SPECIATION AND MINERALOGY OF ARSENIC IN GLACIAL SEDIMENTS AND THEIR EFFECT ON ARSENIC CONCENTRATIONS IN GROUNDWATER, MINNESOTA, USA

In Minnesota, domestic drinking-water wells with dissolved arsenic concentrations exceeding the U.S. Environmental Protection Agency's Maximum Contaminant Level of 10 micrograms per liter are predominantly found within the margins of the Des Moines Lobe glacial advance, which covered the western half of Minnesota from 12-14 thousand years ago. The arsenic concentrations among neighboring water wells can vary over a few hundred meters of horizontal distance. The total arsenic concentrations in the solid aquifer materials are not exceptionally high, 3-10 milligrams per kilogram, compared to the crustal average. This apparent paradox is seen in geologically sourced arsenic contamination worldwide.

A common hypothesis is that the mineral sources of the arsenic are arsenic-bearing pyrites in Cretaceous-age shale fragments common to these glacial sediments. The mineralogy of host geological materials, like iron oxyhydroxides, has been proposed as a primary factor in arsenic release to groundwater. The abundance of arsenic in groundwaters under reducing conditions indicates that the solid source of some of the arsenic may be oxidized, sorbed species.

Our study examines the mineral and chemical species of arsenic in the solid phase within the glacial aquifer sediments to better understand the mechanisms liberating arsenic to groundwater. We use whole-rock chemistry to measure total elemental abundances in the strata, and sequential extractions to identify arsenic species operationally. Bulk and microprobe X-ray absorption spectroscopy (XAS) are used to identify and quantify arsenic and iron species. Microprobe X-ray diffraction (XRD) is used to describe the arsenic-bearing mineralogy.

We sampled and examined glacial sediments from 10 rotary-sonic drill cores from 20-70 meter depths. We measured arsenic speciation in strata above, at, and below transitions between fine-grained glacial tills (confining layers) and sand-gravel deposits (aquifers). We found that arsenic is present in three distinct oxidation states: As⁵⁺, As³⁺, and As^1-. The presence of three arsenic species, with varying proportions in the solids, may explain some of the observed spatial heterogeneity in arsenic concentrations in groundwater in Minnesota. These forms of arsenic will be labile under different redox conditions in waters.