Characteristics of Humic Substances in High Arsenic Groundwater in the Chianan Plain, SW Taiwan
Date Issued
2007
Date
2007
Author(s)
Liu, Li-Chun
DOI
zh-TW
Abstract
It has been well known that there are high contents of arsenic and humic substances in deep groundwater of the Chianan plain. Drinking the groundwater of this area may causes cancers and blackfoot disease. Humic substances have varieties of functional groups and high metal-chelating ability, which forms organo-metallic complexes and emits fluorescence when irradiated by ultra-violet light. Humic substances may play an important role in mobilization of arsenic in sediments and groundwater.
In this study, 13 groundwater samples from 7 monitoring well sites were collected to isolate humic substances by DAX-8 resin or Molecular/Por® stirred cell to explore the relationship between humic substances and arsenic in groundwater. Fluorescence characteristics and UV/visible light absorption of groundwater were also used to investigate the composition and origin of dissolved organic matters (DOMs). The results obtained show that the decay rate of fluorescence intensity decreases in the order of sunlight exposure, lamplight exposure and light-shield. Accordingly, it is suggested that samples must be frozen and store in the dark to prevent decomposition of humic substances.
In average, about 80% of total dissolved organic carbon in these samples is humic substances, and their fluorescence efficiency is about 8.7 QSU per mg/L of humic substances. By combining the results of total alkalinity, C-14 age and δ13C of dissolved inorganic carbon, dissolved organic carbon and fluorescence intensity, the author suggestes that the dissolved inoeganic carbon and humic substances were derived from sedimentary organic matters.
The peaks of excitation-emission matrices (EEMs) and absorption rate of UV-visible light at 250 and 436 nm suggest that that the dissolved humic substances of the Chianan plain are mainly of terrestrial origin with high contents of aromatic ring, which leads to higher metal-chelating ability. Because of the variable compositions of humic substances and background quality of groundwater samples, UV-visible spectrum analysis is an easier method to investigate the origins and compositions of humic substances than EEMs method.
It has been found that dissolved arsenic of the Chianan plain is combined mainly with humic substances with molecular weights greater than 500 Dalton. Arsenic is released from humic substances when groundwater is acidified to pH 2 with HCl during humic substance isolation processes. In addition, the results show that arsenic is mainly chelated with humic substances, forming organo-metallic complex. Samples with high concentrations of arsenic often have high humic substances contents, but not vice versa. Obviously, humic substances content is one of the important factors controlling arsenic concentrations of groundwater in the Chianan plain.
Subjects
嘉南平原
地下水
砷
腐植物質
螢光
Chianan plain
groundwater
arsenic
humic substances
fluorescence
SDGs
Type
thesis