Arsenic distribution and speciation in multiphase media of a lake basin, Tibet: The influences of environmental factors on arsenic biogeochemical behavior in the cold arid plateau lake

Abstract

Widespread arsenic (As) has been found in surface media on the Tibetan Plateau, but few studies of As species have been performed because of the difficult sampling conditions. In this study, As distribution and speciation in multiphase media (including surface water, interstitial water, and sediment) in Lake Yamdrok Basin in Tibet in wet and dry seasons were investigated to allow the biogeochemical behavior of As in the cold arid plateau lake to be understood. The total As (TAs) (mainly containing arsenate (As(V)) and arsenite (As(III))) concentrations were generally higher in surface and interstitial water in the lake zones than in Inflowing rivers. Among the four lake zones, significantly higher As concentrations were found in Chen Co, Yamdrok Tso, and Kongmo Co than in Bajiu Co, and surface sediments from the former three lake zones contained relatively high concentrations of the labile As. Redox potential (Eh) in sediments and HCO3 concentration in surface water primarily controlled labile As mobilization through reductive dissolution of As-bearing iron (hydr)oxides and oxyanion competition for As adsorption sites, and therefore affected the As distributions in aqueous phases. As(III) concentrations in interstitial water accounted for 41% ± 33% of TAs, and positively correlated with the arsenate-reducing microbe population in sediments. In contrast, As(V) was predominant in surface water (accounting for 95% ± 8% of TAs), and even trace amounts of dimethylarsinic acid (DMA) was found in wet season. Notably, lower Eh values in dry season triggered a marked increase in the As concentrations in interstitial water, and this probably increased the risk of As contamination of surface water.