This paper is the result of hydrogeological survey engineering.

Objective Higharsenic (As) groundwater occurred in the west of Hetao Basin. Investigating the spatial distribution and occurrence characteristics of As in aquifer sediments and studying the enrichment mechanism of As ingroundwater are beneficial to ensure the safety of water for local residents.

Methods Twenty five sediment samples from borehole K02 in the piedmont alluvial fan and twenty sixsamples from borehole K01 in the plain were collectedto analyze lithological characteristics and geochemical components. These samples were further to conduct sequential extraction and desorption experiments of As in sediments.

Results The aquifers in the piedmont alluvial fan were in a relatively oxidized environment, while the aquifers in the plain area were in a closed reducing environment. Salinity of the latter sediment was generally higher than that of the former, and salinity of both sediments had a decreasing trend with depth. Total solid As content in the sediments of the piedmont alluvial vans and the plain area displayed little difference, but the occurrence pool of solid Aswas quite different. The former sediment solid As was dominated by the As incorporated in very amorphous Fe−(oxyhydr) oxides, while the latter was dominated by strongly adsorbed As.

Conclusions The differences of As occurrence characteristics in sediments were the main reason why groundwater As concentration in the plain area was higher than that in piedmont alluvial vans. Desorption experiments showed that weak alkalinity or high Na/Ca^0.5 molar ratio could promote the desorption of As.