Role of colloidal particles for hydrogeochemistry in As-affected aquifers of the Hetao Basin, Inner Mongolia

Abstract

High As groundwater was found in shallow aquifers of the Hetao Basin, Inner Mongolia, which has caused serious chronic arsenism. Groundwater particles are ubiquitous in natural aquifers, and significantly affect As mobilization and transportation. This study has investigated their effect on element hydrogeochemistry. Groundwater samples were filtered in the field through a progressively decreasing pore size (10, 5, 3, 1, 0.8, 0.45 μ(m). Unfiltered samples were also taken. Chemical analysis of 43 groundwater samples shows concentrations of most elements (Ba, Co, Mn, Mo, Ni, V, Sr, Mg, Ca, Na, K) in unfiltered samples are well correlated with those in <0.45 μ fraction, and the ratios between them are close to 1.00. Those elements are considered to be present in the form of true dissolved group and are not affected by the filtration. Although saturation indices for calcite and dolomite are greater than 0, they could possibly exist as small size colloids (<0.45 μ) in groundwater. Concentrations of Cd, Pb, Fe, and Al in <0.45 μ fraction are mostly less than those in unfiltered fraction, implying that large-size inorganic-rich (Al/Fe complexing) colloids could be possible carrier for Cd and Pb in the groundwaters. Arsenic, Cu, Zn and Cr are fallen into another group. These elements could mainly be associated with organic-complexing colloids with particle size less than 0.45 μ although As could partially be carried by large-size colloids. In groundwaters saturated with respect to pyrite and siderite, a small proportion of As (about 15%) is associated with large-size Fe complexing particles (<0.45 μ). Larger amounts of As is trapped with small size organic colloids, which has been confirmed by the ultrafiltration. The study suggests that groundwater particles transported in the aquifers could affect element geochemistry, especially for As expected to be associated in small size complexes.Copyright © 2009 by The Geochemical Society of Japan.