from this region. Although health impacts of Mn in drinking waters have yet to be adequately constrained, it is a known neurotoxin (MCL value of 0.4mg/L, WHO Bulletin (2011)). New data on water samples from two localities—Murshidabad, India and Matlab, Bangladesh—reveal concentrations that significantly exceed this proposed MCL. In this study, along with dissolved Mn in groundwaters, modes of occurrences of Mn in sediments have also been examined.

In Murshidabad, we see two distinct hydrogeochemical facies that are strongly dependent upon the sediments in which they reside: high As, low Mn (dark grey colored, Holocene sediments, east of the river Bhagirathi) and low As, high Mn (reddish-brown colored, Pleistocene sediments, west of the river Bhagirathi). Holocene sediments exhibit conditions that are sufficiently reducing that associated groundwaters contain elevated dissolved As and total Fe, yet not quite reducing enough to release Mn. Saturation indices also suggest that Mn may be limited by the precipitation of Mn-carbonates, particularly in regions with significant alkalinity. Sequential extraction results reveal that Mn exists predominantly in the nonspecifically sorbed or easily exchangeable phases in sediments with elevated dissolved Mn.

In contrast, at Matlab, Bangladesh, a different trend is shown in that grey and dark grey sediments (depth 10-45m and 65-95m, respectively) have high As and high Mn (0.5-2 mg/L), whereas light grey sediments (>100m) have both low As and low Mn (<0.2 mg/L). Sequential extractions show that light and dark grey sediments differ in the abundance of bioavailable Mn phases, with dark grey sediments containing ~30-35% of such phases whereas light grey sediments have almost none. Reddish brown sediments (45m-65m) showing < 10µg/L of As, has higher Mn concentrations (0.6-2.5mg/L).

More detailed studies of the redox behavior of Mn are needed in these fluvio-deltaic terrains where high dissolved As is also a problem, in order to understand the different mineralogical reservoirs for coexisting As and Mn in sediments and as well as the contrasting mechanisms for their release.