Geochemical Characteristics Of Groundwater Colloids In Shallow Aquifers From The Hetao Basin, Inner Mongolia

The hetao basin is one of the most severe and largest arsenic poisoning infected patches in China. Since high As groundwaters are mostly used for drinking water in the area and have caused various endemic diseases, investigations have been intensively carried out to study the source, transport and fate of As in aqueous systems. Groundwater colloidal particles were believed to affect As geochemical behavior in groundwaters, which would mobilize it in the groundwater system. Thus, investigation of As geochemistry as impacted by colloids was carried out by using ultrafiltration technology in the Hetao basin, Inner Mongolia.Results reveal that organic and inorganic colloids widely exist in the slightly alkaline and reductive groundwater. The organic colloids occur as a dominant part of the colloids in groundwaters. Organic colloids were found to be presented in all of the fractions, showing that organic components have broad molecular weights between 5 kDa and 100 kDa with > 10 kDa as dominant particles. The water type has no effect on the distribution. The inorganic colloids are mostly made up of Si, Al/Fe/Mn-oxide/hydroxide, in which the silicate colloids are dominant. Aluminum is well correlated with by silicate, indicating the presence of alumino-silicate micro-minerals. Most of the iron colloids have molecular weights over 100 kDa. The constituents of the inorganic colloids include siderite, rubinglimmer, ferric hydroxide, Ca-montmorillonite, chlorite, illite, K-mica and kaolinite.Arsenic existed as both inorganic As(V) and inorganic As(III) in high As groundwater, with As(III) as dominant species(60~100 %). Most of As exist in the true solution (<5 kDa), while about 30 % of total As is combined by colloids.The colloids associated with As mostly have the range of 5~30 kDa. The colloids combining As with molecular weight > 30 kDa is similar in each ultrafiltration site, about 0~20μg/L. The organic colloids and the iron colloids are the main carriers for As, although the attaching mechanism is complex.The organic acids with molecular weights 5~30 kDa are the main part of the colloid which combine As with functional group. In contrast, the iron colloids with molecular weight over 100 kDa would combine As(V) and act as As(V) carriers.