NATURALLY OCCURRING ARSENIC IN HOLOCENE ALLUVIAL AQUIFER AND ITS IMPLICATIONS FOR BIOGEOCHEMICAL LINKAGE AMONG ARSENIC, IRON AND SULFUR FROM SOURCE TO SINK

Naturally elevated levels of arsenic (As) have been recognized in shallow groundwater in a Holocene alluvial aquifer at Kansas City, Missouri. Arsenic concentration ranges from 1 to 167 µg/L in eight monitoring wells in the area. There is a positive correlation between arsenic and dissolved iron and a negative correlation between As and sulfate and nitrate. Arsenic is apparently released into groundwater during the concomitant release of dissolved iron by bacterially mediated reductive dissolution of arsenic-bearing hydrous ferric oxides. Arsenic and iron mobilization is triggered by a linked redox process involving oxidation of organic carbon and reductive dissolution of iron minerals by iron-reducing bacteria (FeRB) in groundwater. The source of organic carbon is natural organic matter that was co-deposited with iron minerals during the Holocene sedimentation of the alluvial floodplain. We are currently conducting molecular biological studies (e.g., DNA sequencing) to identify actual species of bacteria in As-rich waters from the Kansas City site. Culturing of groundwater samples indicates that both FeRB and SRB are present in highly reducing groundwater samples with elevated As, but absent in oxidizing samples with low As concentration. This is the first direct field evidence of As release into groundwater due to FeRB metabolism in Holocene alluvial aquifer .

Other geochemical data from a bioremediation field experiment in a metal-contaminated Holocene alluvial aquifer also support the reductive dissolution hypothesis for arsenic release into groundwater. With the injection of reactive carbons into the subsurface, groundwater at this site quickly changed from oxidizing to reducing condition within a few weeks and arsenic concentration increased from a background value of 0.01 mg/L up to 2.82 mg/L with concomitant increase of iron. The presence of SRB in alluvial aquifer sediments implies that arsenic can be removed from iron and sulfide-rich solution at the onset of sulfate reduction and thereby implies an important biogeochemical linkage of arsenic, iron and sulfur from source to sink in nature.