Biogeochemical Characterization and Cycling of Arsenic in Groundwater of the Littoral Area in the Southwestern Part of Taiwan
Date Issued
2011
Date
2011
Author(s)
Lu, Kuang-Liang
Abstract
Comprehensive biogeochemical studies have been carried out to elucidate the controlling factors on the partition and the cycling of arsenic (As) within coastal aquifers in the southwestern part of Taiwan. Most groundwater samples were characterized as Na-Ca-HCO3 with HCO3- as the dominant anion. Total arsenic (As) concentration, predominantly as As3+, ranged from <1.0-562.7 μg/L. Saline water type, which was mostly concentrated in the uppermost aquifer in the study area, generally retained highly dissolved As concentrations, albeit no correlation was observed among the analyzed parameters and leaching experimental data. Factor analysis proposed a four-factor model, comprising salination, reductive dissolution of Fe/Mn oxyhydroxides, As reduction and chemical potential factor, and explained 89.94% of total variance in groundwater. Clay minerals were evidenced as the main pools for sedimentary As. Sequential extraction data indicated the partitions of As in the distal-fan were distributed evenly, independently of the weakly adsorbed As phase, but high As fractions were concentrated in amorphous and less crystalline Fe hydroxides phase in the mid-fan. Iron oxyhydroxides were regarded as the dominant sinks sorbed As as well as As-bearing sulfides in the well screen level. Stable cultures of iron-reducing bacteria (IRB) were capable of reducing both Fe and As within sediments, and further shed light on the influence of bioavailable Fe minerals. While suspending in a non-sterile, in-situ groundwater, addition of acetate as carbon source would affect the reactive pathways of Fe and As, resulting in the decoupled processes of these two redox couples. At this circumstance, solubilizing Fe(III) accompanied a significant increase of As(III) concentration implicated another release pathway for converting sedimentary As into groundwater during the onset of reducing environment. Desorption behaviors of As were relevant to its valence in the sediments and the co-existence of anions. Although bicarbonate addition resulted in less As desorption than that of phosphate on a molar basis, the contribution of bicarbonate to the total release of As was still greater than phosphate due to the much higher concentration of bicarbonate in groundwater. This result also in part explained the poor correlation between Fe and As concentrations in groundwater. As a consequence, the dissolution of As-related Fe oxyhydroxides mediated by microbial activities was the prerequisite for elevated concentration of As in the subsurface. Two-parameter (pH and Eh) model derived from discriminant analysis can be used for preliminary assessment to determine whether the As concentration exceeds 10 μg/L with simple field measurements in this area.
Subjects
Arsenic
Groundwater
Reducing bacteria
Multivariate statistical techniques
Iron oxyhydroxides
Type
thesis
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