Delineation of spatial redox zones using discriminant analysis and geochemical modelling in arsenic‐affected alluvial aquifers

Abstract

Abstract:This study characterized the redox conditions in arsenic-affected groundwater aquifers of the Lanyang plain, Taiwan. Discriminant analysis was adopted to delineate three redox zones (oxidative, transitional and reductive zones) in different aquifers and yielded 92Ð3% correctness on groundwater quality data. Arsenic is mainly distributed in the reductive zone, and arsenic distribution in the shallow aquifer is mainly affected by surface activities. According to PHREEQC modelling results, possible mechanisms for arsenic release to groundwater in Lanyang plain are explored. Arsenic released to groundwater in the oxidative zone (zone 1) is primarily caused by the oxidations of arsenic-bearing pyrite minerals, and arsenate is the predominant species. While the reductive dissolution of Fe-oxides are responsible for the high arsenic concentration found in the transitional and reductive zones (zones 2 and 3), arsenite is the predominant species. The reduction potential of groundwater rises as the depths and zones increase. Some sulphates may be reduced to form sulphide ions, which then react with arsenic to form arseno-sulphide deposits (such as realgar, orpiment) and then slightly lower groundwater arsenic concentrations. A conceptual diagram which summarized the possible release processes of arsenic in different redox zones along groundwater flow in Lanyang plain is postulated. Arsenic-bearing pyrite and arsenopyrite (FeAsS) are oxidized as they are exposed to the infiltrated oxygenated rainwater, releasing soluble arsenate Fe(II) and SO 4 2 into zone 1. The dissolution of arsenic-rich Fe-oxides due to the onset of reducing conditions in zones 2 and 3 is responsible for the mobility of arsenic and likely to be the primary mechanism of arsenic release to groundwater in the Lanyang plain

Distribution of Inorganic As Species in Groundwater Samples with the Presence of Fe

“…The area is approximately 400 km 2 , with sides of about 30 km each. Detailed hydrogeological settings of these two catchments were described by Wang et al (2007) and Lee et al (2008).…”

“…In Taiwan, high-As groundwater areas have been found in the coastal regions of the Choushui river alluvial fan , the Chianan Plain (Lin et al 2006), and the Lanyang plain (Lee et al 2008). The most plausible release mechanism of As in both areas is the reductive dissolution of As-contained Fe oxyhydroxides (Wang et al 2007; Lee et al 2008;Lu et al 2010).…”

“…The most plausible release mechanism of As in both areas is the reductive dissolution of As-contained Fe oxyhydroxides (Wang et al 2007; Lee et al 2008;Lu et al 2010). Dissolved Fe concentration is hence an important factor affecting As speciation in anoxic groundwater samples.…”

“…Salinization of groundwater in coastal aquifer is also a critical hydrogeochemical factor affecting As speciation in Taiwan Lee et al 2008). The presence of high sulfate concentration may complicate the mobility of As and Fe and alter the distribution of inorganic As species in groundwater (Emett and Khoe 2001).…”

Characterizing hydrochemical properties of springs in Taiwan based on their geological origins

“…The results of DA in different sampling periods clearly showed a seasonal variation of the whole chemistry of groundwater. Lee et al (2008) used DA to delineate redox zones of groundwater according to results of correct assignations. Discriminant analysis is considerably different to factor and cluster analyses because groups of hydrochemical properties need to be determined in advance.…”