Geochemical, mineralogical and statistical characteristics of arsenic in groundwater of the Lanyang Plain, Taiwan

Abstract

High arsenic concentrations (average 0.1 mg/L) of groundwater were found in Lanyang plain of Taiwan. In this study, 39 groundwater samples from 23 wells were collected and 14 hydro-geochemical parameters were analyzed. Factor analysis was applied to determine major influence factors of the arsenic enriched groundwater quality, and PHREEQC was used to calculate the distribution of aqueous species and saturation index of which affected the hydrogeochemistry of groundwater. 393 geological core samples from 9 drilling wells were collected and analyzed the contents of total arsenic and iron. Moreover, core samples associated with high arsenic concentration groundwater were selected, mineralogical phases were analyzed using X-ray fluorescence (XRF), high resolution X-ray photoelectron (XPS) and scanning electron microscope and energy dispersive spectrometer (SEM-EDS). Results of the arsenic enrichment factor determined by factor analysis indicated that infiltration of the organic and nitrogen pollutants from anthropogenic activities to shallow groundwater, and the reductive dissolution from iron oxyhydroxides in the deep aquifer were the main processes of arsenic release to groundwater from the sediment. Total arsenic and iron contents of the core samples were well correlated in marine sequences. The presence of clay layer within the subsurface may increase in the As contamination in groundwater aquifer. However the time for As release from clay layer to lower aquifer may require tens or hundreds years to complete under natural environment condition. Surface analyses of core sample performed by XPS showed that arsenic was adsorbed or co-precipitated with non-crystalline iron oxyhdroxides and sulfides. After a long term burial of sediment, microbial metabolism of organic matter creates a more reducing environment, arsenic may then be gradually released from iron oxyhydroxides by reductive dissolution or desorption to aqueous phase. The framboidal-diagenetic type phase was identified by XPS and the groundwater is supersaturated with respect to pyrite and orpiment determined by PHREEQC are suggesting sulfide minerals co-precipitate As. Arsenic in sediments is released into groundwater primarily by the reductive dissolution of As-bearing Fe-oxyhydroxides in reducing environment in the Lanyang plain.