Hydrogeochemistry of multi-level aquifers of Bengal Basin in Matlab, Bangladesh

Occurrence of natural arsenic (As) in groundwater through drinking water has reduced the access to safe drinking water in Bangladesh. Since the discovery of As in 1993, tens of millions of people are exposed to concentration at levels above the BDWS (50 μg/L) and WHO guideline (10 μg/L). This study deals with a comprehensive and systematic hydrogeological study in the Matlab, Bangladesh which included hydraulic head monitoring using depth-specific piezometers installed to target specific aquifers (shallow, intermediate-deep and deep). Fifteen piezometer nests were drilled targeting the shallow, intermediate-deep and deep aquifers. In most of the nests four wells were drilled within depths up to 70 m, one well was drilled to a depth of 110 m and a deep well down to about 235 m. Groundwater levels were monitored for over a three years period. The hydraulic heads reveal that the shallow aquifers are separated from intermediate and deep aquifers. P1 and P2 piezometers (Aquifer 1) indicated typically high median concentrations of As in groundwater (71-646 µg/L). Derived from the black sand aquifers., these groundwaters were characterized by elevated DOC, HCO3, Fe, NH4-N and PO4-P and relatively low Mn and SO4 justifying the release of As due to reductive dissolution of Fe-oxyhydroxides. Shallow P3 and P4 piezometers revealed median As concentrations ranging from 6-30 µg/L in wells placed in the red and off-white sediments (Aquifer-2). These groundwaters were typically low in DOC, HCO3, Fe, NH4-N and PO4-P, and high Mn and SO4. The As concentrations in the intermediate-deep (P6) and deep (P5) piezometers in Aquifer 3, contained low in As, mostly within the WHO guideline value of 10 µg/L, with low concentration of DOC, HCO3, NH4-N and PO4-P compared to the shallow aquifers. The study thus reveals a distinct hydrogeochemical contrast in the Matlab area in the shallow, intermediate-deep and deep aquifers. The groundwater heads in the shallow aquifers are separated from the intermediate and deep aquifers. The intermediate deep aquifers are connected to the deep aquifers with low As and low Mn to provide safe drinking water to the population.