Soil Science Exit Seminar
Elizabeth Gillispie, Ph.D. Student, Soil Science, NCSU, will present an exit seminar titled:

“Sediment managenese-oxide content as a tool for predicting groundwater arsenic pollution potential in Pleistocene aquifers of Southern Asia “ (under the direction of Dr. Matt Polizzotto)

on 16 October 2017 at 1:30 PM in the McKimmon Room (2223 WMS).
All are welcome to attend.

Abstract

Over 150 million people in South and Southeast Asia consume unsafe drinking water from arsenic(As)-rich Holocene aquifers. Although use of As-free water from neighboring Pleistocene aquifers is a potential mitigation strategy, such aquifers are vulnerable to geogenic (As derived from natural aquifer materials) and allogenic (As transported into the aquifer) As pollution, placing millions more people at potential risk for As exposure.
The goal of this dissertation was to elucidate the roles of sediment variability, manganese(Mn)-oxide mineralogy and redox chemistry in regulating allogenic and geogenic arsenic contamination of well water. To accomplish this goal, chemical analyses of natural, chemically variable Pleistocene sediment from 4 well locations in the Mekong Delta, Cambodia were integrated with well-water sampling, spectroscopic analyses, batch reduction and adsorption experiments, and flow column studies. In general, results show that variability in pore- and well-site- scale sediment chemistry impacts the biogeochemical processes controlling As mobility. Across the aquifer, average groundwater redox potential (Eh = 0.155V ± 0.097 V) was suboxic, around the potential of MnO2(s) reduction and above the potentials of H2AsO4- and Fe(OH)3(am)(s) reduction. According to batch reduction experiments that examined release of geogenic As to solution, when microbial processes were limited, As release was minimized unless solution chemistry was sufficient to promote abiotic desorption or competitive displacement of As from sediment mineral surfaces. Introduction of dissolved organic carbon from external or sediment sources may stimulate As removal to solution through biotic reduction or, in the case of complex humic and fulvic acids characteristic of natural environments, a suite of abiotic and biotic processes. According to batch sorption and flow-column experiments that examined allogenic As transport and retention, the removal of As(III) from solution was controlled by sediment chemistry. In particular, the sorption capacity and maximum amount of As sorbed were directly related to the abundance of crystalline Mn oxides but also correlated strongly with the abundance of crystalline Fe oxides. However, under flow conditions, the retardation and dispersion of As transport were only strongly correlated to the abundance of crystalline Mn oxides, suggesting that the presence of Mn-oxides limits allogenic As transport. Based on our results, incorporating geochemical processes on all scales and assessing the abundance and reactivity of Mn could help provide more accurate models and more easily accessible tools to assess the vulnerability of Pleistocene aquifers to future As pollution.