STRATIGRAPHICALLY AND TEMPORALLY RESOLVED IRON MINERALOGICAL TRANSFORMATIONS IN THE MEKONG DELTA WITH IMPLICATIONS FOR ARSENIC SEQUESTRATION AND RELEASE

Arsenic contamination of drinking water wells in South Asia has received much attention. The association of redox potential (Eh) and iron(II) concentration in well waters has linked the reductive dissolution of aquifer solids to arsenic release. Although the association of sorbed arsenic with iron (oxy)hydroxides is well established, the mechanism of release and/or sequestration under reducing conditions still requires further inquiry. The problem is further convoluted by the spatial heterogeneity of contamination in wells and the inconsistency of the iron(II) and Eh connection. This study seeks to provide a mechanism for such spatial and aqueous geochemical inconsistencies. Lab and field based investigations have shown organic mater drives dissimilatory iron reducing bacteria thereby releasing iron(II) and associated sorbed arsenic. Further reduction may yield competing reprecipitation, however, of iron and arsenic as sulfide phases providing solid phase sinks for arsenic. These complexities could provide cause for the observed geographic and geochemical variability of contamination in wells. This study looks to clarify these competing reactions by exploring solid phase speciation of depth and time-resolved strata. Four cores were drilled in the Kandal Province of Cambodia. Cores were taken from an avulsed scrollbar sequence in the Mekong River Delta along the Bassac River, the first major distributary in the delta. These cores provide sediment that is both stratigraphically resolved with depth and temporally resolved with location in the scrollbar feature. Bulk X-ray absorption spectroscopy (XAS) was used to determine iron and arsenic speciation in preserved sediment collected from the cores. Total organic carbon (TOC) and sediment size analysis was completed. Arsenic, iron, and sulfur solution concentrations were analyzed from the drilled wells and throughout the study area from preexisting wells. Iron mineralogical transformations and arsenic speciation are reported. This study links TOC, relative age of burial and iron and arsenic mineralogy. These interpretations are furthered by combining them with solution data to yield a mechanism for steady state elevated arsenic solution concentrations that can explain well water spatial and geochemical heterogeneity.