Twenty-nine wells were selected for groundwater sampling in the town of Shahai, in the Hetao basin, Inner Mongolia. Four multilevel samplers were installed for monitoring groundwater chemistry at depths of 2.5-20m. Results show that groundwater As exhibits a large spatial variation, ranging between 0.96 and 720μg/L, with 71% of samples exceeding the WHO drinking water guideline value(10μg/L). Fluoride concentrations range between 0.30 and 2.57mg/L. There is no significant correlation between As and F ~- concentrations. Greater As concentrations were found with increasing well depth. However, F ~- concentrations do not show a consistent trend with depth. Groundwater with relatively low Eh has high As concentrations, indicating that the reducing environment is the major factor controlling As mobilization. Low As concentrations(<10μg/L)are found in groundwater at depths less than 10m. High groundwater As concentration is associated with aquifers that have thick overlying clay layers. The clay layers, mainly occurring at depths <10m, have low permeability and high organic C content. These strata restrict diffusion of atmospheric O _2 into the aquifers, and lead to reducing conditions that favor As release. Sediment composition is an additional factor in determining dissolved As concentrations. In aquifers composed of yellowish-brown fine sands at depths around 10m, groundwater generally has low As concentrations which is attributed to the high As adsorption capacity of the yellow-brown Fe oxyhydroxide coatings. Fluoride concentration is positively correlated with pH and negatively correlated with Ca ~(2+)concentration. All groundwater samples are over-saturated with respect to calcite and under-saturated with respect to fluorite. Dissolution and precipitation of Ca minerals(such as fluorite and calcite), and F - adsorption-desorption are likely controlling the concentration of F - in groundwater.
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