USING THE ELECTRON MICROPROBE TO ANALYZE DOLOMITE AND IRON WELL CASINGS TO DETERMINE ARSENIC CONTENT

Arsenic is a notoriously poisonous metalloid sometimes found in Oklahoma groundwater. The element is tasteless and odorless and appears to enter drinking water supplies naturally or through agricultural or industrial practices. In 2001, the Environmental Protection Agency (EPA) adopted new standards for arsenic in drinking water at 10 ppb, replacing the old standard of 50 ppb. The goal for this undergraduate research project was to analyze dolomite and iron precipitate from well casings taken from the Central Oklahoma Aquifer in Norman. The aquifer itself underlies an area of about 2,900 square miles and is an important public supply for several suburban communities in the Oklahoma City area and is a source for numerous domestic water supplies. The aquifer has been noted for its elevated arsenic concentration but the source of the contamination is unknown. Information from this project could provide guidance for the state to improve drinking water and comply with the new EPA standard. As part of the lab portion of the course, a sample of dolomite from the aquifer was mounted in epoxy, polished, cleaned, and carbon coated. Thin sections of the iron casings were commercially prepared, but were cleaned and carbon coated. The samples were imaged using backscattered electrons and X-ray mapped for arsenic and calcium using the Wavelength Dispersive Spectrometers (WDS). Analyzing dolomite for arsenic using the Energy Dispersive Spectrometer (EDS) is problematic, as the magnesium and arsenic characteristic X-ray peaks overlap. The X-ray element maps and EDS spectra indicated that the iron well casing did not contain detectable arsenic using the electron microprobe. However, the dolomite sample appeared to contain small amounts of detectable arsenic within the sample located potentially along grain boundaries or crystal faces. The element may be present as part of the mineral structure. Pits within the dolomite did not contain arsenic, lending support to this hypothesis.