Redox characteristics and zonation of arsenic-affected multi-layers aquifers in the Choushui River alluvial fan, Taiwan

SummaryIn the light of the redox characteristics of groundwater in the arsenic-affected multi-layers aquifers in the Choushui River alluvial fan, an investigation on major redox-sensitive species were carried out to explore their spatial patterns, the controlling factors and the redox zonations. Factor analysis was applied to 16 hydrochemical parameters of 152 groundwater samples. Five factors including seawater salinization, arsenic pollutant, nitrate contamination, alkaline and Fe, Mn accounted for 86.4% of the total variance identified. Among them, arsenic pollutant, nitrate contamination and Fe, Mn were redox-sensitive factors. Arsenic pollutant factor has strong loading on As and TOC, moderate loading on NH4+ and low loading on Alkalinity and Fe. These hydrochemical parameters closely relate to the reductive dissolution processes of arsenic release to groundwater. Discriminant Analysis was adopted to delineate three redox zones (oxidative, transitional and reductive zones) in 4-layers aquifers and yielded 90.4% correctness on groundwater quality data. The spatial distributions of As and NH4+ were similar but showed a reverse pattern with NO3-. Arsenic and NH4+ were mainly distributed in the reductive zone whereas NO3- was majorly distributed in the oxidative zone suggesting the denitrification is partially responsible to the reductive dissolution of As in the groundwater.