Distribution of arsenic in sewage irrigation area of Pearl River Delta, China

Abstract

The aim of the present work is to investigate the distribution of arsenic (As) in sewage irrigation area, to deduce the migration and transformation mechanism of As in soil and groundwater, and to infer the source of As in soil and groundwater. This study is carried out in a sewage irrigation area of the Pearl River Delta, China. Surface water samples, soil samples, and groundwater samples from sewage irrigation area were analyzed for As and other elements. As contents in water samples were analyzed by hydride generation-atomic fluorescence spectroscopy, and As fractionation in soil samples was extracted using a seven-step sequential extraction method according to a seven fraction scheme: water soluble, ion exchangeable, bound to carbonate, weakly bound to organic matter, associated with oxides of iron (Fe) and manganese (Mn), strongly bound to organic matter, and the residual fraction. Waste water has content of As up to 16.8 μg/L in the study area. Soil has enriched As due to the irrigation of soil with waste water, and the total content of As in soil is about 0.7 times higher than the background value. Sequential extraction method reveals that the mean content of residual fraction in soil is more than 70%, releasable fraction (weakly organic fraction, Fe-Mn oxide fraction, and carbonate fraction) is about 20%–30%, whereas strongly organic and mobile fractions (water soluble and ion exchangeable) are within 0.2%. In the soil profile, the contents of water soluble, ion exchangeable, and carbonate fraction decrease with the depth, whereas the contents of other fractions are irregular with the depth. Using correlation analysis, it is concluded that water soluble fraction is easy to change into ion exchangeable and carbonate fraction, ion exchangeable fraction is easy to change into carbonate and Fe-Mn oxide fraction, and carbonate fraction is easy to change into weakly organic and Fe-Mn oxide fraction in the soil of study area. Organic matter and (hydr)oxides of Fe and aluminium (Al) in soil play an important role in controlling the distribution and mobility of As in soil. As concentrations in groundwater range from 2.8 to 21.0 μg/L, and it is inferred that As from waste water and the release of high As sediment (soil and aquifer medium) are the main sources for high As groundwater in study area. Using cluster analysis, it is concluded that reducing groundwater with slightly alkaline is beneficial to enrichment of As in groundwater, and hydroxides of Fe, Mn, and Al also play a key role for the enrichment of As in groundwater of the study area.