Arsenic dissolution and speciation in groundwater of southeast Michigan.

Elevated concentrations of naturally occurring arsenic have been detected in groundwater of southeast Michigan. This arsenic occurrence has been associated with arsenic-containing minerals, such as arsenian pyrite, realgar and orpiment. The main objective of this study is to identify the source of arsenic in groundwater, to investigate the factors controlling its dissolution, speciation and distribution, and to provide a model of arsenic geochemistry in groundwater. Groundwater samples were collected in nine counties of southeast Michigan in 1997. The highest arsenic concentration detected was 278 mug/L, the average being 29 mug/L. Most of the arsenic was found to be As(III). It has been known that the distribution of arsenic species is affected by redox conditions in groundwater. Oxidation of As(III) with ozone and molecular oxygen was investigated in groundwater samples containing 46--62 mug/L total dissolved arsenic. Half-lives of As(III) in experimental solutions were about 4 minutes for ozone reactions and, depending on the Fe concentrations, 2 to 5 days for pure oxygen and 4 to 9 days for air. A new well was drilled in Bad Axe, Huron County, Michigan in 1997, and core samples were taken at different depths. Total arsenic concentrations in core samples varied from 0.8 mg/kg to 72.1 mg/kg. Arsenic leaching experiments were conducted with the core samples using several chemicals under various conditions. It was found that arsenic leaching increased with NaHCO3 concentration in water and with total arsenic concentration in rock. In addition, noticeable arsenic leaching was found at extremely acidic and basic pHs. The arsenic leaching was also dependent on oxic/anoxic conditions. It is concluded that most arsenic is leached as As(III), As(V) or As-HCO 3 from arsenian pyrite or orpiment into the groundwater of the study area. Bicarbonate enhances the rate of arsenian pyrite oxidation or orpiment dissolution by producing As-bicarbonate or Fe-bicarbonate complex as an intermediate. In shallow groundwater, the solubility of arsenic is controlled by the availability of bicarbonate and oxidants, and the dissolution of arsenic is related to iron and sulfur oxidation. In deep groundwater, arsenic is dissolved from orpiment and bicarbonate initiates the dissolution reaction.PhDApplied SciencesEnvironmental engineeringEnvironmental scienceHealth and Environmental SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/131917/2/9938461.pd