The fact that the former highstand whitish sediments of Plio-Pleistocene and lowstand off-white/red sediments of Late Pleistocene containing low As concentrations in the Bengal basin forms the premise for developing a color tool by the ‘Sustainable Arsenic Mitigation Project’ (SASMIT) to guide the local drillers of Matlab, SE Bangladesh, so that they can target low As aquifers.
Groundwater samples collected from the black group of sediments representing a highly reducing environment contains very high concentrations of As, Fe2+ , NH4+, PO43-, HCO3–, and DOC and very low SO42- and Mn2+ concentrations; and vice-versa for the red/offwhite/white group of sediments, thus representing a less reducing condition.
A very strong to good correlation between DOC with HCO3–, Astot, Fetot, NH4+, and PO43- and Astot with HCO3–, Fetot, and NH4+ within the black group of samples indicate that, oxidation of organic matter is driving all the redox reactions and thereby releasing As in solution.
On the contrary, in the case of red/off-white groups of samples, all the aforementioned correlations are poor to negative, reflecting that water samples collected from these sediments are low in organic content, presumably due to weathering of these sediments during the last glacial lowstand and subsequent flushing.
Furthermore, negative correlation between Astot with Mn2+ and SO42- in all groups of samples indicating that mobilization of As is neither related to dissolution of Mn-oxyhydroxides, nor to oxidation of sulfide minerals in the study site. Speciation modeling results show that water samples derived from the back sediments are supersaturated with respect to siderite and vivianite, while near-equilibrium for white and under-saturated for off-white/red groups of samples.
Though all the four groups of samples are slightly under-saturated with respect to the mineral phase rhodochrosite, groundwater is supersaturated with respect to hydroxiapatite and MnHPO4. Three aquifers (Af1, Af2, Af3) intervened by two aquitards (At2, At3) have been identified in the study area. The shallow aquifer (Af1) extending up to a depth of about 40 to 50 meters b.g.l. consisting of black sediments is characterized by mainly Ca-Mg-HCO3 water-type and water-level fluctuation of about 3 – 4 meters.
The intermediate aquifer (Af2) consisting of red/off-white/white sediments underlying At2 is characterized by primarily Na–Ca–Cl water-type and groundwater fluctuation of about ≈ 5 m, indicating water from this aquifer is used for irrigation also. Since well installation in the low As deep aquifer (Af3) consisted of mainly white sediments is costly, the intermediate aquifer (Af2) is the best option to be explored by the local drillers.
The occurrence of the red/off-white sediments are primarily limited to the SE half of the study area because of low preservation potential of the red/off-white LST (Lowstand Systems Tracts) deposits, may be due to raivement erosion and aggradation of TST (Transgressive Systems Tracts) and recent HST (Highstand Systems Tracts) deposits. Depth-specific spatial distribution of lithofacies indicates that installation of wells at a depth of about 70 m b.g.l. anywhere in the SE half of the study area would most likely yield low As groundwater.
Author: Mozumder, Rajib Hassan