UNIVERSALITY OF GEOCHEMICAL AND MICROBIOLOGIC PROCESSES LEADING TO NATURAL ARSENIC CONTAMINATION OF GROUNDWATER IN HOLOCENE ALLUVIAL DEPOSITS

World-wide retreat of continental glaciers at the end of the Pleistocene led to rising sea level, which caused river base levels to also rise, followed by the deposition of Holocene floodplain deposits. These processes commonly led to the codeposition of reactive organic matter and sediments containing hydrous ferric oxides (HFO). Where HFO's in the active stream beds were exposed to even trace amounts of dissolved arsenic in river waters, arsenic became concentrated on the oxidized and reactive HFO surfaces due to the large differences in residence times of the stream-bed alluvium relative to the faster moving waters. Thus the As-concentrating process involves several millenia of river flow and continuous sorption and coprecipitation of As on HFO's. Establishment of shallow water-table aquifers in these low-hydraulic gradient floodplains and degradation of organic matter caused moderately reducing conditions to develop in the saturated zone. HFO's served as terminal electron acceptors for, and were reductively dissolved by, anaerobic dissimilatory iron-reducing bacteria (FeRB). Subsequently, groundwaters locally became enriched in dissolved ferrous iron, bicarbonate, and As associated with HFO's was released. Because these alluvial systems lack abundant sources of sulfate, FeRB out-compete sulfate-reducing bacteria (SRB) which has the effect of keeping As in solution. Locally SRB may remove some Fe, As and all sulfate as As-bearing Fe-sulfide minerals causing the commonly observed inverse relationship between dissolved As and sulfate. These sulfide minerals have been mistaken for the source of arsenic in the past, where in fact they are a SINK for As. Our research has shown that the bacterial-mediated iron reduction and As-desorption process can occur in a matter of weeks in the field when new sources of labile organic matter enter the system. Several natural and anthropogenic situations can cause this to happen. Thus, we propose that the necessary conditons for natural As-contamination of groundwater is similar universally in Holocene alluvial deposits and are not the product of special geochemical or hydrogeologic conditions or rare microorganisms. Recognition of these commonly-linked processes can lead to prediction of problem areas around the world besides the problematic areas in Bangladesh and West Bengal, India.