How Critical Is The Critical Zone To Arsenic: Trying To Solve How And From Where Arsenic Weathers Into The Aquifers Of Asia’s Deltas

While processes in the critical zone are important in soil formation and ecosystem dynamics, they have been shown to influence chemical compositions of groundwater. In the case of areas with elevated groundwater arsenic of natural origin, mobilization of arsenic from the critical zone between soil and aquifer has been invoked to supply much of the As that further transport down the flow path to aquifer. On the other hand, evidence for groundwater accumulating arsenic from aquifer sediment as it flows along the flow path would suggest that critical zone processes may not be important. Is the arsenic coming from the critical zone, such as soil weathering, or is it distributed in heterogeneous sediment in the aquifer itself, or both sources can be important but not at the same time or same location? In looking at the sediment facies and water geochemistry along 1-km high-resolution transects in well-studied shallow aquifer sites in Bangladesh, Vietnam, and Nepal, we use dissolved geochemical and stratigraphic profiles, along with optically dated aquifer ages, to infer the source(s) and rates of arsenic release into these areas’ local shallow groundwater systems. Overall, our results suggest that these shallow aquifer systems have sufficient amounts of labile arsenic and sedimentary organic matter to drive the desorptive and reductive release of arsenic independent of post-depositional soil-forming processes—i.e., arsenic and organic reductant are present at the time of deposition. Although our findings suggest that natural arsenic contamination occurs independent of the critical zone, there still remain unanswered questions about how human development and perturbations to the critical zone will affect arsenic in the groundwater of Asia and other regions.