Distribution and Characteristics of Arsenic in the Hydrogeological Environment of Chianan Plain ,Taiwan:Implication to Arsenic Release Processes

Blackfoot disease was caused by high arsenic concentrations in the groundwater ingested by local inhabitants in several villages in the Chianan Plain of southwestern Taiwan. Generally, arsenic is a natural occurring element found in the earth’s crust. Arsenic contamination of groundwater can occur by geogenic and human activities. Release from natural sources is the dominant cause of elevated arsenic in groundwater. A factorial analysis was employed to evaluate the principal components of the hydrochemical characteristics of 90 groundwater samples in Chianan plain. To determine the correlation between As(s) and Fe(s) in sediment, a total of 463 geological core samples from 9 drilling wells situated at Chianan plain were collected and analyzed the contents of total arsenic and iron. Meanwhile, 42 and 20 samples around high arsenic concentration groundwater were selected, respectively, for additionally sequential extraction and mineralogical characterization. Mineralogical characterization and concentrations of arsenic and iron in the sediment were analyzed using atomic absorption spectrophotometer (AAS) and mineralogical characteristics were evaluated by using X-Ray Fluorescence (XRF) and X-ray Photoelectron Spectrometer (XPS) and Scanning Electron Microscope and Energy Dispersive Spectrometer (SEM-EDS). Results of factor analysis show that hydrochemical characteristics of groundwater samples are grouped by three main factors: salinization, arsenic enrichment, iron. A moderate correlation between As(s) and Fe(s) in sediment is found. The primary and secondary minerals of iron were iron oxy-hydroxides and FeS as shown by XPS analysis. Silicon (Si), aluminum (Al) and iron (Fe) were identified as the main components in the core samples by XRF. The result of XRF revealed that the arsenic correlated well with sulfur (r2=0.77). Accordingly, we postulated that the major processes causing high arsenic levels in groundwater were mainly from reductive dissolution of As-rich Fe oxy-hydroxides and partially from competitive exchange of adsorbed and microbial-mediated oxidation of As-bearing pyrite mineral.