Abstract
The Chianan coastal plain in southwestern Taiwan has long been well known for the prevalence of black-foot disease during the period 1961 to 1985. This disease resulted from drinking groundwater rich in arsenic and dissolved humic substances. However, the occurrence of arsenic and its controlling factors have not yet been studied in detail. Water samples from a total of 86 newly established monitoring wells and sediment samples from 28 fully cored boreholes were analyzed in this study to investigate major chemical [such as SO4 2- , Cl - , redox potential (Eh), pH, and 14 CDIC ages] and geological (such as depositional environment, grain size and arsenic content of sediments) factors which may control the distribution of arsenic in groundwater of the upper ~300-m thick strata. This study shows that arsenic concentrations of almost all groundwater samples exceed the 0.01 mg L -1 limit of the WHO guidelines, with the highest values up to ~1.2 mg L -1 . Only the deep aquifers (> 50 m) deposited before Holocene transgression contain groundwaters with relatively high arsenic contents (> 0.1 mg L -1 ). All the arsenic-rich groundwaters are under strongly reducing conditions with low Eh (< 110 mV) and low SO4 2- (nearly all < 2 mg L -1 ). The different combination of As, Fe, and SO4 2- concentrations of groundwater reflect various redox potentials. The reductive dissolution of As-rich Fe-(hydr)oxides is believed to be the major source of As in the groundwater, but the contents of SO4 2- , humic substances and residence time of water are also responsible for the variation of dissolved As in the Chianan coastal plain.
Highlights
The occurrence and factors controlling the arsenic contents in groundwater have drawn much attention in recent years (e.g., Hering and Kneebone 2001; Inskeep et al 2002; Mandal and Suzuki 2002; Smedley and Kinniburg 2002; Anawar et al 2003; Ahmed et al 2004; Huntsman-Mapila et al 2006; Pedersen et al 2006; Varsanyi and Kovács 2006)
About 50 liters of each water sample were collected for precipitating dissolved inorganic carbon (DIC) for 14C dating using the procedures described by Hackley et al (1992) and Liu (1995)
This study demonstrates that all the arsenic concentrations of the analyzed groundwater samples from the Chianan coastal plain exceed 0.01 mg L-1, with the highest value up to ~1.2 mg L-1
Summary
The occurrence and factors controlling the arsenic contents in groundwater have drawn much attention in recent years (e.g., Hering and Kneebone 2001; Inskeep et al 2002; Mandal and Suzuki 2002; Smedley and Kinniburg 2002; Anawar et al 2003; Ahmed et al 2004; Huntsman-Mapila et al 2006; Pedersen et al 2006; Varsanyi and Kovács 2006). The Chianan coastal plain in southwestern Taiwan (Fig. 1) has been well known for the occurrence of a peculiar peripheral vascular disorder in humans called “black-foot disease”. It has been found that the unconsolidated strata of this plain (more than 1000 m in thickness) are mainly composed of poorly sorted alternating beds of fine-grained clastic sediments such as clay, silt and fine sand (Oung et al 2001) These sediments were deposited in mixed alluvial and delta environments such as fluvial flood plains, estuaries, lagoons, and shallow marines from the Late Pleistocene onwards (Wu 1999). These sediments were mainly derived from the very thick (~5000 m) sequence of slightly consolidated Pliocene marine mudstones (namely Kutingkeng Formation) exposed on the hills to the east (Ho 1986)
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