Download Project Summary Introduction: Geogenic sources of arsenic pose a

Project Summary
Introduction:
Geogenic sources of arsenic pose a health risk to humans through groundwater
and soil consumption. Taiwan has both geogenic and anthropogenic sources of arsenic,
however, the geogenic sources are believed to have caused the majority of groundwater
contamination. There are several possible reasons for the arsenic problem in Taiwan
including the release of arsenic due to reduction of iron and manganese oxides, oxidation
of sulfides, and the preferential sorption of phosphates, organics, carbonates, and other
ions in high concentrations in exchange reactions. Arsenic does not pose a risk as long as
the element does not have the potential for uptake by an organism (bioavailable). The
mobility and chemical availability of arsenic in soil and groundwater is predominantly
controlled by the oxidation state. Naturally occurring arsenic exists as arsenates (As V)
(60%), arsenites (As III), elemental arsenic (As 0), and organically bound arsenic,
however, most drinking water only contains arsenate or arsenite. Taiwan’s groundwater
has reducing conditions and this allows for As(V) to be desorbed from iron oxides as
As(III). The presence of As(III) in Taiwanese groundwater also suggests reducing
conditions, and unfortunately As(III) is the most toxic form of the element. A reducing
environment, like the one present in Taiwan, causes the desorption of arsenic from these
oxides and produces chemically available arsenic.
Objectives:
Arsenic can be deadly to humans when the element is in a chemically available
form. There may be large amounts of arsenic present in an ecosystem, but unless the
element is in the bioavailable available form the arsenic is harmless to humans and other
biological systems. Determining true bioavailability is difficult to measure without
harming test subjects, so instead chemically availability will be used as a proxy for the
amount of bioavailable arsenic in the soil. Studies that determine bioavailability and
chemical availability must also take into account the factors that control the concentration
and speciation of arsenic such as: redox reactions, pH, mineral interactions, microbial
cycling, and the geology of the area. The geology of Taiwan provides a focus of this
research study because of the geologic banding that runs parallel to the western coast.
The underlying geologic banding suggesting that the soils may change as the rock type
changes, which could affect the chemical availability of arsenic.
Experiment:
I will determine specific minerals associated with the varying geology across the
island of Taiwan. This will allow me to determine arsenic speciation as a function of
varying minerals, and determine if the desorption of Arsenic from minerals in specific
areas is responsible for the arsenic contamination in water.
Expected Results:
I expect that the surface sediments under oxidizing conditions with produce a
large amount of available arsenic by the oxidation of sulfides. Under reducing conditions
Arsenic-enriched Fe(III) hydroxides are a likely source of arsenic.
The Coastal Range - turbidites overlaid by alluvium.
The Western Foothills - shallow marine detrital sediments.
The Hsuehshan Range - quartz, carbonate sandstone, argillite, and shale.
The Western Central Range - turbidites.
The Eastern Central Range - a belt of schist, greenschist, gneiss, carbonates, tuffs, and
oceanic pelitic and mafic schists
The Longitudinal Valley - volcanic and sedimentary rocks
The Central Range - volcanic basement rocks, turbidites at the surface.