Solid-phase arsenic speciation in glacial aquifer sediments of west-central Minnesota, USA: a micro-X-ray absorption spectroscopy approach for quantifying trace-level speciation

Abstract

Abstract Arsenic (As) is a geogenic contaminant affecting groundwater in geologically diverse systems. The footprint of the Des Moines Lobe glacial advance in west-central Minnesota, is a regional nexus of drinking-water wells that exceed the US EPA maximum contaminant level for arsenic (As>10µgL-1). Arsenic release from aquifer sediments to groundwater is favored when biogeochemical conditions in aquifers fluctuate. The specific objective of this research was to identify the solid-phase sources and geochemical mechanisms of release of As in aquifers of the Des Moines Lobe glacial advance. The overarching hypothesis is that gradients in hydrologic conductivity and redox conditions found at aquifer-aquitard interfaces promote a suite of geochemical reactions leading to mineral alteration and release of As to groundwater. A microprobe X-ray absorption spectroscopy (µXAS) approach was developed and applied to rotosonic drill core samples to identify the solid-phase speciation of As in aquifer, aquitard, and aquifer-aquitard interface sediments. This approach addressed the low solid-phase As concentrations, as well as the fine-scale physical and chemical heterogeneity of the sediments. The solid-phase Fe and As speciation was interpreted using sediment and well-water chemical data to propose solid-phase As reservoirs and release mechanisms. The results are consistent with three different As release mechanisms: (1) desorption from Fe oxyhydroxides, (2) reductive dissolution of Fe oxyhydroxides, and (3) oxidative dissolution of Fe sulfides. The findings confirm that glacial sediments at the interface between aquifer and aquitard are geochemically active zones for As. The diversity of As release mechanisms is consistent with the geographic heterogeneity observed in the distribution of elevated-As wells. Supplementary file “Nicholas dissertation supplementary files 1 to 5.xlsx” was submitted to the UMN digital conservancy with this thesis. It is an excel workbook with five tables. Tables 1 and 2 are the complete provenance and citation information for all As and Fe reference spectra used. Tables 3, 4, and 5 are the reference spectra fits, fractions, and scores for the sampled spectra from cores OTT3, TG3, and UMRB2.