Arsenic Concentrations of Groundwater in the Chianan Plain, Taiwan
Date Issued
2005
Date
2005
Author(s)
Ou, Tung-Kun
DOI
zh-TW
Abstract
The Chianan plain in SW Taiwan was well known for its high concentrations of arsenic and humic substances in groundwater, which were considered to be responsible for the endemic Blackfoot disease. Eighty-four groundwater samples were collected from aquifers. Dissolved oxygen (DO), redox potential (Eh), pH and electric conductivity (EC) were measured on-site; major anions (Cl- and SO42-), cations (Na+, K+, Mg2+, Ca2+, ΣFe, ΣMn and ΣAs), and fluorescence intensities were measured in the laboratory. The objectives of this study are the following: (1) to determine geochemical conditions of the aquifers; (2) to obtain the distributions of arsenic concentrations and relationship between arsenic concentration and depositional environments; (3) to understand the mechanism of arsenic release to groundwater.
Groundwaters of high arsenic concentration are characterized by low DO (< 1 mg/l), low ORP (< -110 mV) and pH＞7. Arsenic concentrations of the 2nd, 3rd and 4th aquifers, especially in the downstream areas of the Pa-Cheng Stream, Chi-Shui Stream, Tseng-Wen Stream and Yen-Shui Stream, are obviously higher than the 1st aquifer. The arsenic distribution pattern of the 2nd, 3rd and 4th aquifers are similar to that obtained from the private wells. Water samples collected from the estuarine strata were generally high in arsenic contents.
This study used molecular membrane ultrafiltration technique to separate the humic substances into six different molecular weight intervals. Most of the humic substances (ca. 50-90％) have molecular weights of 500-10000 daltons. The distribution pattern of water arsenic distribution in each aquifer was simlar to that of humic substances. The positive correlation between humic substance and arsenic concentrations suggest that arsenic was combined with dissolved humic substances.
The main mechanism responsible for high arsenic concentration of groundwater in the Chianan area was inferred to be the reductive the desorption of arsenic under reducing environment from Fe and Mn oxides previously deposited in the estuary. Estuarine, the border between marine facies and continental facies sediments contain the high content of organic matter and clay minerals, and precipitation of Fe and Mn ions result from alternation of ph and salinity when the river flow into the sea. The very high adsorption capacity of Fe oxides, Mn oxides, clay minerals and organic matters result precipitation of aqueous arsenic. Because after burial of sediment microbial metabolism of organic matter produces a reducing condition. In reducing condition, arsenic was released into solution by Fe- and Mn-oxyhydroxides, clay minerals and organic matters and lead to increase concentration of arsenic. During SO42- reduction, the consequent S2- reacts with Fe2+ to produce FeS and ultimately to pyrite (FeS2), decreasing iron concentraction in groundwater. Because the greater part of arsenic which combines with humic substance dissolve into groundwater, negative correlation exists between arsenic and iron. High HCO3- concentration and high pH mobilize arsenic from surface of iron oxides. The very strong relationship between arsenic and HCO3- concentration and pH support the hypothesis.
Subjects
嘉南地區
地下水
砷
Chianan
groundwater
arsenic
SDGs
Type
thesis
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