Mobilization and Enrichment of Arsenic in Groundwater in the Chianan Coastal Plain, Southwestern Taiwan
Date Issued
2007
Date
2007
Author(s)
Chen, Kuan-Yu
DOI
zh-TW
Abstract
The Chianan coastal plain in southeastern Taiwan has long been well known for the prevalence of black-foot disease since 1920s. Previous literatures showed that this disease resulted from drinking groundwater rich in arsenic and dissolved humic substances. However, the occurrence and enrichment mechanism of arsenic and their controlling factors have not yet been studied in detail.
Well water samples from a total of 84 newly established monitoring wells and 83 pore-water samples from 4 wells were analyzed in this study to investigate major chemical compositions (such as Eh, pH, EC, Na+, K+, Mg2+, Ca2+, SO42-, Cl-, ΣFe, ΣMn and ΣAs, δ13CDIC and 14CDIC-age). Extractable arsenic contents of 534 sediment samples from fully cored boreholes and 24 sediment samples from ground surface were analyzed. Total organic carbon contents of 42 sediment samples were also analyzed.
The technique and equipments we used for pore-water sampling were still difficult to avoid strong oxidization during sampling to understand the distribution of arsenic and humic substances. The average acid extractable arsenic content and total organic carbon (TOC) content in both core and surface sediment samples are about 5 mg/kg and 0.3 %, respectively. Besides, arsenic and TOC contents increase with the proportions of the clay-sized fraction. However, there is no significant correlation between both the arsenic and TOC contents with depositional environment. In addition, no significant correlation was found between concentrations of arsenic and humic substances in groundwater and arsenic and TOC contents of the corresponding core sediments. This demonstrates that both arsenic and TOC contents of the sediments are not direct controlling factors for the concentrations of arsenic and humic substances in groundwater.
Almost all groundwater from the deep aquifers in the down-gradient (western) side have long residence time and are relatively high in arsenic contents (> 0.1 mg/L) and humic substances (>200 QSU). Under strong reduction condition, H2S (or HS-) was generated and precipitated with either arsenic or iron as AsS2(s) or FeS(s). The black precipitates collected from Da-Wen(II) well water is high in arsenic content supporting mineral phases inferred by the PHREEQE program. In-situ generated humic substance competed with H2S (or HS-) for the arsenic. About 90% of arsenic in groundwater is combined with humic substances which are greater than 500 Dalton in molecular weight. It is not uncommon that the groundwater in the Chianan plain are both high in arsenic and humic substances. The relatively high contents of arsenic and iron in groundwater resulted from the lack of H2S (or HS-) under mild reduction condition and the content of chelated arsenic increases with that of the dissolved humic substances.
Subjects
砷
腐植物質
螢光
地下水
沈積物
嘉南平原
arsenic
humic substances
fluorescence
groundwater
sediments
Chianan coastal plain
Type
thesis