The Dynamics and Speciation of Arsenic in Drinking Water Wells in Eastern Wisconsin

Abstract: Arsenic typically develops in Eastern Wisconsin groundwater as a
result of oxidation of sulfide bearing minerals in the limestone bedrock
(Schreiber et al. 2000). Naturally occurring arsenic exists in groundwater
as oxyanions which have two oxidation states, As(III) and As(V). Under
ambient pH conditions As(V) is primarily present as an anion (i.e.,
H₂AsO₄-) while As(III) tends to be uncharged (i.e., H₃AsO₃), making it
much more difficult to remove through the existing treatment
techniques such as adsorption and reverse osmosis (RO). Although
many studies exist establishing arsenic concentrations across
Wisconsin, there is a lack of investigations into the concentrations of
each arsenic species, which is essential for establishing a removal
technique. The primary goal of this research was to establish baseline
concentrations of each arsenic species, accounting for seasonal
variations, and determine how these concentrations could be affected
by water usage. Private drinking water wells were selected at 16
locations across Eastern Wisconsin. The wells were screened at various
depths, in multiple geologic units, and contained a wide-range of total
arsenic concentrations. Analysis of the speciation data indicated that
As(III) was the dominant species of arsenic in all of the wells sampled.
Data from the 11 pumping tests that were conducted, showed 9
exhibiting a downward trend in As(III) concentration and an upward
trend in As(V) concentration as volume purged increased. The pumping
tests also showed a substantial increase in total arsenic in many of the
wells as volume purged increased. There did not appear
to be aseasonal trend in arsenic concentration.