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Bosten Lake, the largest inland lake in China, has been contaminated with arsenic (As) due to anthropogenic activities. Salinity of the lake water has been increasing due to the extremely dry climate and salt drainage from the farmland. Effects of the increasing salinity on the geochemical behavior of arsenic in sediment are still unclear. In this study, vertical variation of geochemical fractions of As in sediment and arsenic concentration in porewater along the salinity gradient of Bosten Lake was studied. Effects of salinity and ions and anions in porewater on arsenic concentration were also examined. Arsenic content in the five Tessier fractions followed the decreasing order of residual ≫ exchangeable > Fe and Mn oxides > organic > carbonate. The ecological risk of arsenic in the sediment was at low or medium levels, indicated by the low percentage of exchangeable and carbonate bound fractions. Contrary to the decreasing electrical conductivity of porewater with depth, arsenic concentration in porewater showed an increasing trend with depth. Arsenic concentration in porewater increased from around zero at the surface layer of sediment to over 600 μg L in deeper layers of some sites. Arsenic concentration inversely correlated with electrical conductivity in porewater. Arsenic concentration generally decreased with increasing concentrations of SO, Ca, Mg and Na but showed no clear relation with concentrations of K and Cl. This study implies that increase of salinity may decrease the release of arsenic from sediment and its ecological risk. [ABSTRACT FROM AUTHOR]