ELEVATED MANGANESE IN GROUNDWATER: A HEALTH CONCERN FOR DRINKING WATER SUPPLIES CONCOMITANT WITH THE GLOBAL GROUNDWATER ARSENIC CRISIS?

The presence of manganese in groundwater has received attention since the latter half of the 20th century among the water scientists. In recent years, focus on the drinking water safety plan has given significant impetus on the assessment of drinking water quality globally and as a consequence manganese has received considerable attention from the drinking water quality perspective duo to its neurotoxic effects. Manganese is transition element with average concentration levels of 600 mg/kg in the upper crust and 1400 mg/kg in the bulk continental crust. Manganese is distributed in a number of rock forming silicate minerals such as pyroxene, amphibole, garnet and olivine, but also common in carbonate minerals when Mn forms solid solutions with Ca, Fe and other divalent cations. In sedimentary environments, oxides of Mn such as pyrolusite, hausmannite and manganite are common besides rhodochrosite as a pure Mn carbonate mineral. Manganese is a redox sensitive element and its mobility in groundwater environment is influenced by the redox conditions in the aquifers. Under oxidizing conditions, Mn is mostly immobile and species such as Mn³⁺ and Mn⁴⁺ predominate and precipitate as hydrous manganese oxides, whereas under reducing conditions Mn²⁺ is the predominant species and mobilized in aqueous media. As a consequence, the behavior of Mn in groundwater environment varies considerably with the redox status of the aquifers and thereby exerts significant control on the quality of drinking water. In general, the alluvial sediments are characterized by Mn concentrations at levels of more than 1000 mg/kg and hence their mobilizations governed by the redox conditions impair the quality of drinking water. In recent years the knowledge on redox chemistry of the sedimentary aquifers have highlighted the global synergies of the distribution of Mn, Fe and As in groundwaters. It has been observed that the reduced groundwaters from the shallow Holocene sedimentary aquifers are characterized by low dissolved Mn concentrations, while the oxidized aquifers are enriched in Mn and thus exert a constraint on the exploitation of groundwater resources thought to be suitable especially from the perspectives of arsenic safe drinking water supplies.