Mechanism of regional enrichment of groundwater by boron: the examples of Bangladesh and Michigan, USA

In Bangladesh, concentrations of boron in groundwater reach 2.1 mg L−1and are high regionally in alluvial aquifers of Late Pleistocene/Holocene age. Concentrations exceed 0.5 mg L−1 across approximately 6700 km2 of the deep aquifer (>150 m depth) and 3000 km2 of the shallow aquifer (<150 m depth). In the Carboniferous sandstone and shale aquifers in Ingham County, Michigan, concentrations reach 6.1 mg L−1 and high values are widespread in the NE of the county. These concentrations exceed the regulatory guideline values for human consumption of 0.5 mg L−1 (WHO) or 0.9 mg L−1 (USA). The boron has desorbed from mineral surfaces as freshwater flushing displaces saline waters from the aquifers. In Bangladesh, desorption is driven by decreasing ionic strength, the equilibrium re-adjustment of mineral sorption sites to the low boron concentration in freshwater, and competitive exchange with HCO3/CO3. In deep Bangladesh aquifers in Barisal District (>250 m deep), boron enrichment delineates a buried estuary marking a previous course of the Ganges and/or Brahmaputra rivers. Boron enrichment is accompanied by ion-exchange that depletes Ca and enriches Na in the flushing freshwater. The patterns of enrichment and depletion indicate the direction of water quality change, in terms of salinization or freshening, with greater sensitivity than absolute chemical parameters.